48435_05a

HEAT DISTRIBUTION

SYSTEMS

SECTION 5Co

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Copyright 1997 by The McGraw-Hill Companies Retrieved from: www.knovel.com

CHAPTER 5.1

STEAM

Lehr AssociatesNew York, New York

5.7.7 INTRODUCTIONTOSTEAM

Nearly any material, at a given temperature and pressure, has a set amount of energywithin it. When materials change their physical state, i.e., go from a liquid to agas, that energy content changes. Such a change occurs when water is heated to agaseous statesteam. When steam is used for heating, a cycle of different energystates occurs. First, water is heated in a boiler to its vaporization point, when itboils off as steam. The vapor is carried to the desired estimation where it is allowedto cool, giving off heat. Usually, the water, now cooled back to a liquid, is returnedto the boiler to be revaporized.

The heat content of water is usually measured in British thermal units (Btu's)or calories. Knowing the temperature is not sufficient to determine the energy con-tent of steamthe pressure must also be known as well as the amount of actualvapor or condensate (moisture). "Steam" can exist as saturated (containing all thevapor it can), dry (at the saturation point or above), wet (below the saturation point),and superheated (capable of holding even more vapor). Wet steamcontainingcondensatehas less energy than dry steam.

These conditions are specified for water in a chart called Mollier diagram (seeFig. 5.1.1). The Mollier diagram specifies the energy content for steam at variousvaporization levels. On the two axes of the diagram are enthalpy (a measure of theheat content of a volume of steam) and entropy (a measure of the energy availablefor work). Rigorous analysis of the thermodynamics of a heating system involvesmeasurements of the specific volume of steam available; its pressure, temperature,and moisture values; and the efficiencies of heat transfer of the elements of theheating system. Usually vendors of steam equipment provide details of their systemsbased on saturated-steam conditions, which simplifies their sizing and use. Satu-rated-steam tables (see Table 5.1) give the values that are necessary to determinethe amount of energy the steam has available for heating.

To calculate the steam consumption of a heating device, the following equationshould be employed:

HQ

~ Wwv(Te ~ Tv) + hfg + SPW(TV - Tc) (5'U)Cop

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qi/rug 'Adjemue

IBIOJ.

EntropyFIGURE 5.1.1 Mollier diagram.

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where H = heating load, Btu/h (W)hfg = latent heat of vaporization, Btu/lb (kJ/kg)

Te = entering steam temperature, 0F (0C)Tv = steam temperature at vaporization, 0F (0C)

SPv^ = specific heat of water vapor, Btu/(lb 0F) [cal/(g 0C)]SPW = specific heat of water, Btu/(lb 0F) [cal/(g 0C)]

Tc = leaving temperature of condensate, 0F (0C)Q = steam rate, Ib/h (kg/h)

TABLE 5.1.1 Saturated-Steam Tables

Gaugepressure

in Hgvacuum

27.925.923.921.819.817.815.713.711.69.67.55.53.51.4

psig

O125

1015202530405060708090

100125150175200

Absolutepressure,

psig123456789

101112131414.715.716.719.724.729.734.739.744.754.764.774.784.794.7

104.7114.7139.7164.7189.7214.7

Temperature,op

101.7126.1141.5153.0162.3170.1176.9182.9188.3193.2197.8202.0205.9209.6212.0215.4218.5227.4239.4249.8258.8266.8274.0286.7297.7307.4316.0323.9331.2337.9352.8365.9377.5387.7

Heat contentSensible

(*/).Btu/lb

69.593.9

109.3120.8130.1137.8144.6150.7156.2161.1165.7169.9173.9177.6180.2183.6186.8195.5207.9218.4227.5235.8243.0256.1267.4277.1286.2294.5302.1309.0324.7338.6350.9362.0

Latent(**)Btu/lb1032.91019.71011.31004.9999.7995.4991.5987.9984.7981.9979.2976.7974.3972.2970.6968.4966.4960.8952.9946.0940.1934.6929.7920.4912.2905.3898.8892.7887.0881.6869.3858.0847.9838.4

Total(*,).Btu/lb

1102.41113.61120.61125.71129.81133.21136.11138.61140.91143.01144.91146.61148.21149.81150.81152.01153.21156.31160.81164.41167.61170.41172.71176.51179.61182.41185.01187.21189.11190.61194.01196.61198.81200.4

Specificvolumeof steamVg9 fVVlb330.0173.5118.690.573,461.953.647.342.338.435.132.430.028.026.825.223.8020.416.513.912.010.69.57.86.75.85.24.74.33.93.22.82.42.1

Note: Metric conversion factors are: 1 in Hg = 25.4 mm Hg: 1 Ib/in2 = 0.07 bar; 0F = 1.8 X 0C +32; 1 Btu/lb = 554 cal/kg; 1 ft3/Ib = 0.06 m3/kg.C

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TJQ =

0.45(7; - Tv) + hfg + TV-TC (5'L2)

or in International System (SI) units,TJ

Q = 0.52(7, - Tv) + 0.2Shfg + 1.16(7, - Tc) (5'L3)

When saturated steam is supplied to the heating unit, Te = Tv, so Te Tv = O.Normally Tc is maintained at or near Tv so that the factor Tv Tc can be omittedfrom the calculation without significantly affecting the outcome.

For a system supplying saturated steam we can simplify the calculation to

Q = ^ -OT Q = ^- (SI units) (5.1.4)fg '^^ "%gThe following formula converts the steam rate Q into gallons per minute (liters persecond) so that the condensate will be in units normally associated with the flowof liquids:

^ = gal/min or ^ = L/s (SI units) (5.1.5)

5.7.2 INTRODUCTIONTOSTEAMHEATING SYSTEMS

Steam systems are used to heat industrial, commercial, and residential buildings.These systems are categorized according to the piping layout and the operatingsteam pressure. This section discusses steam systems which operate at or below200 psig (14 bar).

5.7.3 GENERALSYSTEMDESIGN

The mass flow rate of steam through the piping system is a function of the initialsteam pressure, pressure drop through the pipe, equivalent length of piping, andsize of piping. The roughness of the inner pipe wall is a variable in determiningthe steam's pressure drop. All the charts and tables in this section that outline theperformance of the steam transmitted through the piping assume that the roughnessof the piping is equal to that of new, commercial-grade steel pipe.

5.7.4 PRESSURE CONDITIONS

Steam piping systems are usually categorized by the working pressure of the steamthey supply. The five classes of steam systems are high-pressure, medium-pressure,low-pressure, vapor, and vacuum systems. A high-pressure system has an initialpressure in excess of 100 psig (6.9 bar). The medium-pressure system operates withC

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pressures between 100 psig (6.9 bar) and 15 psig (1 bar). Systems that operate from15 psig (1 bar) to O psig (O bar) are classified as low-pressure. Vapor and vacuumsystems operate from 15 psig (1 bar) to vacuum. Vapor systems attain subatmos-pheric pressures through the condensing process, while vacuum systems require amechanically operated vacuum pump to attain subatmospheric pressures.

5.1.5. PIPINGARRANGEMENTS

The general piping scheme of a steam system can be distinguished by three differentcharacteristics. First, the number of connections required at the heating device de-scribes the system. A one-pipe system has only one piping connection which sup-plies steam and allows condensate to return to the boiler by flowing counter to thesteam in the same pipe. The more common design is to have two piping connec-tions, one for the supply steam and one for the condensate. This arrangement isknown as a two-pipe system.

Second, the direction of the supply steam in the risers characterizes the pipingdesign. An up-feed system has the steam flowing up the riser; conversely, a down-feed system supplies steam down the riser.

Third, the final characteristic of the piping design is the location of the conden-sate return to the boiler. A dry return has its condensate connection above theboiler's waterline, while a wet-return connection is below the waterline.

5.7.6 CONDENSATERETURN

By analyzing how the condensate formed in the heating system is returned to theboiler, an understanding of how the system should operate is achieved. There aretwo commonly used return categories: mechanical and gravity.

If devices such as pumps are used to aid in the return of condensate, the systemis known as a mechanical return. When no mechanical device is used to return thecondensate, the system is classified as a gravity return. The only forces pushingthe condensate back to the boiler or condensate receiver are gravity and the pressureof the steam itself. This type of system usually requires that all steam-consumingcomponents be located at a higher elevation than the boiler or the condensate re-ceiver.

With either mechanical or gravity return systems, the mains are normally pitched1A in (6.3 mm) for every 10 ft (3 m) of length, to ensure the proper flow ofcondensate. The supply mains are sloped up away from the boiler, and the returnmains are pitched down toward the boiler. This allows condensate to flow back tothe boiler.

