Question

Explain step by step how to determine whether the exchanger is suitable for the duty, how to estimate the pressure drop for each stream.

Answer 1

Before starting the design of a heat exchanger for a particular duty or to select a heat exchanger already designed, a screening of some factors needs to be done to check for feasibility. The following factors need to be reviewed:

1. Design temperature and pressure- check if these are possible with the materials that are qualified for manufacture.
2. Process Fluids- check if these are compatible with the materials of construction used/ to be used for the manufacture and if they are free from the excessive blockage causing fouling.
3. Heating/cooling curves- check if they satisfy the energy balance for the specified temperature and flow rates.
4. Allowable pressure drop- check if it is so small that required area for free-flow will tend to infinity. If it is so, the exchanger is not suitable for the given duty.
5. Required thermal effectiveness- check if it is so high that the required heat transfer area will tend to infinity. If it is so, then the exchanger is, again, not suitable for the given heat duty. The equation to calculate thermal effectiveness is given below:

Pressure Drop Calculations:

There are three different types of losses that contribute to the pressure drop in a heat exchanger. All of them are discussed below:

1. Entrance Loss:

The following correlation, based on Bernoulli's equation could be used to calculate pressure drop due to entrance loss:

Where

, the mass flux of the fluid

2. Core Loss:

Pressure drop due to core loss can be calculated using the following correlation:

where f=Friction fanning factor

Pressure Drop due to acceleration or deceleration across the core may be calculated using the following correlation:

where represent the densities of the fluid at exit and entrance of the heat exchanger.

3. Exit Loss:

The pressure drop due to exit loss can be calculated using the following correlation:

The total pressure drop for each stream can be calculated as: