In my previous post, I share about step-by-step design of shell and tube heat exchanger. In this post I want to share you specifically about exchanger layout design. This layout design consists the following items.
Because this is so many, I think I will divide the topic into three topics:
- General design consideration
- Construction detail in tube side
- Construction detail in shell side
My fire water pump calculation post series will be postponed due to these topics. I already prepared simple presentation about design of shell and tube heat exchanger. So, I will share this topic first.
Fluid Allocation
There are at least six consideration to decide if hot fluid or cold fluid is located in shell side or tube side.
- Corrosion. Fluid with higher corrosion tendency is allocated at tube side. The reason is to reduce cost of expensive alloy or clay component
- Fouling. Fluid with higher fouling tendency is allocated at tube side. The reason is tube side is easier to clean and have higher allowable velocity. High velocity will reduce fouling
- Fluid temperature. Fluid with higher fluid temperature is allocated at tube side. The reason is to reduce overall cost if required the use of special alloys
- Operating pressure. Fluid with higher operating pressure is allocated at tube side. This is because high pressure tube is cheaper than high pressure shell
- Pressure drop. Fluid with the lowest allowable pressure drop should be allocated at tube side
- Viscosity. Fluid with more viscous characteristics should be allocated at shell side because it provides higher heat transfer coefficient
- Stream flow rate. Fluid with the lowest stream flow rate is allocated at shell side to give the most economical design.
Fluid Velocity
General rule for fluid velocity at shell side and tube side of heat exchanger is expressed in table below.
| Fluid Phase | Fluid Condition/Description | Tube Side | Shell Side |
|---|---|---|---|
| Liquid | Process Fluid | 1-2 m/s Max 4 m/s | 0.3-1 m/s |
| Water | 1.5-2.5 m/s | 0.3-1 m/s | |
| Vapor | Vacuum | 50-70 m/s | 50-70 m/s |
| Atmospheric | 10-30 m/s | 10-30 m/s | |
| High pressure | 5-10 m/s | 5-10 m/s |
Pressure Drop
Pressure drop at shell side and tube side of heat exchanger is usually defined by Contractor. However, for general rule, table below shows pressure drop as function of fluid phase, viscosity, and operating pressure.
| Fluid | Description | Pressure Drop (kN/m2) |
|---|---|---|
| Liquid | Viscosity < 1 mN s/m2 | 35 |
| 1-10 mN s/m2 | 50-70 | |
| Gas and Vapor | High vacuum | 0.4-0.8 |
| Medium vacuum | 0.1 x abs pressure | |
| 1-2 bar | 0.5 x system gauge pressure | |
| Above 10 bar | 0.5 x system gauge pressure |
When high pressure drop is utilized, ensure that resulting pressure drop does not cause erosion or flow-induced vibration.
Stream Temperature
As a general rule, temperature approach (the difference of outlet temperature of one stream and inlet temperature of the other steam) should be at least 20oC, and the least temperature difference 5-7oC for coolers using cooling water, and 3-5oC using refrigerated brines.
Reference:
Chemical Engineering Design – Coulson & Richardson
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