1

The overall heat transfer coefficient ranges from about 10 W/m2C for gas-to-gas heat exchangers to about 10,000 W/m2C for heat exchangers that involve phase changes.

For short fins of high thermal conductivity, we can use this total area in the convection resistance relation Rconv = 1/hAs

To account for fin efficiency

When the tube is finned on one side to enhance heat transfer, the total heat transfer surface area on

the finned side is

2

Fouling FactorThe performance of heat exchangers usually deteriorates with time as a result of accumulation of deposits on heat transfer surfaces. The layer of deposits represents additional resistance to heat transfer. This is represented by a fouling factor Rf.

The fouling factor increases with the operating temperature and the length of service and decreases with the velocity of the fluids.

Thermal Conductivity of solids

3

Typical value for overall heat transfer coefficient

Shell and Tube

Heat Exchangers Hot Fluid Cold Fluid U [W/m2C]

Heat Exchangers Water Water 800 - 1500

Organic solvents    Organic Solvents    100 - 300

Light oils Light oils 100 - 400

Heavy oils Heavy oils 50 - 300

Reduced crude Flashed crude 35 - 150

Regenerated DEA Foul DEA 450 - 650

Gases (p = atm) Gases (p = atm) 5 - 35

Gases (p = 200 bar) Gases (p = 200 bar) 100 - 300

Coolers Organic solvents Water 250 - 750

Light oils Water 350 - 700

Heavy oils Water 60 - 300

Reduced crude Water 75 - 200

Gases (p = 200 bar) Water 150 - 400

Organic solvents Brine 150 - 500

Water Brine 600 - 1200

Gases Brine 15 - 2504

Heat Exchangers Hot Fluid Cold Fluid U [W/m2C]

Heaters Steam Water 1500 - 4000

Steam Organic solvents 500 - 1000

Steam Light oils 300 - 900

Steam Heavy oils 60 - 450

Steam Gases 30 - 300

Heat Transfer (hot) Oil Heavy oils 50 - 300

Flue gases Steam 30 - 100

Flue gases Hydrocarbon vapors 30 -100

Condensers Aqueous vapors Water 1000 - 1500

Organic vapors Water 700 - 1000

Refinery hydrocarbons Water 400 - 550

Vapors with some non

condensableWater 500 - 700

Vacuum condensers Water 200 - 500

Vaporizers Steam Aqueous solutions 1000 - 1500

Steam Light organics 900 - 1200

Steam Heavy organics 600 - 900

Heat Transfer (hot) oil Refinery hydrocarbons 250 - 550

5

MetalSpecific

HeatThermal

ConductivityDensity

Electrical Conductivi

ty

      cp          

cal/g° Ck        watt/cm

K  g/cm3 1E6/Ωm

Brass 0.09 1.09 8.5  

Iron 0.11 0.803 7.87 11.2

Nickel 0.106 0.905 8.9 14.6

Copper 0.093 3.98 8.95 60.7

Aluminum 0.217 2.37 2.7 37.7

Lead 0.0305 0.352 11.2  

alpha-beta brass - a brass that has more zinc and is stronger than alpha brass; used in making castings and hot-worked products

Sp heat capacity of some metals

Thermal Conductivity of liquids

7

Thermal conductivity of gases

8

9

10

Effectiveness for heat exchangers.

11

12

When all the inlet and outlet temperatures are specified, the size of the heat exchanger can easily be determined using the LMTD method. Alternatively, it can be determined from the effectiveness–NTU method by first evaluating the effectiveness from its definition and then the NTU from the appropriate NTU relation.

13

(e.g., boiler, condenser)

14

Thermal Conductivity of Common Coil Materials

Material Thermal Conductivity

Copper 2724

Aluminum 1536

Steel 314

Cupro-Nickle 90/10 310

Cupro-Nickle 70/30 200

Stainless Steel 108

15

Relative Heat Transfer:Identical Coils w/Different Tube Materials

Tube Material Relative HT Capacity

Copper 1.00

Aluminum 1.00

Steel 0.98

Stainless Steel 0.95

16

Relative Heat Transfer:Identical Coils w/Material Combinations*

Tube Material Fin MaterialRelative HT

Capacity

Steel Copper Keyfin 1.05

Steel Aluminum Keyfin 1.00

Stainless Steel Aluminum Keyfin 0.94

Steel Steel L Fin 0.92

Stainless Steel Stainless Steel L Fin 0.58

*At 800 FPM, 7FPI & 55 psig

Extended Surface (Fin Tube) Heating Coils

18

Which Fin Type?

Standard KeyFin

• Tight joint over wide temperature range

• Best for dissimilar tube and fin materials

19

Which Fin Type?

L-Fin

• L-shaped foot tension wound over knurled tube

• Best for same material tubes & fins

• Large contact area for high heat transfer

• Some corrosion protection for tube

20

Tube MaterialsCarbon Steel Affords both strength & corrosion resistanceStandard 12 Ga A-214 ERW (1” or 1½” OD)

Optional 10 Ga A-214 ERW

10 Ga or 12 Ga A-179 Seamless

Stainless Steel (304L or 316L)When corrosive steam is present or when ideal piping & trapping practices cannot be followed14 Ga (1” OD) & 12 Ga (1½” OD)

21

Fin MaterialsSteel Fins stand up against aggressive cleaning0.024” Thick on 1” OD or 1 1/2” OD Tubes0.036” Thick on 1 ½” OD Tubes

Aluminum Fins provide the best overall value0.020” Thick Keyfin on all Tube Sizes0.030” Thick Keyfin on all Tube Sizes

Stainless Steel Fins fight high external corrosion0.020” Thick Type 304L & 316L on 1” OD Tubes

Copper Fins provide the best heat transfer0.016” Thick on All Tube Sizes

22

Observations from the effectiveness relations and charts

• The value of the effectiveness ranges from 0 to 1. It increases rapidly with NTU for small values (up to about NTU = 1.5) but rather slowly for larger values. Therefore, the use of a heat exchanger with a large NTU (usually larger than 3) and thus a large size cannot be justified economically, since a large increase in NTU in this case corresponds to a small increase in effectiveness.

• For a given NTU and capacity ratio c = Cmin /Cmax, the counter-flow heat exchanger has the highest effectiveness, followed closely by the cross-flow heat exchangers with both fluids unmixed. The lowest effectiveness values are encountered in parallel-flow heat exchangers.

• The effectiveness of a heat exchanger is independent of the capacity ratio c for NTU values of less than about 0.3.

• The value of the capacity ratio c ranges between 0 and 1. For a given NTU, the effectiveness becomes a maximum for c = 0 (e.g., boiler, condenser) and a minimum for c = 1 (when the heat capacity rates of the two fluids are equal).