Basic Concepts of Heat Exchanger Theory

“The compact brazed plate heat exchanger is the most efficient way to transfer heat today and offer the same performance as large shell and tube units in a compact and reliable way”

The basic concept of a heat exchanger is based on the premise that the loss of heat on the high temperature side is exactly the same as the heat gained in the low temperature side after the heat and mass flows through the heat exchanger.

This is expressed by the following formula:

Q(Btu/h)=m(lbs/hr) x ΔT(F) basic heat transfer formula based on the previous premise we conclude

    Q = m1C1ΔT1 - m2C2ΔT2

Where:

Q(Btu/h) = Total Heat Load   
m1, m2 (lbs/hr) = Mass flow rate of circulating water.   
C1, C2 (BTU/F, lbs) = Specific Heat at constant pressure.   
ΔT1 , ΔT2 (F)  = Temperature difference between inlet and outlet of the heating and cooling media.
  

Example: Measuring of flow in the case of a boiler of  80,000 Btu/h

Hot Side             Ts = 176F,           Tr=113F              ΔTα  =63.0(F)
Cold Side             Ti = 59F,            To=104F             ΔTβ  =45.0(F)

(m1)= 80,000/63 = 1,270 lbs/h Flow of the hot side(m1) = 80,000/63=1,270lbs/h (Boiler side)
(m2)= 80,000/45 = 1,778 lbs/h Flow of the cold side (m2) = 80,000/45=1,778 lbs/h this means the production of hot water @ 104F at a flow rate of  3.6 GPM.

Logarithmic average  temp. difference (ΔTα ave)
ΔT promedio = (ΔTα - ΔTβ) / Ln(ΔTα / ΔTβ)
ΔTα  = Ts - To   
           ΔTβ  = Tr  - Ti              

Notes:

Heat Transfer is impossible in case of TsThe Temperature difference between Ts-To( ΔTα ) and Tr-Ti ( ΔTβ  ) should be 1.8F or more.

Coefficient of transmission for the plate heat exchanger    (K: Btu/ft²,h,°F)
 

Coefficients for this type of heat exchangers range from

Water to Water                     615~1230 Btu/ft²,h,°F
Steam to Water                     307~ 615  Btu/ft²,h,°F

Heat Transfer Area Calculation          ( S : ft2 )
 
S = Q/(ΔTave. X K) x α allowance rate)
  

“The compact brazed plate heat exchanger is the most efficient way to transfer heat today and offer the same performance as large shell and tube units in a compact and reliable way”