Heat Exchangers Overview & Sizing

If you currently heat your Swimming Pool with a gas, propane or oil boiler then there is a good chance you have a heat exchanger in your plant room.

You may be interested to know what the item bolted to the wall, or the floor does?

Heat Exchange the Principle:

The above diagram shows the way heat is exchanged. T2 is the primary flow and T1 is the secondary flow.

It is not effective for the hot water from your heat source eg. boiler to mix with your pool water, so the pool water T2 is pumped through a pipe and your heat source hot water is pumped thru T1 circulating very close to the pool water flow. This is where the heat transfer happens using the principles of thermodynamics in essence the heat passes to the pool water whilst the cooled boiler water returns to the boiler to be re-heated and the process continues again.

The most common types of exchanger in domestic or commercial pool plant rooms are outlined below.

Shell and Tube, Finned and Tube Stack Exchangers.

The following heat exchange units are the most common types found in domestic plant rooms.

The 1st image is a Tube Shell unit, whilst, the 2nd is of a Finned Exchanger and the final image is of a Tube Stack Exchanger.

The design of the tube shell is simple, tubes sit inside a sealed shell where the pool water flows through the sealed vessel where the tubes carry the hot primary flow allowing the exchange to take place. The finned exchangers pass the primary flow through fins rather than tubes as this gives the greatest surface area for exchange to happen. The disadvantage of these exchangers is they have a set primary input temperature and unless this is reached the exchange will not work.

The final option is a tube stack type, where a bundle or stack of tubes surrounds the secondary flow and as there are lots of tubes giving a much greater surface area making the exchange very efficient.

The market leaders of this type are Bowmans, a UK company, they have over many decades refined and developed this effective technology.

We would always advise when possible to replace an existing unit or a new unit with this type of exchanger. Their quality, effectiveness and durability offset the higher initial outlay cost.

The below is a plate and frame exchange, this normally is used in larger commercial pools and is not really found in a domestic situation.

This system is not recommended for use in a home pool application and as such is not the real focus of this guide.

How to Size Your New Heat Ex-Change unit.

In layman's terms to size or select an exchange unit we assume that the heat supplied is via a domestic gas boiler, a dedicated pool boiler will in almost every case have a built-in heat exchange unit.

An air-source heat pump that is used to heat a property requires a different system in conjunction with the exchange unit, we can supply specific units made primarily for this application.

Please email our technical team on info@poolandspacentre.co.uk to talk through any aspect of the design or installation of this system. As a note typically if you have a dedicated air source heat pump for your pool it will have the exchange unit built into it.

When sizing a unit the pool volume is a parameter but mainly the sizing of the primary heat source (the boiler) is of primary concern. The boiler should be sized (Kw rating) to have enough capacity to heat the volume of water in a timely fashion. When sizing the exchange unit the MOST CRITICAL factor is the sustainable primary input temperature that can be maintained.

FOR EXAMPLE, You have a 35kw capacity boiler that can give a maximum output temp of 75oC most exchange units have an optimum primary input temp of 82oC (most dedicated pool boilers will reach this temp) however a domestic boiler will struggle over extended periods to maintain the max output @75oC. This can lead to the boiler having a much shorter life as it's working flat out all the time it's heating.

The solution to this is to reduce the primary input temp. to a more sustainable say 60oC. The idea is to get the 35kw rating of the boiler into the heat exchange unit and get as much of the 35kw rated output into the pool water. To this end, you would ensure that the reduced temp gives as close to the rated Kw of the boiler.

A 35Kw boiler would be mated with a 70kw Bowman's exchange unit as @ 60oC will have an exchange rate of 40kw so you are getting the maximum from the boiler without causing undue wear by making it work flat out.

This is shown in the table below:

As you can see the 70kw unit is advised for a pool of 26,000 gallons, as a guide we say selecting the larger unit protects your boiler and then you maximise the energy used, it's as effective as it can be.

What should my exchanger be made of?

This depends on certain factors, such as, if you use a salt chlorinator, the saltwater mix is very corrosive, therefore, a titanium exchange unit is recommended as it provides more resistance to the corrosive nature of the water. The other options are Cupro-nickel and Stainless steel. With Chlorine or Bromine or Active Oxygen, you would not need a Titanium unit.

You must always be aware that to keep on top of your pool chemistry, as a result of PH going out of range can cause damage to your exchange unit and lead in some cases to failure of the unit and damage to other pool equipment.

Armed with this information you should be able to narrow down your choice and make an informed decision on the unit that best suits you.

We have supplied hundreds of units for varying applications since 2003 and our dedicated technical team are here to help at any stage of the design or installation of the unit. Please email our technical team at info@poolandspacentre.co.uk, we are here to help.