The temperature has been dropping, so is it time to add antifreeze to protect your heat exchanger?
The benefits of protecting a heat exchanger from freezing are well known, but are there any downsides to using anti-freeze?
If the heat exchanger was originally selected using calculations based on water only, then the answer is “yes”.
1) Increases the viscosity of the water.
This means that it is going to be harder for the pump to push it around the system, and so what usually happens is that the either the pump slows down, and/or the power consumption goes up.
2) Reduces the thermal conductivity of the water.
A lower conductivity value effectively reduces the speed in which heat passes through a medium. Therefore, when inside a heat exchanger, the amount of heat being transferred across to the other liquid circuit goes down and so not all of the heat has had long enough inside the exchanger to travel across to the other circuit.
3) Alters the specific heat and the density of the water.
The specific heat and the density values are used in the equation to calculate a liquid flowrate. Antifreeze solutions do not have the same values as plain water. Unless the flow rate is changed to allow for what is no longer just water (because antifreeze has been added), then the temperature rise or fall inside the heat exchanger changes. This in most cases lowers what is called the Log Mean Temperature Difference, which in turn decreases the performance of the heat exchanger.
4) The overall thermal characteristics and behaviour of the water circuit inside the heat exchanger is affected.
In many cases, this reduces what is known as the heat transfer co-efficient a figure that largely determines the heat transfer area required). When this value goes down then the performance of the heat exchanger falls with it, and it is necessary to use a larger heat exchanger surface to achieve the same results..
15% anti-freeze solution used instead of plain water: performance drops by 7%
20% anti-freeze solution used instead of plain water: performance drops by 10%
25% anti-freeze solution used instead of plain water: performance drops by 12%
30% anti-freeze solution used instead of plain water: performance drops by 14%
40% anti-freeze solution used instead of plain water: performance drops by 19%
50% anti-freeze solution used instead of plain water: performance drops by 24%
The above is a indicative guide only and the use of non-toxic antifreeze solutions can vary the results slightly. The results also give a suggestion as to how much larger your exchanger would need to be in systems that need to have protection from freezing.
Air to water heat exchanger / Heating or cooling coils: If a tube or bend has a split that looks like it has been cut with a knife then suspect ice damage.
A Gasketed plate heat exchanger would generally have a gasket pushed out
A brazed plate heat exchanger would either crack, or have a “bulge” out of the back of the unit (opposite end to wher the connections are located)
A shell and tube would split a tube, or distort (or burst open) the shell. They may be cases when the tube plate has distorted and the tube to plate welds have split as a result, but these are rare because usually the shell would go first.