Heat Transfer Coefficients 
                 
It 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*DTlm  
            
Where:
Q = heat load
U = overall heat transfer coefficient
A = heat transfer area
DTlm = 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 FluidCold FluidOverall U
(BTU/hr-ft2-F)
SteamAir10 – 20
SteamWater250 – 750
SteamMethanol200 – 700
SteamAmmonia200 – 700
SteamAqueous solutions100 – 700
SteamLight hydrocarbons
(viscosity < 0.5 cP)
100 – 200
SteamMedium hydrocarbons
(0.5 cP < viscosity < 1 cP)
50 – 100
SteamHeavy hydrocarbons
(viscosity > 1)
6 – 60
SteamGases5 – 50
DowthermGases4 – 40
DowthermHeavy oils8 – 60
Flue gasAromatic hydrocarbon and steam5 – 10
                  
              

Evaporators

Hot FluidCold FluidOverall U
(BTU/hr-ft2-F)
SteamWater350 – 750
SteamOrganic solvents100 – 200
SteamLight oils80 – 180
SteamHeavy oils (vacuum)25 – 75
WaterRefrigerant75 – 150
Organic solventsRefrigerant30 – 100
                 
                 

Coolers (no phase change)

Cold Fluid Hot FluidOverall U
(BTU/hr-ft2-F)
WaterWater150 – 300
WaterOrganic solvent50 – 150
WaterGases3 – 50
WaterLight oils60 – 160
WaterHeavy oils10 – 50
Light oilOrganic solvent20 – 70
BrineWater100 – 200
BrineOrganic solvent30 – 90
BrineGases3 – 50
Organic solventsOrganic solvents20 – 60
Heavy oilsHeavy oils8 – 50
           
             

Condensers

Cold Fluid Hot FluidOverall U
(BTU/hr-ft2-F)
WaterSteam (pressure)350 -750
Water Steam (vacuum)300 – 600
Water or brineOrganic solvent (saturated, atmospheric)100 – 200
Water or brineOrganic solvent (atmospheric, high non-condensables)20 – 80
Water or brineOrganic solvent (saturated, vacuum)50 – 120
Water or brineOrganic solvent (vacuum, high non-condensables)10 – 50
Water or brineAromatic vapours (atmospheric with non-condensables)5 – 30
WaterLow boiling hydrocarbon (atmospheric)80 – 200
WaterHigh 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   + Rout  +  Rwo   +  Rio   +  1
 U           hout                                      hio 
         
Where:
U = overall heat transfer coefficient
hout = film coefficient on outside surface
Rout = resistance due to fouling on outside surface
Rwo = resistance due to metal wall of heat transfer area (corrected to the outside)
Rio = resistance due to fouling on inside surface (corrected to the outside)
hio = = 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

FluidFilm Coefficient
(BTU/hr-ft2-F)
Water300 – 2000
Gases3 – 50
Organic Solvents60 – 500
Oils10 – 120
            
           

Condensing

FluidFilm Coefficient
(BTU/hr-ft2-F)
Steam1000 – 3000
Organic Solvents150 – 500
Light Oils200 – 400
Heavy Oils (vacuum)20 – 50
Ammonia500 – 1000
            
             

Evaporation

FluidFilm Coefficient
(BTU/hr-ft2-F)
Water800 – 2000
Organic Solvents100 – 300
Light Oils150 – 300
Heavy Oils10 – 50
Ammonia200 – 400
        
      
The information is provided for educational use only – use at your own risks.