A warning to Companies and Installers wishing to heat or cool the contents of a tank over a period of time using an externally mounted heat exchanger.

When a tank of liquid is either heated or cooled over a period of time, it is commonly referred to as a “batch process”.

The tank contents are pumped out of the tank, through a heat exchanger, and then back into the tank – in this way the entire contents are gradually heated or cooled.

In any tank heating or cooling process, the temperature of the contents varies continuously throughout the heating and the cooling process. A heat exchanger has a fixed amount of surface area. The ideal heat exchanger would have a continuously varying amount of surface area – a small amount at the start of the batch process and a large amount at the end. As this is currently in the realms of fantasy, suppliers of heat exchangers have to approximate the selection by basing the figures upon an assumed steady state condition in order for the heat exchanger to be sized on some fixed points. This is because the selection software only allows a single inlet temperature and a single outlet temperature to be entered. In real terms, this means that the heat exchanger is usually oversized at the start of the cycle, and undersized at the end of the cycle (this allows an “average” to be taken where you have two figures, one being higher and the other being lower than the average). The calculated heating or cooling time for the tank is derived on that basis and the exchanger selected accordingly.

Example equations:

Average heat load required to process the tank

Using Metric units

Tank volume in Litres
Specific Gravity (SG) in Kg/Litre
Specific Heat (SH) in kcal/kg C
Temp diff = tank start temperature – final tank temperature in Deg C
Time in hours

(To convert to KW then divide the answer in Kcal/hour by 860)

How to calculate theoretical tank inlet temperature to select the Heat Exchanger

Example for Cooling the tank contents:

Using:
Tank Start temp = 53 C
Tank End temp = 10 C
Chilled water inlet = 5 C

The tank circuit inlet temperature to the exchanger is calculated using:

Inlet temp to use to select Heat Exchanger = X + chilled water temp.
 
Example for Heating the tank contents:

Using:
Start temp = 10 C
End temp = 53 C
Hot water inlet = 80 C

The tank circuit inlet temperature to the exchanger is calculated using:

Inlet temp to use to select Heat Exchanger = Hot water inlet temp – X

These calculations are relatively trustworthy.
However, it is not always the case that you can trust the selection of the heat exchanger from a supplier.

As with many things these days, the actual understanding of a batch heating and cooling process can be lost as there is a reliance upon selection software to provide the answer. As with most software, “if rubbish goes in, then sometimes rubbish comes out”.

Common Mistakes

1) Using a dedicated chiller or boiler to serve the heat exchanger

As mentioned above, heat exchanger suppliers sometimes select the surface area required based on an assumed inlet and outlet temperature (the outlet temperature being a function of what flow rate is used to circulate the liquid out of the tank, through the PHE and then back into the tank again).

This is where the error comes into play…

If the average heat duty calculated by the heat exchanger supplier for the heat exchanger is 100 kW, then it would seem reasonable that an installer would consider a chiller or a boiler of similar output plus a little bit extra for good measure, say an extra 10 kW. However, the batch calculation used by some heat exchanger suppliers would expect the heat exchanger to transfer in excess of 200 kW at the start of the cycle albeit a considerably lower duty at the end of the cycle.

This higher amount of heat transfer is now impossible using the example above as the services are only based upon 110 kW!

Now because this extra heat energy / removal is not being made available to the heat exchanger at the start of the cycle, the batch calculation carried out by the heat exchanger supplier becomes inaccurate. This is because when the tank temperature gets to a point where the heat exchanger starts to become undersized (this is when the liquid in the tank is colder than the theoretical inlet temperature used), this loss of heat transfer at the start of the cycle cannot catch up because the heat exchanger is becoming less capable of transferring the heat that the services can provide or remove. The result being, the expected heating or cooling time is increased and so instead of the tank contents reaching the desired end temperature in what was quoted / calculated to be for example 1 hour, it increases to 1 hour 5 minutes or more.

Ok this may be acceptable if there is only one batch cycle to carry out, but in other cases, if this extra 5 minutes (or more) is multiplied up over a period of a year then the lost production time can soon become worrying high !

