Tag: effectiveness

Heat Exchanger Analysis

Heat Exchanger Analysis

Heat Exchanger Analysis based on Effectiveness (ε) – NTU method is done when inlet temperatures are known and outlet temperatures are to be determined.

Obtain Process Parameters

Get process stream mass flowrate (M), specific heat (Cp) and inlet temperature (T). Obtain the heat transfer area (A) and overall heat transfer coefficient (U) for the given dimensions of heat exchanger.

Calculate heat capacities and obtain the minimum heat capacity.

CH = MH * CpH
CC = MC * CpC
CMin = Minimum (CH , CC)
CR = CMin / CMax

where MH, MC are hot and cold fluid mass flowrate; CpH, CpC are hot and cold fluid specific heat.

Calculate NTU and QMax

Number of transfer units ( NTU ) is calculated using following equation :

NTU = U.A/ CMin

Maximum heat transfer rate ( QMax ) is calculated using following equation :

QMax = CMin.(THot In - TCold In)

Determine Effectiveness

Based on NTU and CR (Ratio of heat capacities) determine heat exchanger effectiveness (ε) from Effectiveness – NTU curves available in literature.

ε – NTU Curve for Cross Flow exchanger Both stream unmixed

Effectiveness NTU Curve for Cross Flow Both stream unmixed

Calculate Outlet Temperature

Heat exchanger duty is calculated as:

Q = ε * Q Max

Outlet temperature are estimated as following :

THot Out = THot In - Q /( MH.CpH)
TCold Out = TCold In + Q /( MC.CpC)

Example
Hot exhaust gases 1.5 kg/s, enter a finned-tube, cross-flow heat exchanger at 250°C and is used to heat pressurized water at a flow rate of 1 kg/s available at 35°C. Exhaust gas specific heat is 1000 J/kg.K and water specific heat is 4197 J/kg.K. Overall heat transfer coefficient is 100 W/m².K and area is 40 m². What is rate of heat transfer by exchanger and gas and water outlet temperatures ?

Calculate Heat Capacities

CH = 1.5 kg/s * 1 kJ/kg.°K 
   = 1.5 kW/ °K
CC = 1.0 kg/s * 4.197 kJ/kg.°K 
   = 4.197 kW/ °K
CR = 0.36

Determine NTU and Q Max

NTU = 2.67
QMax = 322.5 kW

Determine Effectiveness
Based on ε – NTU curves for cross flow heat exchanger, determine ε

ε = 0.845

Calculate Outlet Temperature
Heat duty is calculated as following –

Q = 272.36 kW
THot Out = 68.43 ° C
TCold Out = 99.89 ° C

Spreadsheet

All above calculation for some common exchanger geometries have been provided in below spreadsheet.

Spreadsheet for Heat Exchanger Analysis