5.7.7 PIPE-SIZING CRITERIA

Once the heating loads are known, the steam flow rates can be determined; thenthe required size of the steam piping can be specified for proper operation. Thefollowing factors must be analyzed in sizing the steam piping:

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Initial steam pressure Total allowable pressure drop Maximum steam velocity Direction of condensate flow Equivalent length of system

For different initial pressures, the allowable pressure drop in the piping varies.Table 5.2 gives typical values in selecting pressure-drop limits. To ensure that theparameters from the table are suitable for an application, check that the total systempressure drop does not exceed 50 percent of the initial pressure, that the condensatehas enough steam pressure to return to the boiler, and that the steam velocity iswithin specified limits to ensure quiet and long-lasting operation.

When steam piping is sized, there is a trade-off between quiet, efficient operationand first-cost considerations. A good compromise point exists when the steam sup-ply pipe is sized for velocities between 6000 and 12,000 ft/min (30.5 and 61m/s). This allows quiet operation while offering a reasonable installed cost. If thepiping is downsized so that the velocity exceeds 20,000 ft/min (101 m/s), thesystem may produce objectional hammering noise or restrict the flow of condensatewhen it is counter to the steam's direction. It is recommended that the piping besized so that the velocity will never approach 20,000 ft/min (101 m/s) in any leg.

As condensate flows into the return line, a portion of it will flash into steam.The volume of the steam-condensate mixture is much greater than the volume ofpure condensate. To avoid undersizing the return lines, the return piping should besized at some reasonable proportion of dry steam. A maximum size would be toassume that the return is 100 percent saturated steam. An acceptable velocity forthe design of the return lines is 5000 ft/min (25.4 m/s).

5.1.8 DETERMINING EQUIVALENT LENGTH

The "equivalent length" of pipe is equal to the actual length of pipe plus the frictionlosses associated with fittings and valves. For simplicity's sake, the fitting and valvelosses are stated as the equivalent length of straight pipe needed to produce thesame friction loss. Values for common fittings and valves are stated in Table 5.1.3.

The equivalent lengthnot the actual lengthis the value used in all the figuresand charts for pipe sizing. Common practice is to assume that the equivalent lengthis 1.5 times the actual length when a design is first being sized. After the initialsizing and layout are completed, the exact equivalent length should be calculatedand all the pipe sizes checked.

5.1.9 BASIC TABLES FOR STEAM PIPE SIZING

Figure 5.1.2 is used to determine the flow and velocity of steam in Schedule 40pipe at various values of pressure drop per 100 ft (30.5 m), based on O psig(1-bar) saturated steam. By using the multiplier tables, it may also be used at allsaturated pressures between O and 200 psig (1 and 14 bar). Figure 5.1.2 is validonly when steam and condensate flow in the same direction.

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TABLE 5.1.2 Pressure Drops for Steam Pipe Sizing

Total pressure drop inreturn piping

Pressure drop for mainsand risers

Total pressure drop insupply pipingInitial steam pressure

barlb/in2bar/100 m(lb/in2)/100ftbarlb/in2barpsig

0.0690.0040.0170.0690.2760.3450.691.031.37

1Vl6V4145

101520

0.028-0.0570.0070.0280.0570.2280.455

0.455-1.140.455-1.140.455-2.28

V*-V4Vl2VsV412

2-52-52-10

0.069-0.1380.004-0.0170.017-0.0520.069-1.380.276-0.4140.345-0.0690.069-1.031.03-1.721.72-2.07

1-2V6-1/41/4-3/41-24-65-10

10-1515-2525-30

O0.1380.3451.032.073.456.90

10.3

VacuumO25

153050

100150

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TABLE 5.1.3 Length of Pipe to Be Added to Actual Length of RunOwing toFittingsto Obtain Equivalent Length

Size ofpipe, in

V2y*

iIViIy222V233V24568

101214

Standardelbow

1.31.82.23.03.54.35.06.589

111317212730

Side outletteet

345678

11131518222735455363

Gatevalve$

0.30.40.50.60.81.01.11.41.61.92.22.83.74.65.56.4

Globevalve$

14182329344654668092

112136180230270310

Anglevalve$

7101215182227344045566792

112132152

Length to be added to run, ft*

*Metric conversion: 1 in = 2.54 cm and 1 ft = 0.31 m.fValues given apply only to a tee used to divert the flow in the main to the last riser.$ Valve in full-open position.Example: Determine the length in feet of pipe to be added to actual length of run illustrated.

Measured length 132.0 ft4-in gate valve 1.9 ft4-4 in elbows 36.0 ft2-4 in tees 36.0 ftEquivalent 205.9 ft

Source: Reprinted by permission from ASHRAE Handbook1989 Fundamentals.

5.1.10 TABLES FOR LOW-PRESSURE STEAMPIPE SIZING

Table 5.1.4, derived from Fig. 5.1.2, gives the values needed to select pipe sizes atvarious pressure drops for systems operating at 3.5 and 12 psig (0.24 and 0.84 bar).The flow rates given for 3.5 psig (0.24 bar) can be used for saturated-steam pres-sures from 1 to 5 psig (0.07 to 0.34 bar), and those for 12 psig (0.84 bar) can beused for saturated pressures from 8 to 16 psig (0.55 to 1.1 bar) with an error notto exceed 8 percent.

Table 5.1.5 is used for systems where the condensate flows counter to the supplysteam.

Lastriser orradiator

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PRESSU

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TOO FT.

PRESSU

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PER

SO.

IN.

PER

100 FT.

SATURAT

ED SRE

AM PRE

SSURE

-

PSIG

FLOW RATE - POUNDS PER HOURBASED ON MOODY FRICTION FACTOR WHERE FLOW OF CONDENSA7E DOES NOT INHIBIT THE FLOW OF STEAM

FIGURE 5.1.2 Basic chart for flow rate and velocity of steam in Schedule 40 pipe, based onsaturation pressure of O psig (O bar). (Reprinted by permission from ASHRAE Handbook1989Fundamentals.)

To size return piping, Table 5.1.6 is used. This table gives guidelines for returnpiping for wet, dry, and vacuum return systems.

5.1.11 TABLES FOR SIZING MEDIUM- ANDHIGH-PRESSURE PIPE SYSTEMS

Larger, industrial-type space-heating systems are designed to use either medium-or high-pressure steam at 15 to 200 psig (1.03 to 14 bar). These systems often

METRIC CONVERSIONS

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TABLE 5.1.4 Flow Rate of Steam, Ib/h, in Schedule 40 Pipe* at Initial Saturation Pressure of 3.5 and 12 psigt

Pressure drop, lb/in2 per 100-ft length^

2 lb/in2 Sat.press., psig

1 lb/in2 Sat.press., psig

3/4 lb/in2 (12 oz)Sat. press.,

Psig

V2 lb/in2 (8 oz)Sat. press.,

psigV4 lb/in2 (4 oz)Sat press., psig

Vfe lb/in2 (2 oz)Sat. press.,

psig

Vi6 lb/in2 (1 oz)Sat. press.,

psigNom.pipesize.

3.5 123.5 123.5 123.5 123.5 123.5 123.5 12in60 73

114 137232 280360 430710 850

1,150 1,3701,950 2,4002,950 3,4504,200 4,9007,500 8,600

11,900 14,20024,000 29,50042,700 52,00067,800 81,000

42 5081 95

162 200246 304480 590780 950

1,380 1,6702,000 2,4202,880 3,4605,100 6,1008,400 10,000

16,500 20,50030,000 37,00048,000 57,500

36 4368 82

140 170218 260420 510680 820

1,190 1,4301,740 2,1002,450 3,0004,380 5,2507,200 8,600

14,500 17,70026,200 32,00041,000 49,500

29 3554 66

111 138174 210336 410540 660960 1,160

1,410 1,7001,980 2,4003,570 4,2505,700 7,000

11,400 14,30021,000 26,00033,000 40,000

20 2437 4678 96

120 147234 285378 460660 810990 1,218

1,410 1,6902,440 3,0003,960 4,8508,100 10,000

15,000 18,20023,400 28,400

14 1626 3153 6684 100

162 194258 310465 550670 800950 1,160

1,680 2,1002,820 3,3505,570 7,000

10,200 12,60016,500 19,500

9 1117 2136 4556 70

108 134174 215318 380462 550640 800

1,200 1,4301,920 2,3003,900 4,8007,200 8,800

11,400 13,700

1X41IV4iy222V233V24568

1012*R*Based on Moody friction factor, where flow of condensate does not inhibit the flow of steam.tThe flow rates of 3.5 psig can be used to cover saturated pressure from 1 to 6 psig, and the rates at 12 psig can

be used to cover saturated pressure from 8 to 16 psig with an error not exceeding 8 percent. The steam velocitiescorresponding to the flow rates given in this table can be found from the basic chart and velocity multiplier chart, Fig.5.2.

