Why Plate Heat Exchangers are not always the best solution

Companies manufacturing and/or selling Plate Heat Exchangers (PHE’s) often advertise and promote the following features of this type of heat exchanger:

1) they can be easily expanded
2) they are easily maintained
3) they are more efficient than shell and tube exchangers

But are these sales claims completely accurate? In my opinion, no…

Claim: “Easily expanded”

The suggestion is that heat transfer and performance can be increased by adding extra plates. “Excellent”, you might say, “That means I do not need to change my heat exchanger if I increase my throughput”. Sounds good on paper, but the reality is different. There are several issues:

Some engineers and consultants writing specifications do not seem to fully understand how a heat exchanger works. It is quite typical to read specifications advocating that the heat exchanger required should have capacity to handle an increased duty by the addition of extra plates. This makes good reading for the end user as he or she can then be led to believe that the heat exchanger on offer will not only perform the duty required in the first instance but also take care of future expansions without having to change the plant except by adding a few more plates. But whether or not it is possible to achieve a greater performance largely depends on the temperatures specified which in turn determine the Log Mean Temperature Difference (LMTD for short).

If the gap between the primary and secondary circuits are close together then this results in a small LMTD. As this gap increases, then the LMTD also becomes larger;

Example: A small LMTD
Primary 80 C flow, 50 C return with a secondary of 75 C flow and 45 C return gives you an LMTD of 5 Deg. C.

Example: A larger LMTD
Primary 80 C flow and 60 C return with a secondary of 45 C flow and 35 C return gives you an
LMTD of 30 Deg C.

With smaller LMTDs as in the example above, if you add 25% extra plates to the PHE then the increase in duty you can expect is typically around 1 to 2%. Adding 50% more plates would only give you around 3 to 5% extra heat transfer duty above the original amount. Another factor that can influence performance and is when the inlet and the outlet temperatures cross over. Using the smaller LMTD example again, where the secondary flow temperature of 75 C is higher than the primary return temperature of 50 C, the temperatures of the primary and the secondary circuits have effectively “crossed over”.

Larger values of LMTD, especially if you do not have temperature cross over, can give you much more heat transfer by adding more plates to the PHE. Considering the larger LMTD example above, then adding 25% extra plates will give as much as 20 to 30% extra heat transfer, and by adding 50% more plates can typically give around 30 to 60% extra heat transfer*.

*NOTE – the performance figures can vary depending upon the plate design, number of plates added (thermal performance), whether or not the complete pack is reconfigured and what fluids are being passed through the PHE. Therefore the figures in these examples are illustrative because in practice the final figures depend on the aforementioned variables.

It should be noted that the extra heat transfer performance calculation was achieved by lowering the primary return temperature and increasing the secondary flow temperature whilst maintaining the original figures for flow rates. Widening the temperature differences can in itself cause problems with boilers, chillers and the like, i.e. adding extra plates could cause the boiler or chiller to trip out if no other adjustments are made. Which leads on to the next point.

It is much easier to increase the performance of the PHE by adding extra plates when both the primary and the secondary flow rates are increased as well. If you add for example 30% extra plates to the PHE and you also increase both flow rates by the same amount (30%), then you can expect the heat transfer duty to follow suit and be increased by the same level. But it is unlikely that you will be able to increase a pump flow rate by 30% and not make any major changes to the pumps (larger motors and /or impellors etc.)? And the existing pipework (including the PHE connection sizes), and fittings (control valves, bends, orifice plates etc) may have to be changed to avoid excessive velocities (and hence higher pressure losses).

It can legitimately be argued that if a future expansion is going to require 25 to 50% higher heat transfer duty then it is going to require a major overhaul to the entire system. Therefore specifying or purchasing a PHE that has the capacity to accept extra plates with a view to passing more heat across for a future project with minimum fuss/cost, is basically nonsense. And might mean that other, possibly more suitable and thereby cost effective heat exchangers, were not considered.

When the PHE is opened to have the extra plates added, it is highly recommended (and sometimes essential) that the gaskets are changed in the original pack to avoid mixing older compressed /set gaskets with the new ones fitted to the later added plates. Or, if the old plates are nevertheless used, then there is a high chance they will have to be removed and cleaned before being put back into service. This can cost dearly!

It is also traditional for PHE manufacturers and sales outlets to charge premium prices for any additional/spare parts. Therefore a scenario can arise when the cost of adding the extra plates will cost more than if a completely new larger unit was purchased instead.

Claim: “Easily Maintained”

Ninety-nine per cent of the time, carrying out maintenance on a PHE is going to necessitate opening up the plate pack. This requires good access to both sides of the PHE to allow for the tie bolts to be loosened and removed. As space for installation tends to be at a premium, it is common for a PHE to be tucked away in a corner, or tightly surrounded by pipe work and/or other pieces of plant. But, unless attention is paid to achieving good access and space around the PHE on installation, and sadly this is not always the case, then removing the tie bolts down either side of the PHE can be very time consuming and difficult. And, as this is a manual job, then this time is going to cost money.

