hydraulics in high rise - ashrae · pdf file... head- building services hydraulics in high...

Hydraulics in High Rise Buildings Satnam Singh, Head- Building Services

Hydraulics in High Rise Buildings

1. Introduction

The cities are moving towards High Rise Buildings. This has imposed challenges for designers.

Hydraulics in High Rise buildings deals with Water. Unlike Air, water is heavy and a vertical column

of water contained in various pipelines of the buildings imposes pressures at the lower levels. This

pressure becomes such a culprit that designers need to find ways to deal with it. In the subsequent

sections we will discuss some key challenges faced in Hydraulics of a high rise building that need

attention and how can they be addressed.

2. Chilled Water Circuit

The equipment employed in Chilled Water circuits has limitation in handling pressures beyond a

certain limit. Higher pressure rating of Chilled water System would also mean increase in the cost of

components. Therefore a thorough attention by owner on the various options to limit the pressures

in the circuit is essential. Lower operating pressures also allow increased operating life of the

component.

2.1. Point of No Pressure change

A Chilled Water Circuit is a closed loop Circuit and the location of Expansion tank defines the

point of no pressure change. Pressures elsewhere in circuit can be calculated from this

reference point by adjusting for the pressure variations due to elevation difference, Friction

losses and Pump Gain. It is well known that locating expansion tank at the highest level in the

Circuit helps to reduce the expansion tank sizing but in a high rise building it is noteworthy that

locating Expansion Tank at the highest level could reduce the pressure experienced at the

lowest level by an amount equivalent to the pressure drop in the Return piping starting from

the topmost level. Even though the pressure saving could only be small but this can become the

first step to counter pressure.


2.2. Location of Pumps

It is a convention that Pumps are placed upstream of the Chillers. This means Chillers (when

placed in bottom of the circuit) experience highest pressure in chilled water circuit. Variable

Primary flow system where the Primary pump heads are substantial the pressure exerted on

the chillers also increase substantially. Since Chillers are most critical and costly equipment in

circuit therefore limiting pressures on it should make a lot of sense. It is absolutely possible to

place the primary pumps downstream of Chillers. This allows the pressure on the chiller to

further reduced by an amount equivalent to primary pump head. In Variable Primary where the

Primary pump head could be as high as 5 bar this would mean a substantial reduction in

pressure on chillers.

Similarly Condenser Water Pumps and Heat Exchanger Pumps can be employed on downstream

of Condensers and Heat Exchangers respectively to limit pressures on them.

Having the complete plant room placed on upper floors can help reducing the pressures

experienced by the critical plant equipment but this is not always possible.

2.3. Zoning

Excess pressures because of very high heights need to be handled by breaking pressures using

Heat Exchangers. Heat Exchangers allow transfer of heat from one zone to other but pressure

of higher zones is isolated from the lower zones. Heat Exchangers also bring in the challenge of

temperature drop from low temperature zone to high temperature zone, additional pressure

drop on Heat Exchanger and controls for optimized and efficient heat transfer from one zone to

other.

In the two figures below for option1 & 2 the Valve controller senses temperature in Zone 1 and

modulates 2 way control Valve across Heat Exchanger to allow only as much water from pump

P2 as is required. As in any conventional secondary pumping system, P2 and P1 pumps can a be

made to operate on variable speed based on feedback Differential pressure sensors in Zone 2


and Zone 1 respectively. Either of Option 1 and 2 can be employed based on the total size of

the chilled water circuit, difference in hydraulic and load characteristics of AHUs in Zone 2 and

the Heat Exchanger.

In relatively larger capacity circuits a separate dedicated pumping system P3 to feed to the Heat

Exchanger could become viable (Option 3). In this case Pump P3 could operate on variable

speed directly based on the Temperature feedback from the Zone 1 and pressure drop in 2 way

valve across Heat Exchangers could be avoided. These pressure drops across 2 way Control

Valves could otherwise become substantially high to maintain the required valve authority.


2.4. Kind of Pump

Double Suction Pumps are considered much more robust to end suction Pumps. In high-rise

buildings that impose pressure challenges and flow rates are also expected to be higher

because of higher loads double suction pump should become a favored choice.

