- Logarithmic Mean Temperature Difference
- Number of transfer units
1. Heat exchanger: heat exchanger is a device which is designed for effective heat transfer between two fluids. the flow of heat transfer is from high temperature to low-temperature fluid.
Ex. Boilers, condenser, etc.
2. Logarithmic mean temperature difference (LMTD): the temperature difference which, if constant would give the same rate of heat transfer as actually occurs under variable conditions of temperature difference.
heat transfer by the heat exchanger
Q = UAdTm
# LMTD in parallel flow heat exchanger
ΔT1 = Th1-Tc1
ΔT2 = Th2-Tc2
# LMTD in counter flow heat exchanger
ΔT1 = Th1-Tc2
ΔT2 = Th2– Tc1
- ΔTm = ΔT1-ΔT2 / ln(ΔT1/ΔT2)
- Q (heat transfer) = UAΔTm
3. Heat capacity ratio: it is denoted by C, and defined by the ratio of minimum heat capacity rate to maximum heat capacity rate of two fluids.
C= Cmin/ Cmax
4. Effectiveness: it is defined as the ratio of actual heat transfer rate to the maximum possible heat transfer rate of any heat exchanger.
ε = actual heat transfer/ maximum possible heat transfer
# if (mCp)hot > (mCp)cold,
ε = (Tc1-Tc1)/ (Th1-Tc1)
# if (mCp)hot < (mCp)cold,
ε = (Th1-Th2)/(Th1-Tc1)
5. Number of transfer units(NTU): NTU is the measure of heat transfer size of a heat exchanger.
NTU = UA/Cmin
# effectiveness from NTU method for parallel flow
ε = 1 – e-NTU(1+C) / (1 + C)
# effectiveness from NTU method for counter flow
ε = 1 – e-(1-C)NTU / 1 + Ce-(1-C)NTU
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