Heat Transfer Coefficients

               

Heat Transfer Coefficient is often useful to determine values for overall heat transfer coefficients while performing non-exact activities such as early project cost estimating and basic heat exchanger performance assessments. 

              

The equation which relates the overall heat transfer coefficient to the heat duty and the heat transfer area is:  

                      

Q= U*A*DT_lm  

            

Where:

Q = heat load

U = overall heat transfer coefficient

A = heat transfer area

DT_lm = log mean temperature difference 

             

Overall heat transfer coefficients are dependant on many parameters such as the nature of the fluid, fluid velocities, type of heat exchanger, temperatures and fouling.  Despite all these determining parameters, typical overall heat transfer coefficients are available for common applications and fluids.   If little information about the process and the parameters outlined above is available, the following values can be used as a guide for overall heat transfer coefficients: 

              

Sensible Vapour:                                                    30 Btu/hr-ft²-F

Sensible Heating/Cooling or Condensing:                100 Btu/hr-ft²-F

Boiling:                                                                120 Btu/hr-ft²-F

     

       

When more information about the fluids and process is available, one can use the overall heat transfer coefficient values in the tables below as a guide as to the order of magnitude.  Actual overall heat transfer coefficients may be smaller or larger than the values listed.    

           

Heaters (no phase change)
Hot Fluid	Cold Fluid	Overall U (BTU/hr-ft2-F)
Steam	Air	10 – 20
Steam	Water	250 – 750
Steam	Methanol	200 – 700
Steam	Ammonia	200 – 700
Steam	Aqueous solutions	100 – 700
Steam	Light hydrocarbons(viscosity < 0.5 cP)	100 – 200
Steam	Medium hydrocarbons(0.5 cP < viscosity < 1 cP)	50 – 100
Steam	Heavy hydrocarbons(viscosity > 1)	6 – 60
Steam	Gases	5 – 50
Dowtherm	Gases	4 – 40
Dowtherm	Heavy oils	8 – 60
Flue gas	Aromatic hydrocarbon and steam	5 – 10

                  

              

Evaporators
Hot Fluid	Cold Fluid	Overall U (BTU/hr-ft2-F)
Steam	Water	350 – 750
Steam	Organic solvents	100 – 200
Steam	Light oils	80 – 180
Steam	Heavy oils (vacuum)	25 – 75
Water	Refrigerant	75 – 150
Organic solvents	Refrigerant	30 – 100

                 

                 

Coolers (no phase change)
Cold Fluid	Hot Fluid	Overall U (BTU/hr-ft2-F)
Water	Water	150 – 300
Water	Organic solvent	50 – 150
Water	Gases	3 – 50
Water	Light oils	60 – 160
Water	Heavy oils	10 – 50
Light oil	Organic solvent	20 – 70
Brine	Water	100 – 200
Brine	Organic solvent	30 – 90
Brine	Gases	3 – 50
Organic solvents	Organic solvents	20 – 60
Heavy oils	Heavy oils	8 – 50

           

             

Condensers
Cold Fluid	Hot Fluid	Overall U (BTU/hr-ft2-F)
Water	Steam (pressure)	350 -750
Water	Steam (vacuum)	300 – 600
Water or brine	Organic solvent (saturated, atmospheric)	100 – 200
Water or brine	Organic solvent (atmospheric, high non-condensables)	20 – 80
Water or brine	Organic solvent (saturated, vacuum)	50 – 120
Water or brine	Organic solvent (vacuum, high non-condensables)	10 – 50
Water or brine	Aromatic vapours (atmospheric with non-condensables)	5 – 30
Water	Low boiling hydrocarbon (atmospheric)	80 – 200
Water	High boiling hydrocarbon (vacuum)	10 – 30

                       

                

When the process is well defined, one can use film heat transfer coefficients to calculate the overall heat transfer coefficient. 

        

The overall heat transfer coefficient can be calculated from the film coefficients using the equation:

          

 1   =      1   + R_out  +  R_wo   +  R_io   +  1

 U           h_out                                      h_io 

         

Where:

U = overall heat transfer coefficient

h_out = film coefficient on outside surface

R_out = resistance due to fouling on outside surface

R_wo = resistance due to metal wall of heat transfer area (corrected to the outside)

R_io = resistance due to fouling on inside surface (corrected to the outside)

h_{io =} = film coefficient on inside surface (corrected to the outside) 

            

In order to use the equation above, values for the film heat transfer coefficients must be determined.  Film coefficients, just like overall coefficients, are influenced by many parameters such as nature of the fluid, type of heat exchanger, fluid velocity, transport properties and temperature. The tables below provide examples of film coefficients values for various applications.  Again, these should be used as a guide as to the order of magnitude and the actual film coefficients may be smaller or larger than the values listed. 

             

           

no phase change
Fluid	Film Coefficient (BTU/hr-ft2-F)
Water	300 – 2000
Gases	3 – 50
Organic Solvents	60 – 500
Oils	10 – 120

            

           

Condensing
Fluid	Film Coefficient (BTU/hr-ft2-F)
Steam	1000 – 3000
Organic Solvents	150 – 500
Light Oils	200 – 400
Heavy Oils (vacuum)	20 – 50
Ammonia	500 – 1000

            

             

Evaporation
Fluid	Film Coefficient (BTU/hr-ft2-F)
Water	800 – 2000
Organic Solvents	100 – 300
Light Oils	150 – 300
Heavy Oils	10 – 50
Ammonia	200 – 400