3. Fire Fighting

After 9/11 fire in Timesquare buildings, the codes have become extra vigilant on the Firefighting

requirements of High Rise Buildings. NFPA has included some specific minimum requirements on the

High Rise buildings. Given below are some of the requirements that have surfaced specifically for

high rise buildings.

3.1. Water Tank

For all the floors that are beyond the reach of fire departments the building should have water tank

of the fire protection capacity. Water tank should

be divided in two chambers with each having 50%

of more capacity so that when one chamber is

being serviced then building could draw water

from other chamber in the event of fire. Each Tank

chamber should have an auto fill of capacity equal

to the fire protection demand rate. This ensures

full redundancy in case one of the tank chambers

is under service.


3.2. Pumps

An auxiliary power supply to the electric motor of each fire pump is required. High Rise Buildings

may require pumps to generate excessive pressures and sometimes this may require pumps to

operate in series. The question is whether the pumps operating in series shall be on the same level

or should they be on different levels. Having pumps at different levels will ensure that the suction

pressure of pump in upper zone will be lesser. There are also some critical disadvantages on placing

the pumps at different levels. In case the pump at lower level fails to operate then the pump at

upper level will cavitate. The communication between people controlling the two stations in

emergency will be very difficult. It is recommended that three pumps can be put in series but all the

pumps shall be in a single room and access is required through external door or 2 hr fire rated

passage.

3.3. Pumps & Tanks

It is also required to be ensured that sufficient redundancy is built into the system ie water supply,

pumps and tanks, so that even if one of the component in the system is not in service the system is

still capable to ensure that the fire water requirements is met.


4. Water Distribution

4.1. Water Distribution

Water at fixtures should get a minimum pressure of 1.5 to 2 bar for the smooth flow. The higher

limit of the pressure on these fixtures is 5 bar. A standing height of around 30 m (ie 6 to 8 floors)

can add 3 bar static pressure to the lowest level. In a single zone 30m height wherein pumps

maintain a pressure of 2 bar at the highest floor, the pressure at the lowest level in the zone will be

5 bar. Therefore in a high rise building the zone needs to be formed after every 30m (or so) height.

Pressure reducing valves play an important role in High Rise Buildings. The flow to each 30m Zone

from the main riser passes through a pressure reducing valve so that pressure at the highest level in

the zone is limited to 2 bar.

Different kinds of pipes, fittings in the main risers have varying pressure sustaining capability. Based

on this capability a limit is imposed and the maximum height that can be covered by single riser. In

such a scenario it becomes necessary to break the pressures by putting intermediate break pressure

tanks.

To allow the air purge to happen the horizontal piping in each zone can be done on floor below the

topmost zone so that the air gets purged from the taps of the topmost floor.

4.2. Pressure Boosting – One of the Multiple Pumps on VFD vs Dedicated VFD on each Pump

It is very common to use Pressure Boosters to boost the water from lower level to higher levels in a

high rise building. Conventionally constant speed pump boosters were used with large size of

pressure vessels but recently the trend has changed to energy efficient Variable speed pumping. It

has been a common practice to use Variable Speed Drive only on one of the multiple running

pumps. This approach has certain limitations and it is recommended for the reasons below that all

the pumps shall be on Variable Speed.

1. It is very common that engineers consider a 10 t0 15% safety factor while defining pump head

for the boosters. This ultimately would mean that a pump selected for 100 m will actually end


up working against a system head of say 85m only. Other mistakes in sizing could make this

difference even bigger. The pump on variable speed is able to adjust to the changes in the

pressure requirement however the pumps at constant speed cannot readjust and may end up

going beyond the pump curve. This could be detrimental for the pump.

2. Having a single VFD attached to one pump would mean that the pump will be required to run

all the time and the other pumps come on/off at full flow depending on the water requirement.

If the single pump is locked on to VFD then it can be under constant abuse therefore VFD needs

to actually toggle to other parallel pumps after a predefined period. Therefore controller needs

to build in it sophisticated controls to make this toggling happen. Moreover follower pumps

starting on constant speed generally through a DOL or a Star Delta starter can create pressure

jerks whenever they start. On the other hand the designer will be very convenient if all the

multiple pumps will have dedicated VFD.