^Metric conversions: 1 in = 2.54 cm, 1 lb/in2 = 0.07 bar, and 1 Ib = 0.46 kg.Source: Reprinted by permission from ASHRAE Handbook 1989 Fundamentals.

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TABLE 5.1.5 Steam Pipe Capacities for Low-Pressure Systems, Ib/hFor use on one-pipe systems or two-pipe systems in which condensate flows against the steam flow

Radiator and riserrunouts

One-pipe systemsRadiator valves andvertical connections

Two-pipe systems

Supply risers up-feed

Condensate flowing against steamHorizontalVertical

Nominal pipe size,in

FtED$CtB*A77

1616234265

119186278545

7162342

611203872

116200286380

714274293

132200288425788

1,4003,0005,7009,500

19,000

814314897

159282387511

1,0501,8003,7507,000

11,50022,000

V41IViIV222V233V24568

101216

*Do not use column B for pressure drops of less than Vie lb/in2 per 100 ft of equivalent run. Use Fig. 5.2 or Table5.4 instead.

fPitch of horizontal runouts to risers and radiators should be not less than l/2 in /ft. Where this pitch cannot beobtained, runouts over 8 ft in length should be one pipe size larger than called for in this table.$Do not use column D for pressure drops of less than VTA lb/in2 per 100 ft of equivalent run except on sizes 3 inand over. Use Fig. 5.2 or Table 5.4 instead.

Note: Steam at an average pressure of 1 psig is used as a basis of calculating capacities. Metric conversion factorsof 1 in = 2.54 cm and 1 Ib = 0.46 kg can be used.

Source: Reprinted from ASHRAE Handbook 1989 Fundamentals.

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TABLE 5.1.6 Return Main and Riser Capacities for Low-Pressure Systems, Ib /hl/i lb/in2 or 8-oz drop

per 100 ft1A lb/in2 or 4-oz drop

per 100 ft1Xs lb/in2 or 2-oz drop

per 100 ftVi6 lb/in2 or 1-ozdrop per 100 ft

1/24 lb/in2 or %-ozdrop per 100 ft

Vfc lb/in2 or 1Xs-OZdrop per 100 ftPipe

size-Wet Dry Vac.Wet Dry Vac.Wet Dry Vac.Wet Dry Vac.Wet Dry Vac.Wet Dry Vac.inW X YT U VQ R SN O PK L MH I JG

Return Main283494848

1,3402,8304,7307,560

11,30015,50027,30043,800

200350 115 350600 241 600950 378 950

2,000 825 2,0003,350 1360 3,3505,350 2500 5,3508,000 3580 8,000

11,000 5380 11,00019,40031,000

142250 103 249425 217 426675 340 674

1400 740 1,4202350 1230 2,3803750 2250 3,8005500 3230 5,6807750 4830 7,810

13,70022,000

100175 80 175300 168 300475 265 475

1000 575 1,0001680 950 1,6802680 1750 2,6804000 2500 4,0005500 3750 5,500

9,68015,500

42145 71 143248 149 244393 236 388810 535 815

1580 868 1,3602130 1560 2,1803300 2200 3,2504580 3350 4,500

7,88012,600

125 62213 130338 206700 470

1180 7601880 14602750 19703880 2930

-V41I1X4I1X2221X2331X2456

Riser494848

1,3402,8304,7307,560

11,30015,50027,30043,800

48 350113 600248 950375 2,000750 3,350

5,3508,000

11,00019,40031,000

48 249113 426248 674375 1,420750 2,380

3,8005,6807,810

13,70022,000

48 175113 300248 475375 1,000750 1,680

2,6804,0005,5009,680

15,500

48 143113 244248 388375 815750 1,360

2,1803,2504,4807,880

12,600

48113248375750

%1I1X4I1X2221X2331X245

Note: This table is based on pipe size data developed through the research investigations of The American Societyof Heating, Refrigerating and Air-Conditioning Engineers. Metric conversion factors of 1 in = 2.54 cm, 1 lb/in2 =0.07 bar, and 1 ft = 0.31 m can be used.

Source: Reprinted by permission from ASHRAE Handbook 1989 Fundamentals.

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involve unit heaters and/or air-handling units. Figures 5.1.3 to 5.1.6 provide tablesfor sizing steam piping for systems of 30, 50, 100, and 150 psig (2, 3.5, 6.9, and10.5 bar).

5.7.72 AIRVENTS

The presence of air in the steam supply line impedes the heat-transfer ability of thesystem due to the high insulating value of air. Air also interferes with the flow ofsteam by forming pockets at the ends of runs that prevent the steam from reachingthe system's extremities.

A valve that releases air from the system while restricting the flow of all otherfluids is known as an "air vent." Air vents should be located at all system highpoints and where air pockets are likely to form. Venting should be done continuallyto prevent the buildup of air in the system.

Air enters the system by two means. First, when cold makeup feed water issupplied to the boiler, air is present in the water. As the water is heated, the air

FLOW ANDVELOCfTY BASEDON 30 PSIG

PRESSU

RE DRO

P -

PSI

PER

100 FT.

PRESSU

RE DRO

P -

OUNCES

PER

SO.

IN.

PER

100 FT.

FLOW RATE - POUNDS PER HOURBASED ON MOODY FRICTION FACTOR WHERE FLOW OF CONDENSATE DOES NOT INHIBIT THE FLOW OF STEAM

(MAY BE USED FOR STEAM PRESSURE FROM 23 TO 37 PSlG WITH AN ERROR NOT EXCEEDING 9%)METRIC CONVERSIONS:1 Ib = 0.45 kg; 1 Ib/in2 = 0.07 bar; 1 ft = 0.3 m;1 in2 = 6.5 cm2; 1 oz - 28.1 g.

FIGURE 5.1.3 Chart for flow rate and velocity of steam in Schedule 40 pipe, based on saturationpressure of 30 psig (2.1 bar). (Reprinted by permission from ASHRAE Handbook7959 Funda-mentals.}C

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PRESSU

RE DRO

P -

PSI

PER

100 FT.

PRESSU

RE DRO

P -

OUNCES

PER

SO.

IN.

PER

100 FT.

FLOW RATE - POUNDS PER HOURBASED ON MOODY FRICTION FACTOR WHERE FLOW OF CONDENSATE DOES NOT INHIBIT THE FLOW OF STEAM(MAY BE USED FOR STEAM PRESSURE FROM 40 TO 60 PSIG WITH AN ERROR NOT EXCEEDING 8%)

METRIC CONVERSIONS:1 Ib = 0.45 kg; 1 Ib/in2 = 0.07 bar; 1 ft = 0.3 m;1 in2 * 6.5 cm2; 1 oz = 28.1 g.

FIGURE 5.1.4 Chart for flow rate and velocity of steam in Schedule 40 pipe, based onsaturation pressure of 50 psig (3.5 bar). (Reprinted by permission from ASHRAEHandbook1989 Fundamentals.}

tends to separate from the water. Second, when the system is turned off, steam istrapped in the pipes. Eventually the steam cools and condenses. Since the volumeof the condensate is negligible compared to the initial volume of the steam, avacuum is formed in the piping. Air leaks into the system through openings in thejoints until the internal pressure equalizes. Upon restarting the system, the air isswept along with the steam and becomes entrained in the system.

5.7.73 STEAMTRAPS

When steam is transmitted through the piping or the end-user equipment, it losespart of its heat energy. As heat is removed from saturated steam, a vapor-liquidmixture forms in the pipe. The presence of liquid condensate in the steam linesinterferes with the proper operation of the system. Liquid condensate derates thesystem's heating capacity because water has a much smaller amount of availableCo

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PRESSU

RE DRO

P -

PSI PER

100

FT.

PRESSU

RE DRO

P -

OUNCES

PER

SO.

IN.

PER

100 FT.

F-LOW RATE - POUNDS PER HOURBASED ON MOODY FRICTION FACTOR WHERE FLOW OF CONDENSATE DOES NOT INHIBIT THE FLOW OF STEAM

(MAY BE USED FOR STEAM PRESSURE FROM 85 TO 120 PSIG WITH AN ERROR NOT EXCEEDING 8%)METRIC CONVERSIONS:1 Ib - 0.45 kg; 1 Ib/in2 = 0.07 bar; 1 ft = 0.3 m;1 in2 = 6.5 cm2; 1 oz = 28.1 g.

