Process Design Process Equipments Solved Sample Problems

Problem solving – Heat exchanger tube side pressure drop calculation

Problem Statement -

Calculate the tube side pressure drop for the following heat exchanger specification,
Process fluid = water
Inlet pressure = 4 barg
Inlet temperature = 500C
Outlet temperature = 300C
Tubeside flowrate = 50000 kg/hr
Number of tubes = 25
Tube ID (internal diameter) = 1 inch
Tube length = 3.5 m

Solution -

Calculation of tube side pressure drop for a heat exchanger is quite straightforward. Since the pressure drop in individual tubes is very easy to calculate using established practices for calculation of pressure drop across tubes and pipes. Total flow on the tubeside can first be divided by number of tubes and pressure drop across a single tube can be calculated using Darcy-Weisbach equation. This corresponds to the tubside pressure drop. Problem solving is performed in following 2 basic steps.

Step 1.

First step of problem solving requires determination the important physical properties of given fluid (water) at given temperature and pressure conditions. Since, water density will be lowest at inlet temperature (500C), which corresponds to highest volumetric flow. Hence for conservative pressure drop estimate, physical properties of water are calculated at the inlet conditions. Using EnggCyclopedia's Liquid Density Calculator,
water density at 500C = 988.0 kg/m3 Using EnggCyclopedia's Liquid Viscosity Calculator,
water viscosity at 500C =0.53 cP

Step 2.

Total volumetric flow = 50000 kg/hr ÷ 988.0 kg/m3 = 50.61 m3/hr Volumetric flow in each 1" tube = 50.61 ÷ 25 = 2.02 m3/hr Pressure loss per unit length of the tube is then calculated using EnggCyclopedia's pressure drop calculators for pipes and tubes. This calculator is based on Darcy-Weisbach equation.

Pressure loss across a single tube (ΔP/L) = 6.17 bar/km

Tube length (L) = 3.5 m
Tubeside pressure drop (ΔP) = 6.17 × 3.5 / 1000 = 0.0216 bar

Alternative Solution

Another alternative is to directly use EnggCyclopedia's Heat Exchanger Tube side Pressure Drop Calculator. All the inputs given in the sample problem statements are given to the calculator and pressure drop across the tubeside is calculated as output. This calculator uses the same basic steps discussed above and hence the answer also matches with the figure above (0.0216 bar) . The following image is a snapshot of this direct calculation of tubeside pressure drop.



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