3. When the booster set only contains a single VFD then designer also needs to check as to how

booster will run in case the VFD is under service. In such a scenario booster controller needs to

ensure that pumps will keep operating on constant speed. To ensure smooth operation the

designer needs to size the pressure vessel for constant speed application. This means that size

of pressure vessel will be required to be much higher. With each pump having a dedicated VFD

allows designer to consider minimal size of pressure vessel.

4. It is very common in chilled water application to use sensor at the farthest zone to ensure that

pump operates to satisfy the minimum pressure requirement at changing flow rates thereby

allowing energy saving. Reduced flow rates in the pipes mean that the pressure drop will reduce

in a square relationship. Pressure boosting is considered as a constant pressure application but

the fact is that at lower flows the friction losses are lesser and the pumps can actually operate at

lesser pressures still satisfying the system requirements. Such friction loss compensation is

possible if all the pumps are on VFD.

5. Hot Water Circulation

5.1. When users open hot water fixtures they instantaneously need hot water. In case hot water is

not available then they get inconvenienced and are forced to keep the tap open and waste

water till they start getting hot water. This is a typical problem when hot water is not

recirculated properly. In high rise buildings recirculation of hot water imposes bigger challenge.

Consider first figure without brazed plate heat exchanger and you will note that each zone has

got recirculation pump and each recirculation pump has a pressure reducing valve in the loop.


What this means is that the recirculation pump head of the pumps in the lower zones is much

higher (because of pressure drops imposed by pressure reducing valves). Pressure reducing

valves in high rise building are inevitable for the reasons discussed in previous sections. These

high head and very low flow pumps are often selected in inefficient zone and pose problems

related to hot water recirculation.

5.2. When Heat Exchangers are used in the recirculation circuit then Pressure reducing valves can

be avoided from the recirculation loop and single larger pump for bigger water recirculation

can be employed. For individual zones recirculation pumps of fractional horsepower can be

used.

6. Drainage – Venting

6.1. Waste water drainage pipes are not only required to deal with water but also air that is

entrapped in it. When the water flows down in the pipes then air needs to fill the space that

water has left otherwise the pockets of vacuum create nuisance in the flow. Appropriate

venting becomes critical in such a scenario.

High rise buildings are most susceptible to problems associated with improper venting. It

becomes necessary that a vent stack is made to run parallel to the drainage pipes so that the

drainage pipes are suitably vented at right places. In case multiple fixtures are used close to


each other and proper venting is not in place then drainage from one fixture could tend to draw

water seal through the other nearby fixtures causing greater difficulties.

7. Conclusion

While increasing height of the buildings is imposing greater challenges for the designers but

increasing capabilities of designers and products is making it possible for us to reach greater heights.


Mr.Satnam Singh, Head - Building Services, Xylem India

Mr. Satnam Singh is Head - Building Services for Xylem Water

Solutions India Private Limited.

Xylem was launched in 2011 from the spinoff of the water-related

businesses of ITT Corporation. Xylem (XYL) is the largest water

company listed on NYSE and has replaced ITT in Fortune 500

index. A leading global water leader enabling customers to

transport, treat, test and efficiently use water in public utility, residential/commercial building,

industrial and agricultural markets. The company does business in more than 150 countries

through a number of market-leading product brands, and its people bring broad applications

expertise with a strong focus on finding local solutions to the world’s most challenging water

and wastewater problems. Xylem is headquartered in White Plains, N.Y., with projected 2011

annual revenues of $3.8 billion with 12,500 employees worldwide.

Mr. Satnam Singh is BE Mechanical Engineer and has done his PGDBM in Marketing & Finance.

In his 15 years of experience in Industry he has worked on many of the building services

disciplines and has thorough involvement in Hydraulics, HVAC & Treatment subjects. He has

been highly successful in understanding new technologies and presenting it to industry for

successful incorporation. In his career he has worked on applications like Clear and Waste

Water Pumping, Treatment, Piping, Off Peak Cooling, Ventilation, Chilled Beams and so on.

Mr. Satnam has been involved in Xylem products from year 2001 through its channel partner.

He has joined Xylem directly from Year 2012 and is heading Building Services business of Xylem

in India. He can be contacted on his email id [email protected]

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