When assembling a PHE plate pack there are rules about how it needs to be put together in terms of which way up the plates are placed inside the frame and the sequence that they are put together. It may be “easy” for a trained fitter that has built packs many times before, but not all plate designs have gaskets that stay correctly in place – especially now when most manufacturers use an adhesive free gasket attachment system (and some of these clip type arrangements are lacking to say the least). Consequently, careful attention has to be paid to how the plates are put into the frame followed by a close inspection of the gaskets (especially the diagonal running portions of the gaskets that are near to the port holes) to make sure that they are seated correctly before squeezing the pack together.

The tightening of the tie bolts compresses the plate pack thus setting the correct gasket thickness and ensuring the gap between the plates is the right distance apart. Tightening of these bolts is difficult manually and generally requires power tools. The tie bolts do become very “tight” when approaching the correct compression setting of the pack and on larger units it is sometimes necessary to use elongated spanners to make it physically possible to turn the nuts on the bolts. There are some parts available today such as bearing boxes fitted to the tie bolts which do reduce the friction to aid tightening, but these are usually fitted as an “extra” and there are many PHEs where these are not present.

Gasketed PHEs do require gasket changes. It is the gaskets that are preventing the liquids from escaping. Hence they perform an extremely important function and must be changed as often as necessary. The frequency of these changes depends on the fluids used, the temperatures, the operating pressures, the operating regimes/conditions, where the unit is installed and the number of times the pack is opened.

Although it appears easy and straight forward in a PHE assembly factory using new parts, power tools, plenty of space /easy access and no pipe work attached, as we all know, these conditions are extremely rare when on site and this is going to make maintenance /gasketed changes more of an issue. Therefore to make a claim that gasketed PHEs are easy to maintain is very debatable. It may be true that it is possible to access and maintain all of the heat transfer surfaces, but at what trouble and cost when on site as opposed to in a specially set up work space?

Taking a PHE that is around one metre tall and a third of a metre wide with 50 plates in the pack, there are going to be around 130 to 150 metres of gasket length. To ensure no leaks once the pack is compressed, it is important that the gasket seating and sealing areas are free from dirt and grit. This generally means that it is necessary to clean the plates if the gaskets need to be renewed.

Plate cleaning on site is seldom practical and re-gasketing on site can be a nightmare due to space limitations, keeping things clean, health and safety reasons and access to water supplies and drains. Therefore unless a replacement pack is purchased (bearing in mind the potential costs involved as mentioned above}, then there are going to be higher than expected labour costs plus transportation of parts to and from the cleaners or service company. Even if the plates are clean enough, once opened it is usually necessary to wipe the sealing areas and gasket faces clean prior to re-assembly. If we take the pack that has around 150 metres of gasket length, the actual distance to wipe down is doubled as the sealing area on the back of the adjacent plate has to also be cleaned. Even with a relatively small plate pack, it does take some time to do this manually which adds to the labour and downtime costs.

Claim: “More efficient than shell and tubes”

Efficiency is one of those terms that can mean different things depending on context. If you talk of efficiency in the way that this term is generally used (i.e. energy in, compared to energy out) then for most types of heat exchanger that use no energy to power or drive them, the efficiency of heat transfer is very high in terms of what you put into the exchanger, you get out on the other side. PHEs achieve typically 99% efficiency with only a small amount of heat being lost from around the plate pack and through the end frames. Shell and tubes have similar efficiencies. Fitting insulation to either type reduces the heat loss and thus increases the efficiency.

If we take efficiency to mean the amount of heat that can be transferred compared to the material used, then yes, you can argue that a PHE can achieve higher heat transfer coefficients and therefore use a lower amount of heat transfer area than an old fashioned shell and tube. However, with the advent of corrugated tubes, spiralled tubes, rifled tubes and dimpled tubes, some relatively high heat transfer coefficients can be obtained and hence some designs of shell and tubes are catching up.

Conclusion

Gasketed plate heat exchangers are a brilliant and compact way to transfer heat for low viscosity liquids and now with the advent of free channel plates (basically a flattened corrugated tube), dimple pressings and the like, the possibility of using plates for more viscous products and fluids containing particles has increased.

PHEs are relatively low priced when compared to other heat exchanger types, primarily due to the lower amount of material used, the use of mass produced parts, minimal welding (if any) and the reduced amount of man-hours required to put them together in the factory. But this does not make them always the best choice.

Adding extra plates can sometimes result in a very small difference in performance.

Gasketed PHE units can be easily maintained in an ideal environment.

It can be argued that the efficiency of a PHE is roughly the same as e.g. shell and tube. This all depends on how the meaning of the word efficiency is being defined at the time. Such a definition is rarely proffered.

When deciding what type of heat exchanger to purchase, careful consideration should be given to where the heat exchanger is going, what the operating parameters are going to be, what fluids are being used and the safety and maintenance issues rather than immediately opting for a gasketed PHE (which sometimes draws you in because of the lower initial cost).

And if you are tempted to buy a gasketed PHE because a salesman or an advertisement is claiming that you can easily expand and maintain this type of equipment, then be warned!

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