FIGURE 5.1.5 Chart for flow rate and velocity of steam in Schedule 40 pipe, based on saturationpressure of 100 psig (7 bar). (Reprinted by permission from ASHRAE Handbook1989 Funda-mentals.}

energy than steam does. Furthermore, the accumulation of water in the supply steampiping can obstruct the flow of the steam through the system.

A valve that permits condensate to flow from the supply line without allowingsteam to escape is known as a "steam trap." All steam traps should be located suchthat condensate can flow via gravity through them. Through mechanical means, thesteam trap recognizes when steam is present by sensing the density, kinetic energy,or temperature of the fluid at the trap. When conditions indicate that steam is absent,the trap opens and allows the condensate to drop to the return line. As soon as thetrap senses the presence of steam, it slams shut.

5.1,14 STEAMTRAPTYPES

There are six types of steam traps normally employed in the heating, ventilating,and air-conditioning (HVAC) industry. Since traps differ in their operational char-Co

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PRESSU

RE DRO

P -

PSI

PER

100 FT.

PRESSU

RE DRO

P -

OUNCES

PER

SQ.

IN.

PER

100 FT.

FLOW RATE - POUNDS PER HOURBASED ON MOODY FRICTION FACTOR WHERE FLOW OF CONDENSATE DOES NOT INHIBIT THE FLOW OF STEAM

(MAY BE USED FOR STEAM PRESSURE FROM 127 TO 180 PSIG WITH AN ERROR NOT EXCEEDING 8%)METRIC CONVERSIONS:1 Ib = 0.45 kg; 1 Ib/in2 = 0.07 bar; 1 ft = 0.3 m;1 in2 = 6.5 cm2; 1 oz = 28.1 g.

FIGURE 5.1.6 Chart for flow rate and velocity of steam in Schedule 40 pipe, based on saturationpressure of 150 psig (10.5 bar). (Reprinted by permission from ASHRAE Handbook1989 Fun-damentals.)

acteristics, selection of the proper trap is critical to efficient operation of the system.Different applications require specific types of traps, and no one type of trap willperform satisfactorily in all situations.

Three of the six basic types of traps operate thermostatically be sensing a tem-perature difference between subcooled condensate and steam: liquid-expansion, bal-anced-pressure thermostatic, and bimetallic thermostatic traps. Two othertypesthe bucket trap and the float-and-thermostatic trapare activated by differ-ences in density between steam and condensate. These are also known as blast typetraps. Finally, the thermodynamic steam trap operates on the differences in thevelocity at which steam passes through the trap. This velocity difference can alsobe considered as a change in kinetic energy.

5.7.15 BALANCED-PRESSURE STEAM TRAPS

The balanced-pressure steam trap (Fig. 5.1.7) employs a bellows filled with a fluidmixture that boils below the steam temperature. When steam is present at the trap

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Bellows

Valve

FIGURE 5.1.7 Balanced-pressure steam trap.

inlet, the liquid in the bellows is vaporized and expands to seal the trap. Condensateaccumulates at the trap and starts to subcool. When the condensate cools enoughto condense the fluid in the bellows, the trap opens and the condensate flowsthrough the trap.

This type of trap has two possible drawbacks. First, it must allow condensate tosubcool 5 to 3O0F (2.8 to 16.70C) below the steam temperature to operate. Second,it discharges condensate intermittently.

Advantages of the balanced-pressure trap are that it is freeze-proof, can handlea large condensate load, does a good job of air venting, and is self-adjustingthroughout its operating range. These traps are typically used in conjunction withsteam radiators and sterilizers.

5.1.16 BIMETALLICTHERMOSTATICSTEAM TRAPS

These traps operate on the same principle as the balanced-pressure steam trap. Thebellows mechanism is replaced by a bimetallic strip formed from two dissimilarmetals that have very different coefficients of expansion. As the bimetallic strip isheated, the difference in the expansion rate of the metals causes the strip to bend.The trap is fabricated so that when the strip is heated to the steam's temperature,there is enough movement to close off the valve. The bimetallic thermostatic trap(Fig. 5.1.8) has a slow response to load conditions, requiring as much as 10O0F(55.50C) of subcooling, and is not self-adjusting to changes in inlet pressure.

These traps are suited for superheated steam applications and situations wherea great deal of condensate subcooling is required to prevent flashing in the returnline. Normally these traps are applied to steam-tracing lines that can tolerate partialflooding.

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Bimetal

Valve

FIGURE 5.1.8 Bimetallic steam trap.

5.7.77 LIQUID-EXPANSION STEAM TRAPS

The liquid-expansion steam trap (Fig. 5.1.9) is designed with an oil-filled cylinderwhich drives a piston. When steam is present, the oil expands, thrusting the pistonout. The end of the piston acts as the valve and seals the port to the return line.As condensate collects in the trap and cools, the oil starts to contract. The con-traction of the oil causes the piston to move away from the port and permits theflow of condensate from the trap.

These traps are freeze-proof and are used for freeze protection of system lowpoints and heating coils. Their limitations are that they are not self-adjusting tochanges of inlet pressure and that they require condensate subcooling by 2 to 3O0F(1.1 to 16.70C).

5.7.78 BUCKETSTEAMTRAPS

Bucket traps operate by gravity, utilizing the density difference between liquid andvapor. When the body of the trap is filled with liquid and a vapor enters the bucket,the bucket will float. As the bucket fills with liquid, the bucket sinks. The bucket's

Valve Piston Liquid-filled chamber

FIGURE 5.1.9 Liquid-expansion steam trap.Cop

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movement activates a valve. If the bucket rises due to the vapor pressure, the valvecloses; and when the bucket sinks, the valve opens, permitting condensate to flowfrom the trap. The most common type of bucket trap is the inverted bucket (Fig.5.1.10), so named because the bucket has its open side facing down.

Bucket traps are capable of working at very high pressures, can discharge con-densate at the saturated-steam temperature, and are resistant to water hammer. Un-fortunately, if the water seal is lost, the bucket trap will continuously allow steamto pass through. Other disadvantages of these traps are their susceptibility to freeze-up, their lack of good air-venting capability, and their intermittent discharge.

Inverted-bucket traps are usually installed on high-pressure indoor steam maindrips.

5.1.19 FLOAT-AND-THERMOSTATICSTEAM TRAPS

A float-and-thermostatic steam trap (Fig. 5.1.11) is actually two distinct traps inone unit. The balanced-pressure steam trap, outlined previously, is located at thetop of the trap body and acts as an air vent. The rest of the unit consists of a floatthat rises and falls based on the level of condensate in the trap. The trap inlet islocated above the outlet. The float position operates a valve that controls flow tothe return line. As the condensate level rises above the outlet, the float causes thevalve to open. If the condensate level drops enough, the float causes the valve toclose. Since the float allows the valve to open only when the condensate level is

Valve

Air ventBucket

FIGURE 5.1.10 Inverted-bucket steam trap.Cop

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ThermostaticAir vent

Float

ValveFIGURE 5.1.11 Float-and-thermostat steam trap.

above the outlet, a water seal is maintained to prevent steam from passing throughthe outlet when the valve is open.

The float-and-thermostatic steam traps cannot be used on a superheated-steamsystem unless they are modified and are usually not installed outdoors because theyare subject to freeze-up. These types of traps will continuously vent air. They donot require subcooling of condensate and are unaffected by changes in systempressure. Typically float-and-thermostatic traps are used in conjunction with heatingdevices, such as unit heaters, water heaters, and converters.

5.1.20 THERMODYNAMICSTEAMTRAPS

The design of the thermodynamic steam trap (Fig. 5.1.12) is based on the theorythat the total pressure of fluid passing through the trap will remain constant. Sincethe total pressure equals the sum of the static and dynamic pressures, any increase

Disk

FIGURE 5.1.12 Thermodynamic steam trap.Copy

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in dynamic pressure will cause a decrease in the static pressure, and vice versa.These traps have only one moving part, a disk that can seal off both the inlet andthe outlet of the trap. Steam entering the trap accelerates radially over the disk,causing a reduction in static pressure under the disk. As the steam dead-ends abovethe disk, the static pressure above the disk increases. This difference in pressureinduces the disk to seal off the trap's openings. The trap will remain closed untilthe steam in the trap condenses sufficiently to reduce the pressure above the diskto an amount less than the inlet steam pressure. At that point, the disk moves awayfrom the inlet port.

Thermodynamic steam traps should not be used on systems operating below 5psig (0.34 bar) or on those that have back pressures equal to or greater than 80percent of their supply pressure.