It is therefore important that when the heat exchanger is selected for use with a dedicated boiler or chiller, the figures used in the selection, and subsequent checks made on the selection, take into account that the time period indicated is not based upon an exchanger that is transferring more heat than the services can handle.

2) Circulation Rates

The faster the circulation rate, then the more times the liquid goes through the exchanger, which also means that the temperature difference of the liquid across the exchanger is less, and hence the exchanger size is reduced. An inexperienced heat exchanger supplier may opt for this as their prime concern is offering the cheapest piece of equipment possible.

Problems – if you make the circulation rate too fast on a tank that is relatively small, then too high a circulation rate causes large splashes, etc in the tank from the return pipe. Although the cost of the heat exchanger can be made more attractive (as usually a higher circulation rate means a smaller heat exchanger surface area is required), there is going to be a need for larger pipes, extra safety measures installed to reduce splashing and higher running costs due to the larger pump motors etc.

Conversely, if the circulation is too low, then this will limit the removal or addition of heat to the tank liquid, hence the time period is going to be longer just because not enough of the contents of the tank are being passed through the heat exchanger over a given time period. A circulation rate that is too low tends to catch many people out as they consider low energy usage pumps etc but fail to appreciate that if you do not put all of the tank contents through the heat exchanger enough times, then the overall process time is going to be increased.

Another issue that is sometimes overlooked with batch cooling is that the circulation pump is going to add some energy into the system and the higher the circulation rate, then the larger the pump motor and the more heat this is going to add.

Ambient gains are also not always considered. If the tank is subjected to ambient heat and process cooling is being carried out, then the selection of the heat exchanger and the subsequent size of the services need to allow for ambient heat influencing the temperature of the tank contents (as will any pipework to and from the heat exchanger).

3) Temperature cross overs

Example: Tank liquid cooling from 15 down to 4C using glycol at 0 C up to 5 C.

There is a crossover of temperatures (the 4 C final tank temperature asked for is lower than the 5 C glycol outlet of the chiller). At the start of the cycle this is not a problem for the heat exchanger as the temperatures are going to all be above 4 deg C. Heat travels across the exchanger as all is “in balance”. However, at the end of the cycle the demand on the exchanger to heat the glycol to a higher temperature than the tank (unless religion comes into play, this is never going to be possible !).

Therefore, it makes sense to be careful when looking at what services to use and the optimal operational temperatures because otherwise, a situation could arise at the end of the tank cycle, the chiller (in this example) may start to cut out.

The heat exchanger supplier should point this out, but the selection software may not have highlighted this and so, temperature cross-overs in batch heating and cooling can be overlooked.

Conclusion

If there is a large services ring main that is capable of providing or removing much more heat than the heat exchanger can transfer then the basic equations above can be used with some degree of confidence. However, if there is going to be a dedicated heater and chiller used specifically for the tank duty, then make sure that the heat exchanger has been sized correctly and ask the supplier to check the performance before making any purchases.

Look at where the tank is going to be located and make sure that if the contents are going to be influenced by external heat sources (sunlight, cooking processes etc) then these are taken into account, as these factors can have a significant effect on the heat transfer duty necessary to heat, or cool, the tank contents. The same applies if the tank is being heated and is located outside thus being exposed to winter air and cooling winds.

Circulation rates – for a starting point, look at 3 to 4 times the tank volume per hour going through the exchanger, higher for shorter periods, and lower for longer periods (for instance for large tanks being processed over 24 hours, then once or twice each hour through the exchanger may be sufficient).

Remember that the quicker the time to heat or cool the tank requested, then the greater energy the services have to provide and so if time allows, then go for the longest time period in order to reduce the size and running cost of the equipment.

An experienced heat exchanger supplier can help out to ensure that the heat exchanger proposed will actually work “as it says on the tin” thus minimising any performance issues once installation is complete. In tank heating and cooling, the smallest heat exchanger (and therefore most likely the cheapest) is not always the correct choice.

TimSmith Heat Exchangers Ltd

 
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