Plate Heat Exchangers used in free cooling applications – in some cases, do you end up with what you have paid for instead of what was asked for?

To take full advantage of potential energy savings that can be gained from free cooling, the heat exchanger must be selected correctly but it is not always easy to determine the best choice of supplier due to a surprising variation of sizes and prices available.

Once closely examined it can transpire that the cheapest price is not always the best choice.

As per the title suggests, if care is not taken then what is purchased may be good for the budget in the first instance but may end up costing much, much more in a relatively short time span.

Why is this? – to explain:

Due to the rising cost of energy, the increase in popularity of the use of “free cooling” (this is when a cooling tower or other naturally cooled liquid source is used in the colder winter months to act as a chiller, or to pre-cool a chilled water circuit prior to using refrigerants. This method has shown to save on energy as it costs less to run than a refrigeration plant. The request for heat exchangers to operate with a high degree of temperature cross over and very close approach temperatures is ever on the increase as these are necessary to provide the greatest amounts of energy savings. Plate heat exchangers are usually the best choice for these applications as they function in a pure counter-current way and can run with close temperatures without ending up the size of a building in order to do so.

Putting in figures to illustrate this:

Assume water both circuits (although it could be water/glycol or anything else)

Circuit 1 cooling from 20 C down to 13 C

Circuit 2 heating from 12 C up to 19 C.

The temperatures of the respective circuits “cross over” (i.e. the outlet temperature of circuit 1 is lower than the outlet temperature of circuit 2),

In this example the approach temperatures are 1 deg C in each case. The only way to achieve this is to flow both liquids through the heat exchanger in opposite directions (counter-current).

As part of the process to calculate the required heat transfer area for the heat exchanger, there is a factor called the Log Mean Temperature Difference (LMTD for short). The closer the approach temperatures, the lower the LMTD.

Once the LMTD is calculated it can then be applied in another equation to determine the heat transfer area:

The heat transfer coefficient is determined by the types of fluids used, the flowrates/velocities through the plate channels, the heat transfer surface profile and shape, heat transfer wall thickness and a myriad of other factors unique to the heat exchanger being considered.

Heat transfer area is the figure that all manufacturers want to make as small as possible so the cost of the equipment is reduced.
Again, putting in some numbers to illustrate

• Assume that the required heat transfer is 500 kW (this is 500,000 watts)
• The LMTD = 1 deg C based upon the temperatures in the previous illustration
• Assume that the heat transfer coefficient is calculated out to be 4000 w/m²C

Using these figures in the equation:

With small values of LMTD there is a potential opening to look at the selection and decide if it can be made cheaper without anyone really noticing.

If in the above equation, the LMTD is “tweaked” a little bit and hence the temperatures used for the selection, there can be a reduction to the surface area without making it too obvious due to the difficulties in measurement and to then subsequently prove that a heat exchanger is undersized if the “tweaking” is only subtle.

Example

The LMTD needs only to be adjusted by a small amount to reduce the heat transfer area (in this example by just over 27 m²).

The actual “true” selection parameters would then be:

Circuit 1: 20 C inlet / 13 C outlet

Circuit 2: 11.8 C inlet / 18.8 C outlet instead of the required 12 C inlet / 19 C outlet

In reality, it is difficult to measure this loss of 0.2 Deg C especially as all of the other parameters and flows have to be exactly correct in order for a heat exchanger to perform as specified. However using the example above, if the heat exchanger that needs 125m² is reduced down to 97.5m² then it is never going to perform the required amount of heat transfer of 500 kw when the real operating inlet temperatures actually match the correct design condition (i.e. 20 inlet side 1 and 12 deg C on side 2). The “tweaked” heat exchanger operating at these conditions may be as much as 10 to 40% down on heat transfer depending upon the extent of the tweaking. This loss of energy transferred is then going to have to be paid for if this is a free cooling application.

If the heat exchanger is down 20 kW on heat transfer and operates for an 8 hour daily period, then this could be as much as 58 mega Watts of annual energy lost that needs to be paid for. If we take £0.3 per KW/hour then this equates to a staggering annual loss of £17,520.00.

Conclusion

When the operating temperatures are close together then a small adjustment to the figures used in the heat exchanger selection can have a large impact on the heat transfer area calculated and hence the cost of the final piece of kit. When considering temperatures alone the reduction in surface area can be hidden due to the mechanics of physically measuring temperatures, but a small LMTD and an undersized heat exchanger will result in potentially noticeable higher Plant running costs.

The heat transfer area offered by most suppliers of the same type of heat exchanger should be in the same “ball park” . There will be some differences due to recent advances in heat transfer technologies and the shapes of the plates and in these cases the suppliers will be forth coming with all necessary selection information.

Some quotations that have been seen over the years do not always provide the surface area, the heat transfer coefficients and sometimes not even the number of plates fitted or the liquid hold up volume which does make it difficult to actually compare the different quotations and to question the differences.

At, TimSmith Heat Exchangers Ltd we provide an honest, reliable and transparent service and include all of the information on our selection data sheets. After all, when a new car is purchased, it is rarely ever purchased on price alone, and in both cases of either a heat exchanger, or a car, you are in danger of ending up with exactly what you paid for instead of what you actually thought that you were purchasing.

An example of a typical “transparent” data sheet showing number of plates, the surface area and the coefficients used

 
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