These traps are compact and have a long life due to the simplicity of their design.They can operate under high pressures, responding quickly to load and pressurevariations while discharging condensate without requiring subcooling. Thermody-namic traps are usually installed in main drips and steam tracer lines.

5.7.27 STEAMTRAPLOCATION

Steam traps are located either in the return line or in drip legs. A "drip leg" (shownin Fig. 5.1.13) is a piping assembly that hangs below the supply main; its purposeis to remove condensate and sediment from the main. Gravity allows condensateand sediment to leave the main and accumulate in the drip leg. When the condensatein the leg rises to the level of the trap intake, the trap fills and then discharges thecondensate to the return line. The drip leg pipe should be of sufficient size to permitcondensate to drain freely from the main. For mains of 4 in (102 mm) or less indiameter, the drip leg should be the same size as the main pipe. For mains largerthan 4 in (102 mm), the pipe diameter of the drip leg should be half of the main'ssize, but not less than 4 in (102 mm). Where possible, all drip legs should be atleast 18 in (45.7 cm) long. A trap should be installed in the return line after everysteam-consuming device. Each device should have its own trap to prevent possible"short-circuiting" that could occur if multiple devices share a common trap. A dripleg should be located before risers, expansion joints, bends, valves, and regulators.System low points, end of mains, and untrapped supply runs of over 300 ft (100m) are additional locations where drip legs should be installed.

5.1.22 STEAMTRAPSIZING

A steam trap must be properly sized to handle the full load of condensate. Forheating devices, the method of determining the amount of condensate was discussedin Sec. 5.1. (See also Sec. 5.24. See also "determining condensate load for a sys-tem," next page.) Mains have their largest condensate loads during startup. Table5.7 gives values for the condensate load of mains at startup.

The performance of a steam trap is affected by the inlet pressure and backpressure of the system. Therefore, when a trap is chosen, it is prudent to oversizethe trap by a reasonable amount. Table 5.1.8 gives a guideline on how large to sizetraps. Grossly oversizing a trap will cause the system to operate improperly.Co

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Steam supply main

18-in.minimum

Strainer

Float andthermostaticsteam trap

Servicevalve

Dirtpocket

Condensate return main

FIGURE 5.1.13 Typical drip-leg piping assembly.

TABLE 5.1.7 Startup Condensate Loads in Steam Mains, Ib/h per 100-ft Length

Pipe Steam pressure, psig*tsize,

in O 5 15 30 50 100 150 2002 6 7 8 9 10 13 15 162V2 10 11 12 14 16 20 23 253 13 14 17 19 22 27 30 334 18 20 23 27 31 38 43 475 25 28 32 37 42 51 58 646 32 36 41 48 55 67 75 838 48 54 62 72 82 100 113 125

10 68 77 88 102 116 142 160 17712 90 101 116 134 153 188 212 234

*Based on 7O0F (210C) ambient air. Schedule 40 pipe uninsulated.fFor metric equivalents, use the following conversion factors: 1 in = 2.54 cm = 25.4 mm; 1 Ib/in2 =

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TABLE 5.1.8 Steam Trap Selection: Safety Factor

Trap type Safety factor multiplierBalanced-pressure thermostatic trap 3Bimetallic trap 2.5Liquid-expansion trap 3Inverted-bucket trap 2.5Float-and-thermostatic trap 2Thermodynamic trap 1.5

5.7.23 STEAMTRAPSELECTION

Once the size of the steam trap is known, the type of trap which will provide thebest performance must be selected. When a trap is chosen, care must be taken toselect a type that will operate over the full range of pressures that the system willexert.

The best operating economy based on trap life and minimization of waste steammust be considered. If the trap will be subjected to low ambient temperatures, itshould be of a freeze-proof design. For traps serving heating devices, continuousgas-venting capability is desirable. When the application is examined, the need forsteam trap construction which is resistant to corrosion and water hammering shouldbe considered.

5.7.24 DETERMININGCONDENSATELOADFORA SYSTEM

The steam consumption of a system over time is equal to the amount of condensateformed during that period. Unfortunately, only when traps of the modulating type(such as float-and-thermostatic traps) are employed does the condensate return si-multaneously equal the steam consumption.

If a blast type, say a bucket trap, is installed, the flow of condensate will beintermittent and equal to the trap's discharge rate, not the steam consumption rate.Since blast-type traps discharge intermittently, you can safely assume that not allthe traps will discharge at once. For sizing purposes, the rule of thumb is that nomore than two-thirds of the blast-type traps will discharge at any given time. Thiscondensate load and the design steam consumption for the equipment utilizingmodulating-type traps should be combined to determine the peak condensate loadof the entire system. When the piping is sized, consider oversizing the condensatereturn main by one pipe size. This can be beneficial when future increases in thesystem's steam consumption are anticipated.

5.7.25 WATER DAMAGE

Water hammering is a phenomenon that occurs when condensate remaining in apipe flashes into steam. The sudden expansion of the condensate causes a vibrationC

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in the pipe which can lead to premature failure of joints and can cause an objec-tional noise throughout the structure the pipe is serving. A more dangerous situationcan develop if enough condensate accumulates in the pipe to block the passage ofsteam. The steam pressure behind the blockage will build up. Eventually the block-age may be transmitted through the pipe at a speed approaching the design velocityof the steam. When water travels at such a high velocity, it can damage the firstobstruction it comes to, such as a valve or elbow. Both water hammering anddamage from blockages can be prevented by proper trapping and pitching of thesteam lines.

When certain gases, such as carbon dioxide (CO2), are trapped in steam lines,the gases tend to mix with the condensate and form unwanted by-products, suchas mild acids. These by-products will accelerate the rate of erosion in the systemand cause premature failure in the system's components. Proper air venting willreduce the amount of gas in the system and increases its operating life.

5.1.26 WATERCONDITIONING

The formation of scale and sludge deposits on boiler heating surfaces creates aproblem in generating steam. Water conditioning in a steam generating systemshould be under the supervision of a specialist. Refer to Chap. 8.5 of this handbookfor a discussion of water treatment.

5.7.27 FREEZEPROTECTION

Whenever a steam system is servicing an area whose outdoor temperature will dropbelow 350F (1.70C), the designer must make provisions to prevent freezing. Analarm should be installed to alert the building operator of a loss of steam pressureor exceptionally low condensate temperatures. If air-handling units are used, thealarm should also terminate the supply fan's operation. The following recommen-dations will help to minimize freezing problems in steam systems:1. Select traps of nonfreezing design if they are located in potentially cold areas.2. Install a strainer before all heating units.3. Do not oversize traps.4. Make sure that condensate lines are properly pitched.5. Keep condensate lines as short as possible.6. Where possible, do not use overhead return.7. If heating coils are used, allow only the interdistributing tube type.8. Limit the maximum tube length of heating coils to 10 ft (3 m).9. All coils and lines should be vented and drainable.

5.1.28 PIPINGSUPPORTS

All steam piping is pitched to facilitate the flow of condensate. Table 5.1.9 containsthe recommended support spacing for piping. The data are based on Schedule 40pipe filled with water and an average amount of valves and fittings.

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TABLE 5.1.9Recommended Hangar Spacing

Distancebetween

supports, ftPipe size, in Length

3/4 41 7iy4 7I1X2 92 1021X2 113 1231X2 124 145 156 178 19

10 2012 2314 25

Note: Figures are based on Schedule 40 steelpipe filled with water including a normal amount ofvalving and fittings. These conversion factors can beused: 1 in = 2.54 cm and 1 ft = 0.3 m.

5.1.29 STRAINERS

Strainers (Fig. 5.1.24) should be located in the supply main before all steam-consuming devices and as part of the drip-leg assembly to collect particles andsediment carried in the system. Strainers located in areas not susceptible to freeze-

Stroinerscreen

FIGURE 5.1.14 Typical strainer.Cop

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up should extend down directly under the steam lines to allow sediment and par-ticles to collect at the bottom of the strainer. In areas where freezing is possible,strainers should be installed at about a 20 angle below the horizontal plane. Thiswill form an air pocket which will allow for expansion if the water in the strainerfreezes.

The strainers should be cleared regularly as part of a routine maintenance sched-ule.

5.1.30 PRESSURE-REDUCINGVALVES

As steam pressure increases, the specific volume of the steam decreases as well asthe heat of vaporization.

