Question

We are designing a double pass heat exchanger to heat ethanol from 25 C to 75 C using steam. The flowrate of ethanol is 175 gpm = 0.011 m3 /s. We will start by using tubes with OD = 0.625”, ID = 0.50”. The ethanol is heated by low pressure saturated steam with Tin,s = 150 C, and Tout,s = 140 F. We eventually (Problem 8) want to calculate the heat needed to raise the temperature of the ethanol, and the area needed for the exchanger. Let’s assume we have 85 copper tubes (each is a double pass) for this problem, which carry all the flow; this is a design parameter that we would normally iterate for. Here are a few properties of …

ETHANOL: density = 789 kg/m3 heat capacity = 2440 J/kg thermal conductivity = 0.171 W/m-K visc at 50 C = 0.670e-3 Pa-s = 0.000670 Pa-s CONDENSING STEAM (WATER): heat of vaporization hfg = 2260 kJ/kg k = 0.60 W/m-K viscosity = 0.30 Pa-s at the temperature of condensation

a) Calculate the required heat load, based on the change in ethanol temperatures.

Qthermo = _________________ W

b) Calculate the heat transfer coefficients.

inner (ethanol)

h = __________ W/m2

outer (condensing steam)

h = __________ W/m2

overall U = __________ W/m2

2 - Calculate then the required area and length of the heat exchanger, disregarding the values previously calculated and instead using Q=1200 kW and U = 1400 W/m2-C

Delta T = ____________ C

F = __________ (correction factor)

A = __________ m^2

L = __________ m

Answer 1

The accuracy of the answer depends on the type of correlation used for heat transfer coefficeint calculation and also on the accuracy of phyical data provided