heat exchanger training 02. 25. 15

Process Training - Heat Exchanger

Heat Exchanger Calculations

By Sharon Wenger

February 26, 2015

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2Overview

Introduction Basic Heat Transfer

Conduction Convection Radiation

Types of Exchangers Shell & Tube Hairpin Plate & Frame Brazed Plate Welded Plate Finned Tube TEMA Heat Exchangers

Heat Transfer Calculation Formulas General Sensible Heating or Cooling of Fluids Steam Condensing Heating of Cooling a Solid Terminology and Definitions

Working Problems Work Example 1 - Shell and Tube Heat Exchanger Calculation Work Example 2 - Mozambique LNG PJ FEED - DeC2 OVHD Condenser (251-E-1006)

3Basic Heat Transfer

There are three forms of heat transfer. Conduction Convection Radiation

Conduction Heat flow through a solid medium due to the temperature difference across the solid. The temperature difference is the driving force for heat transfer. Conduction heat transfer is governed by Fourier's Law

Where k is the thermal conductivity (W/m.K) and is a characteristic of the wall material. The minus sign is a consequence of the fact that heat is transferred in the direction of decreasing temperature.

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Basic Heat Transfer - Convection Heat Transfer

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Basic Heat TransferCombined Conduction and Convection Coefficients

6Overall Heat Transfer Coefficients

7Fouling

Over a period of time, deposits or coatings form on the tube surfaces. This is called fouling or scaling of heat exchangers. The deposits usually have low thermal conductivity and offer additional high conductive resistance to heat transfer. This additional resistance reduces the overall heat transfer coefficient for the exchanger. Therefore, the fouling resistance factor also known as dirt resistance (Rd) is added to the overall coefficient under clean conditions to obtain the overall coefficient under service conditions

8What is Heat Exchanger

Heat exchangers are equipment that facilitate heat transfer between fluids. The heat transfer surface area is either the inner or outer surface area of the inside pipe, depending on which is chosen as the reference.

9Types of Heat Exchangers

Shell & Tube Hairpin Plate & Frame Brazed Plate Welded Plate Finned Tube TEMA Heat

Exchangers

Plate and Frame Heat Exchanger

Casketed Plate & Frame Heat Exchanger

Brazed Plate Heat Exchanger

Hairpin Heat Exchanger

Twisted Tube Heat Exchangers

10TEMA Types of Heat Exchangers

TEMA TYPE Heat Exchangers

11Shell and Tube Heat Exchangers

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Shell and Tube Heat Exchangers

Temperature Difference

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Temperature Difference

FT is the LMTD correction factor. The value of FT is depends upon the stream inlet and outlet temperatures and the exchanger geometry

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Approach of a Temperature Difference (ATD)

What is Approach of a Temperature Difference (ATD)That is to transfer heat from the refrigerant coolant temperature difference between the tow, and that is the approach of a temperature difference, Shown in Fig. below. It must be large enough to provide a flow of heat, necessary to achieve the required system capacity.

17Main heat transfer equation Q = UA ΔTm

where, Q = rate of heat transfer U = mean overall heat transfer coefficient A = heat transfer surface area ΔTm = logarithmic mean temperature difference

Q = m Cp T Q = rate of heat transfer, Btu/hr m = flow rate, pounds/hr Cp = heat capacity, Btu/pound/F T = temperature change, F

Q = mphase_change Q = rate of heat transfer, Btu/hr mphase_change = mass that changes phase, pounds/hr = heat of vaporization, Btu/pound

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Estimating Heat Transfer Calculation FormulasMost of the formula listed below are “rules of thumb” for quick estimation purposes and are limited in their application. The estimation is under standard temperatures and pressures.

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“Quick” Heat Transfer Calculation Formulas – for Estimation

General Liquid:

– = GPM x Density 8.022 = GPM × 501.375 × Specific GravitySpecific Gravity = Density/62.4 Psia = Psig+14.7

– = Evaporative Cooling Tower Tons x 15000– SCFM of air = [ACFM x (psig + 14.7) x 528] / [(Temp + 460) x 14.7]– SCFM of air = Lbs/ Hr of air / 4.5 (at atmospheric temperature and

pressure)

21Heat Transfer Calculation Formulas

Sensible Heating or Cooling of Fluids

– Btu/hr = Lbs./ Hr × Specific Heat × Specific Gravity × Temp Rise (K)– Btu/hr = GPM × Temp Rise x K

water = 500 K 30% glycol = 470 K 40% glycol = 450 K 50% glycol = 433K hydraulic oil = (210 – 243) K

– For Air, Btu/hr = 1.085 x SCFM × Temp Rise

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Heat Transfer Calculation Formulas

