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## Quick Launch

Heat exchangers are often bought oversized for the required duty. This tutorial looks at the reasons why, the effects this has and related requirements, such as trap sizing for oversized exchangers.​

# Oversized Heat Exchangers

The previous calculations (Module13.2) assumed that the heat exchanger had been sized on the perfect heating area to meet the specification. This would mean that the heat exchanger was exactly sized for the duty.

This is highly unlikely in practice as the designer or specifier will usually add other factors, including those for fouling and uncertainty of maximum operating loads. It is also unlikely that manufacturers can supply heat exchangers to match a specification exactly. As undersized heat exchangers are impractical they are usually bought oversized.

The operating conditions laid down in Example 13.2.1, Part ‘C’, have been reconsidered in Example 13.3.1 by adding 15% to the required heating area to account for contingencies.

Required heating area is calculated to be 1.09 m² (Example 13.2.1, Part ‘C’) therefore the specified heating area for Example 13.3.1 is to be 1.09 + 15% = 1.254 m².

The minimum size that the manufacturer can supply has a heating area of 1.31 m², representing an actual heating area of some 20% above that required. A larger heating area requires less steam pressure for the same heat transfer rate, and because of this the steam pressure in an oversized heat exchanger will be lower for the same heat load.

As the steam pressure is less, the steam temperature is less, and the heat exchanger LMTD (Logarithmic Mean Temperature Difference) will also be less.

To determine the steam temperature for the design condition, it is first necessary to find the new LMTD (TLM) for the larger heating area (see Example 13.3.1).

## Example 13.3.1

The TLM can be found by re-arranging Equation 13.2.1 to give Equation 13.3.1​

## From Example 13.2.2, at full-load:

The secondary inlet temperature (T1) = 10°C
The secondary outlet temperature (T2) = 60°C
The new steam design temperature can now be determined using Equation 2.5.5:​

This temperature corresponds to a steam pressure of 1.95 bar g. When the heat exchanger was perfectly sized in Module 13.2, the steam pressure was 4 bar g. In this example, with a heat exchanger 20% oversized, the steam pressure is 51% less.
Now that the steam pressure has been predicted at the full-load condition, it is possible to calculate the steam flow at full-load.