Many times the boiler is designed to operate at a higher steam pressure than theheating components. The higher boiler pressure allows the supply-main size to bereduced because of the smaller specific volume of the steam. At a convenient pointin the main near the heating devices, a "pressure-reducing valve" is installed. Thisvalve reduces the pressure and allows the steam to expand. As the steam expands,its heat of vaporization increases, allowing for greater system efficiency. The pipesize directly downstream of the pressure-reducing valve should be increased toaccommodate the steam's expansion. This should be done even if the reducing-valve connections for the inlet and outlet are the same size.

5.1.31 FLASHTANKS

A reservoir where condensate accumulates at low pressure before it returns to theboiler is normally provided. Another name for this reservoir is the flash tank. Asthe hot condensate reaches a low-pressure area, some of the liquid will flash intosteam.

At the top of the flash tank, a steam line routes the steam that has just formedback into the system to be utilized. The flash tank improves the efficiency of thesystem and guarantees that only liquid condensate is returned to the boiler.

5.1.32 STEAMSEPARATORS

The need for pure steam without the presence of water droplets is imperative topermit control devices to operate properly. A device that allows vapor to pass whileknocking water droplets from the stream is known as a steam separator.

Steam separators should be installed before all control devices and anywhereelse in the system where small water droplets cannot be tolerated. Obviously, steamseparators are not required on superheated-steam installations.

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CHAPTER 5.2

HOT-WATER SYSTEMS

Lehr AssociatesNew York, New York

5.2.1 INTRODUCTION

The predominant method of heating today's buildings, whether single-family dwell-ings or large structures, uses hot water to convey heat from a central generatingsource throughout, the building. In nearly all new construction, the water is circu-lated through a piping distribution network by an electrically driven pump; this typeof system is classified as a forced-circulation system. Heat from the circulatingwater is transferred to radiators, finned tubes, cabinet heaters, or other types ofterminal units (see Chaps. 5.9 to 5.13) distributed strategically throughout the struc-ture.

Older systems used gravity to circulate the hot water, by utilizing the differencein density between supply and return columns of the piping network. Since thistype of system is rarely installed today, this chapter confines itself to forced-circulation systems. As a matter of fact, the latest American Society of Heating,Refrigeration, and Air-Conditioning Engineers (ASHRAE) guide refers readers toeditions published before 1957 for details on designing gravity hot-water systems.

All hot-water heating systems rely on some form of central generating facilityas the source of heat. This facility can be in the form of a boiler that consumesoil, gas, or electricity as the prime energy source or steam-to-water and water-to-water heat exchangers that derive heat from a utility or district-heating network.

This chapter gives details on the basic types of hot-water systems, as character-ized by their temperature rating, general principles of system design, and specialconsiderations of the equipment that comprises hot-water systems.

5.2.2 CLASSES OF HOT-WATER SYSTEMS

Hot-water systems are classified by operating temperature into three groups: low,medium, and high temperature. The 7957 ASHRAE Handbook provides the follow-ing distinctions among these systems:1. Low-temperature water (LTW) system: A low-temperature hot-water system op-

erates within the pressure and temperature limits of the American Society ofMechanical Engineers' (ASME) Boiler Construction Code for low-pressure heat-ing boilers. The maximum allowable working pressure for such boilers is 160

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lb/in2 (11 bar) with a maximum temperature of 25O0F (1210C). The usual max-imum working pressure for LTW systems is 30 lb/in2 (2 bar), although boilersspecifically designed, tested, and stamped for higher pressures frequently maybe used with working pressures to 160 lb/in2 (11 bar). Steam-to-water or water-to-water heat exchangers are often used, too.

2. Medium-temperature water (MTW) system: MTW hot-water systems operate attemperatures of 35O0F (1770C) or less, with pressures not exceeding 150 psia(10.5 bar). The usual design supply temperature is approximately 250 to 3250F(121 to 1630C), with a usual pressure rating for boilers and equipment of 150lb/in2 (10.5 bar).

3. High-temperature water (HTW) system: When operating temperatures exceed35O0F (1770C) and the operating pressure is in the range of 300 lb/in2 (20.7bar), the system is an HTW type. The maximum design supply water temperatureis 400 to 45O0F (205 to 2320C). Boilers and related equipment are rated for 300lb/in2 service (21 bar). The pressure and temperature rating of each componentmust be checked against the system's design characteristics.

LTW systems are generally used for space heating in single homes, residentialbuildings, and most commercial- and institutional-type buildings such as officestructures, hotels, hospitals, and the like. With a heat-transfer coil or similar deviceinside or near the boiler, LTW systems can supply hot water for domestic watersupplies. Terminal units vary widely and include radiators, finned-tube fan-coilunits, unit heaters, and others. Typically overall heat loads do not exceed 5000 to10,000 MBtu/h (1.5 to 3 MW).

MTW systems show up in many industrial applications for space heating andprocess-water requirements. Overall loads range up to 20,000 MBtu/h (6 MW).Generally HTW systems are limited to campus-type district heating installations orto applications requiring process heat in the HTW range. System loads are generallygreater than 20,000 MBtu/h (6 MW).

The designs of MTW and HTW systems resemble each other closely. The sys-tems are completely closed, with no losses from flashing. Piping can run in prac-tically any direction, since supply and return mains are kept at substantial pressures.Higher temperature drops occur in MTW and HTW systems, relative to LTW sys-tems, while a lesser volume of water is circulated (depending on the heat load ofthe system). LTW systems lend themselves better to combined hot-water/chilled-water heating/cooling systems. Extra care and expense must be devoted to fittings,terminal equipment, and mechanical components, especially for HTW systems.

Finally, often a combined system is desirable: an MTW or HTW circuit forprocess heat and an LTW circuit for space heating. The hot water for the LTWsystem can be obtained via a heat exchanger with the main heating system.

5.2.3 DESIGNOFHOT-WATERSYSTEMS

Design hot-water systems involves a complex interplay of heat loads and the typeof generating system. A traditional starting point, primarily for residential LTWsystems, was the assumption of a 2O0F (U0C) temperature drop through the circuit,from which the overall flow rate could be determined. A more recent practice is toperform a rigorous analysis, because the 2O0F (U0C) assumption can lead to over-sized pipes and flow rates.

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TABLE 5.2.1 Typical Ratings of Wall Fin Elements

Hot-water capacity, Btu/(h - ft),* at 650F (17.40C), en-tering air with average water temperature of:

Element typeSteel, 1V4 in (32 mm)!Copper-aluminum, 1 in ](25.4 mm)Steel, grilled. enclo-\sure, 1 in (25.4 mmy

Rows12t

I A1

2t

22O0F104.40C

126020501000148013102080

21O0F98.90C

11401850900

134011901880

20O0F93.30C

10301680820

121010801700

19O0F87.80C

9401520740

1100980

1540

18O0F82.20C

8301350660970860

1370

17O0F76.70C

7301190580860760

1210

*1 Btu/(h ft) = 0.0768 kcal/(h m).t4-in (10.2-mm) center-to-center gap.

System design can be broken down into five elements:

1. Determining the heat load2. Selecting terminal units or convectors based on the average water temperature

and temperature drop and locating them on the architectural plan3. Developing a piping layout, including the choice of return system4. Locating mains, side branches, and other piping elements5. Specifying mechanical components, the expansion tank, and the boiler

A good initial point is to run the flow main from the boiler to the terminal unitor units with the largest heat load and then to select branch runs to connect otherterminal units. Common space-heating terminal elements are convectors of wallfins, both of which contain a length of finned tube over which air can be fanned ifdesired. The air entering temperature is usually assumed to be 650F (180C). Mostmanufacturers supply tables showing heat ratings of the convectors, based on theassumed temperature drop, and the average entering water temperature (AWT). SeeTable 5.2.1 for an example for finned-tube convectors.

An alternative approach is to assume a constant-temperature water flow (basedon the leaving temperature of each class of terminal equipment) and to computethe required flow rate.

Both daily and annual variations in heat loads should be evaluated in order toarrive at a suitable design. This is especially true when LTW systems combininghot-water heating and cool-water cooling are envisioned. Figure 5.2.1 shows theseasonal effects of outside temperature on one type of piping design, the two-pipesystem.

5.2.4 PIPINGLAYOUT

Once a preliminary evaluation of heat load and terminal units has been performed,a piping layout can be undertaken. The usual starting-point optionsrunning theflow main by the shortest and most accessible route to the larger heat loadscanbe explored for the type of overall piping arrangement desired.

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J0 'airuEjadujai

JOIBM uuejsXs Sys

tem wa

ter tem

peratu

re, 0 C

Outside temperature, 0FFIGURE 5.2.1 Seasonal operating characteristics of a two-pipe forcedhot-water system. (Courtesy of The Industrial Press.)

Pipe circuits generally are organized into one- or two-pipe arrangements. One-pipe systems with radiators or similar terminal units often have a feed and returnpipe that diverts water from the flow main to the radiator and back to the flowmain; even though two pipes are present, the system is still considered a one-pipearrangement (see Fig. 5.2.2). Finned-tube heating elements running along the outerwalls of small residencesa common arrangementare true one-pipe systems, asshown in Fig. 5.2.3. Each terminal unit in the circuit receives progressively lower

CompressiontankReliefvalve

Pressure -reducingvalveDrain

Air vent

Air vent

Flow mainDiverter fittingsor reducing teesReturn main

Boiler

Circulating pumpFIGURE 5.2.2 Arrangement of piping for a one-pipe forced hot-water system with closedexpansion tank. (Courtesy of The Industrial Press.)C

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From second floor

Door loop

Square head,balancing

cocks

Circulator

Drain

Supply

Hot-waterboiler

A'r vent

KitchenNipple and cock for draining

Air ventsTo second floor

Bathroom

Closet

90 elbowBedroom

ThermostatBedroom

Closet

Livingroom

Door capsAir vent

Doorloop

FIGURE 5.2.3 Typical installation of one-pipe forced-circulation "loop" hot-water system using baseboardradiators. (Courtesy of The Industrial Press.)

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CompressiontankReliefvalve

Pressure -reducingvalveDrain

Air vent

Flow main

Return mainBOILER

Circulating pumpFIGURE 5.2.4 A two-pipe forced hot-water heating system with reverse-return piping.(Courtesy of The Industrial Press.)

water temperature; thus the units are sized larger as they are located farther fromthe heat source.

Two-pipe systems allow for parallel heating arrangements, whereby terminalunits can receive hot water at roughly similar inlet temperatures. The cooled waterreturns via a second pipe. The flow of this pipe can be specified to run in direct orreverse fashion back to the heat generator. Choosing between these options allowsfor better balancing of heat supplies among various terminal units and for somevariation in overall system capital cost. Reverse-return systems specify that thedistance that the water travels to a particular unit is the same as the return distancefrom that unit (Fig. 5.2.4).

5.2.5 PRESSUREDROPANDPUMPING REQUIREMENTS

All hot-water systems require some type of pumping to overcome friction losses ofthe flowing water, because whatever head is developed by the height of the watersystem (static pressure) is offset by the return pressure. Some more complex sys-tems are better served economically by two or more pumps strategically located,rather than one large pump.

Standard charts provide data on friction loss for runs of common types of piping(Fig. 5.2.5). To this should be added pressure losses from elbows, fittings, and otherelements (Table 5.2.2). Similarly, manufacturers of radiators and other terminalunits provide data on friction losses through their equipment.

Pump specifications are arrived at by first computing the overall pressure dropand the amount of desired water flow. "Pump curves"charts which show thepressure developed by pumps as a function of the flow ratecan be used to arriveat the correct sizing. Many designers prefer to work with mass flow rate [pounds(kilograms) per hour] rather than gallons per minute (liters per second), units com-Co

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Flow,

gal/m

in

Friction loss, [ft (m) of water per 100 ft (m)]Metric conversion: gal/min to L /min = 3.78

FIGURE 5.2.5 Friction loss for open-system piping. (From Carrier Air Conditioning Company,Handbook of Air Conditioning System Design, McGraw-Hill, New York, 1965. Used withpermission.)

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TABLE 5.2.2 Fitting Losses in Equivalent Feet* of PipeScrewed, welded, flanged, flared, and brazed connections

Mitre elbowsSmooth bend teesSmooth bend elbowsNom-inalpipeor

tubesize,in*

30Ell

45Ell

60EH

90Ell

Straight-through flowRe-

ducedV2

Re-duced

V4

Nore-

duction

Flow-throughbranch

180Std.t

45Streetf

45Std.t

90Streett

90LongRad.$

90Std.t

*Conversion factors: 1 ft = 0.31 in; 1 in = 25.4 mm.^R/D approximately equal to 1.$/?/D approximately equal to 1.5.Source: Carrier Air Conditioning Company, Handbook of Air Conditioning System Design, McGraw-Hill, New York, 1965. Used with permission.

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mon to pump curves. The conversion between the two is temperature-dependent;two quick conversions commonly used are

Water at 4O0F (4.40C): 1 Ib/h = 0.002 gal/min (1 kg/h = 1.26 E - 4 L/s)Water at 40O0F (204.50C): 1 Ib/h = 0.0023 gal/min (1 kg/h = 1.45 E - 4 L/s)

The next step is to determine the system curve for the hot-water circuit. Thefollowing formula is employed:

t/0.5 EJ0.5^- = ^ - (5 2 nW1 W2 p

TABLE 5.2.3 Allowable Flow Rates for Closed SystemPiping, Standard-Weight Steel Pipe

Flow range, Pressure dropPipe size, in gal/min range, ft per 100 ft

V2 0-2 0-43A 3-4 2.5-41 5-7.5 2.0-41% 8-16 1.25-4IV2 17-24 2-42 25^8 1.25-421X2 49-77 2-43 78-140 1.5^4 141-280 1.25-45 281-500 1.5-46 501-800 1.75-48 801-1700 1.0-4

10 1701-2500 1.25-2.7512 2501-3600 1.25-2.2514 3601-4200 1.25-2.016 4201-5500 1.0-1.7518 5501-7000 0.9-1.5020 7001-9000 0.8-1.2524 9001-13,000 0.6-1.00

Note: The above capacities are based on a maximum pressure dropof 4 ft per 100 ft and a maximum velocity of 10 ft/s. Conversions: 1in = 25.4 mm, 1 gal = 3.8 L, and 1 ft = 0.31 m.

5.2,7 VENTINGANDEXPANSIONTANKS

Hot-water systems require pressures greater than atmospheric at all times to preventair infiltration. Flashing or boiling of water is also minimized by maintaining thesystem above the water vapor pressurepreventing this also minimizes waterhammer.

Maintaining this pressure, as well as allowing for the expansion and contractionof water as it is heated and cools, is most frequently carried out by means of anexpansion tank. The expansion of medium- or high-temperature water systems canbe calculated by consulting steam tables. The specific volume of water at its initialconditions is subtracted from its volume at the highest temperature, to calculate thevolume change. To a certain limited extent, the water's expansion and contractionare offset by the similar changes that system piping and heating units undergo.These changes can be calculated from coefficients of expansion of the materials ofthe piping.

The simplest type of expansion tank is open to atmosphere at an elevation thatprovides the pressurization (head) the system requires. Open tanks have the dis-advantage of allowing air to enter the system via absorption in the water. Closedtanks are more common now, especially with larger systems. Three common typesof expansion/pressurization tanks are in use today:

1. Adjustable expansion tank. This tank employs an automatic valve along with aclosed tank that has water and air feeds. As the temperature in the system rises,C

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the pressure rises. A control valve releases air in the tank to the atmosphere.When the pressure and the water level drop, high-pressure air is injected intothe tank. High-temperature systems should use nitrogen rather than plain air toreduce corrosive effects.

2. Pump-pressurized cushion tank. This design involves a makeup tank which isfed by a pump and a back-pressure control valve. For small systems (dependingon local codes and on the water pressure available) the pump is skipped andcity water pressure is used to feed a makeup tank that pressurizes the heatingcircuit. In principle, either type of pressurized tank can be roughly sized byassuming the expansion and contraction rates of the water to be equal.

3. Compression tank. A compression tank employs a specified volume of gaswithin an enclosure. As the water temperature and volume increase, the pressureon the gas volume rises, causing that gas volume to decrease. In this manner,the tank accommodates changing water volumes while keeping the system withina specified range of upper and lower pressures.

In low-temperature systems, the compression tank is usually connected to thesystem through an air separator situated between the boiler exit and the suctioninlet of the circulating pump. Air separated from the water will rise into the com-pression tank. When the compression tank is located at a system's high point, itcan be smaller in volume since the pressure is at its lowest. Tank sizing is alsodependent on the location of the circulation pump relative to the tank.

One commonly used formula for sizing the compression tank, when operatingtemperatures are below 16O0F (71.10C), is

EVV = (5 2 3)v* p i p _ p IP v-J-^>;M)7M "o/r2

where Vt = compression tank volumeVx = volume of circulating system, exclusive of compression tankE = coefficient of expansion from initial to operating temperature

P0 = absolute pressure in compression tank prior to fillingP1 = absolute static pressure after fillingP2 = absolute pressure at system operating temperature

For operating temperatures between 160 and 28O0F (71.1 to 137.80C), this formulais used:

(0.00041* - 0.0466)V,V< = P I P -PIP (USCS units) (5'2'4)* Q / r l rO/r2

(0.000738r - 0.03348)V,V< = P I P -PIP (metric units) (5'2'5)"Q'" I ro'*2

where t = maximum operating temperature.Compression tanks can be supplied with an impermeable membrane (diaphragm)

to prevent air from being drawn into the circulating water when the system tem-perature drops. The diaphragm also allows the compression tank to be smaller involume.

Diaphragm compression tanks are equipped with sight glasses or similar devicesto monitor the water level. Too low a water level prevents the air behind the dia-phragm from affecting the system's pressure.

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5.2.8 MECHANICAL AND CONTROL EQUIPMENT

Mechanical components for low-temperature systems are under less severe servicethan those for medium- and high-temperature units; correspondingly, the care withwhich components are specified should increase with the higher-temperature sys-tems.

ASME and ASHRAE rules should be observed for dealing with pressure vessels.Specifically, the chemical condition of the circulating water in high-temperaturesystems should be checked periodically by an expert. Pressure gauges should belocated at both ends of the circulation pump. Modulating combustion controls,rather than straight on/off controls, are necessary to minimize pressure swings thatlead to flashing. Where compression-type expansion tanks are used, an interlockwith the system's heat generator should be installed to prevent operation when thewater level in the tank is too low or insufficient air is present to maintain the tankcompression. Valves and fittings for high-temperature systems should be specifiedwith materials that resist corrosion and erosion, such as stainless steel.

The primary control factor for a hot-water system is the operating temperaturerange, which in turn is based on outside air temperatures. Electronic thermosensorsand thermostats function to keep the room air temperature within the desired range.The system should also be equipped with a manual on/off control.

The electronic control for moderating the room air temperature can be of severaltypes. Most are based on a solenoid device, which sends a signal current on thebasis of a temperature reading. The control can be a simple on/off device or canhave various modulating schemes to minimize large temperature swings. Temper-ature controls can also be set for zone heating of certain rooms or areas within alarge room, depending on the piping layout. In this case, the electronic control isconnected with various flow control valves that will reduce or expand water flowto the heating units.

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Front MatterTable of ContentsPart A. System ConsiderationsPart B. Systems and Components3. Components for Heating and Cooling4. Heat Generation Equipment5. Heat Distribution Systems5.1 Steam5.1.1 Introduction to Steam5.1.2 Introduction to Steam Heating Systems5.1.3 General System Design5.1.4 Pressure Conditions5.1.5 Piping Arrangements5.1.6 Condensate Return5.1.7 Pipe-Sizing Criteria5.1.8 Determining Equivalent Length5.1.9 Basic Tables for Steam Pipe Sizing5.1.10 Tables for Low-Pressure Steam Pipe Sizing5.1.11 Tables for Sizing Medium- and High-Pressure Pipe Systems5.1.12 Air Vents5.1.13 Steam Traps5.1.14 Steam Trap Types5.1.15 Balanced-Pressure Steam Traps5.1.16 Bimetallic Thermostatic Steam Traps5.1.17 Liquid-Expansion Steam Traps5.1.18 Bucket Steam Traps5.1.19 Float-and-Thermostatic Steam Traps5.1.20 Thermodynamic Steam Traps5.1.21 Steam Trap Location5.1.22 Steam Trap Sizing5.1.23 Steam Trap Selection5.1.24 Determining Condensate Load for a System5.1.25 Water Damage5.1.26 Water Conditioning5.1.27 Freeze Protection5.1.28 Piping Supports5.1.29 Strainers5.1.30 Pressure-Reducing Valves5.1.31 Flash Tanks5.1.32 Steam Separators

5.2 Hot-Water Systems5.2.1 Introduction5.2.2 Classes of Hot-Water Systems5.2.3 Design of Hot-Water Systems5.2.4 Piping Layout5.2.5 Pressure Drop and Pumping Requirements5.2.6 Pipe Sizing5.2.7 Venting and Expansion Tanks5.2.8 Mechanical and Control Equipment

5.3 Infrared Heating5.3.1 Introduction5.3.2 Types of Heaters and Applications5.3.3 Physiology of Infrared Heating5.3.4 Spacing and Arrangement of Electric Heaters5.3.5 Gas Infrared Radiant Heating

5.4 Electric Heating5.4.1 Introduction5.4.2 System Selection5.4.3 Central Hot-Water Systems5.4.4 Warm-Air Systems5.4.5 Convector with Metallic Heating Element5.4.6 Unit Ventilators5.4.7 Unit Heaters5.4.8 Baseboard Heaters5.4.9 Infrared Heaters5.4.10 Valance, Cornice, or Cove Heaters5.4.11 Radiant Convector Wall Panels5.4.12 Integrated Heat Recovery5.4.13 Heat Pumps (See Also Chap. 6.3)5.4.14 Specifying Electric Heating Systems5.4.15 Electric Circuit Design5.4.16 Heat Pump Types

5.5 Solar Space Heating5.5.1 Introduction5.5.2 Types of Distribution Systems5.5.3 General Design5.5.4 Heat-Transfer Media5.5.5 Water Drainback Systems5.5.6 Pumping Considerations5.5.7 Additional Fluid System Considerations5.5.8 Materials and Equipment

5.6 Snow-Melting Systems5.6.1 Introduction5.6.2 Determination of the Snow-Melting Load5.6.3 Piping Layout5.6.4 Determine the Gallons/Minute (Liters/Second) Requirement and Specify a Heat Exchanger5.6.5 Select Specialties5.6.6 Electrical Snow Melting5.6.7 Electric Heat Output5.6.8 Infrared (Radiant) Snow Melting5.6.9 System Controls

5.7 Heat Tracing5.7.1 Introduction5.7.2 Basic Design Considerations5.7.3 Electric Heat-Tracing Design5.7.4 Accessory and Control Equipment

5.8 Unit Heaters5.8.1 Introduction5.8.2 Unit Heating System Differences5.8.3 Classification of Unit Heaters5.8.4 Typical Unit Heater Connections5.8.5 Calculating Heat Loss for a Building5.8.6 Selecting Unit Heaters5.8.7 When Quietness is a Factor5.8.8 Controls for Unit Heater Operation5.8.9 Locating Unit Heaters5.8.10 Seven Good Reasons for Replacing Rather Than Repairing Unit Heaters5.8.11 References

5.9 Hydronic Cabinet Heaters5.9.1 Cabinet Unit Heaters-Heating Only5.9.2 Fan-Coil Units-Heating and Cooling5.9.3 Unit Ventilators-Heating, Cooling, and Ventilating5.9.4 Selection5.9.5 Applications5.9.6 References

5.10 Heat Exchangers5.10.1 Introduction5.10.2 Shell-and-Tube Heat Exchangers5.10.3 Nonremovable (Fixed-Tubesheet) Tube Bundles5.10.4 U-Tube Removable Tube Bundles5.10.5 Packed Floating Tub Sheet Removable Bundles5.10.6 Internal Floating Head Removable Bundles5.10.7 Tubes for Shell-and-Tube Design5.10.8 Tube Joints5.10.9 Headers for Shell-and-Tube Design5.10.10 Plate-and-Frame Heat Exchangers5.10.11 Brazed Plate Heat Exchangers5.10.12 Coils5.10.13 Maintenance of Heat Exchangers5.10.14 References5.10.15 Bibliography

5.11 Radiators for Steam and Hot Water Heating5.11.1 Introduction5.11.2 Heating Elements5.11.3 Enclosures5.11.4 Architectural Enclosures5.11.5 Ratings5.11.6 Selection5.11.7 Application5.11.8 Piping Arrangements5.11.9 Automatic Control5.11.10 References

5.12 Door Heating5.12.1 Introduction5.12.2 Characteristics of Door Heating Loads5.12.3. Types of Door Heating Equipment Available5.12.4 Controls and Control Systems5.12.5 Selection of Door Heaters5.12.6 Alternatives to Door Heating5.12.7 Door Heater Installation5.12.8 Door Heating Worksheet-Explanation5.12.9 Door Heating Worksheet-Sample Form for Use

5.13 Radiant Panel Heating5.13.1 Introduction5.13.2 Definitions and Terms5.13.3 History and Applications5.13.4 Design Considerations5.13,5 System Components5.13.6 System Design5.13.7 Installation Methods5.13.8 Summary5.13.9 References

6. Refrigeration Systems for HVAC7. Cooling Distribution Systems and EquipmentPart C. General ConsiderationsAppendices

Index

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