Steam Condensing

Btu/hr = Lbs./ Hr x Latent Heat

Heating or cooling a solidBtu/hr for Solids = Lbs./Hr x Specific Heat x Delta-T

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Work Example 1 - Shell and Tube Heat Exchanger CalculationSolved Example 1:Given:

24Work Example 1 - Shell and Tube Heat Exchanger Calculation

Find:

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Work Example 1 - Shell and Tube Heat Exchanger Calculation

Solve Steps:

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Working Problem 2 – Mozambique LNGFEED - DeC2 OVHD Condenser (251-E-1006)

Do we have to have all the required information to design a heat exchanger? Not necessary. That is where we use the quick calculation formulas to design it

Example 2 - Working Problem from Mozambique LNG FEED - DeC2 OVHD Condenser (251-E-1006)

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What are the Min Information Req. to Design a Heat Exchanger

Depends1) Pre-FEED2) FEED3) EPC detailed design.

In case 1) and 2) we only have minimum amount of information, can we prepare a quick budget design? Yes!In case 3) we have all the detail information to design for a particular application, by using program such as HTRI will help us optimize a design result.

36Information Required to Design a Heat Exchanger

Fluid Properties1. Fluid Composition and Percentage 2. Specific Heat3. Viscosity, cp 4. Specific Gravity or Density5. Thermal Conductivity6. Latent Heat, (if phase change)7. Operating Pressure and Temperature

Support Information 8. Allowable Pressure Drop9. Fouling Factor10. Design Pressure11. Design Temperature

Work Example 2 - Mozambique LNG FEED – Deethanizer OVHD Condenser (251-E-1006)

Steps to solve the problem1. Gather all the Information Required

to Design a Heat Exchanger2. PFD3. HMB4. Find the streams in and out to the

Exchanger

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CalculatedWhat information are available?

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Approach of a Temperature Difference (ATD)Condensate Supply Temp. (°C) = achieve T (-16) – rise T (-3) – ATD (-3) = -22.3 (°C)

StreamsHot Side Cold Side112 113 374 375

Temperature [C] -0.14 -16.3 -22.3 -19.3Pressure [bara] 26.7 26.7 2.5 2.5Mass Flow [kg/h] 3980 3980 4500 4500

Calculate Heat Exchanger Duty Required

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Streams 1 2 3 4

Temperature [C] tc1 tc2 Th1 Th2

-0.14 -16.3 -22.3 -19.3

Pressure [bara] 26.7 26.7 2.5 2.5

Mass Flow [kg/h] 3980 3980 4500 4500ΔTLM (°C) 11.35

UA (Kw/°C) 33 - 40 From Go-By

For UA = 33 Q (Kw) = 375

For UA = 40 Q (Kw) = 454

Result UniSim Results

Q = UA ΔTLM

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Simulation Results

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Information Required to Design a Heat Exchanger

Streams

Stream No. 374 375

Pressure bar_a 2.50 2.50Temperature oC -19.32 -19.32

VAPOR PROPERTIES

Flow kgmole/hr 25.30 102.16kg/hr 1,116 4,505MMSCF60F/day 5.080E-01 2.05

Volumetric Flow m3/hr 198.11 799.99Enthalpy kJ/kgmole -107,494 -107,494Entropy kJ/kgmole*K -289.614 -289.614Molecular Weight kg/kgmole 44.097 44.097Density kg/m3 5.631 5.631Heat Capacity kJ/kg*K 1.604 1.604Viscosity cpoise 6.744E-03 6.744E-03Thermal Conductivity W/m*K 1.365E-02 1.365E-02Specific Heat Ratio 1.198 1.198Gas Compressibility 9.286E-01 9.286E-01LIQUID PROPERTIES

Flow kgmole/hr 76.86 0.00kg/hr 3,389

Volumetric Flow m3/hr 6.12 0.00Enthalpy kJ/kgmole -125,135Entropy kJ/kgmole*K -359.113 0.000Molecular Weight kg/kgmole 44.097 0.000Density kg/m3 553.422 0.000Heat Capacity kJ/kg*K 2.364 0.000Viscosity cpoise 1.512E-01 0.000E+00Thermal Conductivity W/m*K 1.183E-01 0.000E+00Specific Heat Ratio 1.567

TOTAL PROPERTIES

Flow kgmole/hr 102.16 102.16kg/hr 4,505 4,505MMSCF60F/day 2.05 2.05

Enthalpy kW -3,427 -3,051Entropy kJ/hr*K -34,930 -29,588Molecular Weight kg/kgmole 44.097 44.097

42Thanks & Questions