In this post we'll look at an important topic: choosing a heat exchanger for a glycol air heater - although it doesn't necessarily have to run on glycol. Heating with air heaters isn't really common in single-family homes, but it's fairly often used in warehouses or company premises. We'll briefly explain how air heaters work and what the operating principle of this type of heating is - and in the second part of the post we'll get to the title topic itself, namely selecting a Nordic Tec plate heat exchanger for such an air heater.
If you're only interested in the matter of selecting a heat exchanger for the air heater, and you generally already know how an air heater and this heating model work - you can skip ahead to the second part of the article by clicking the relevant paragraph in the table of contents.
An air heater is a type of device whose job is to heat the air stream flowing through it using a hot medium circulating in the system. This heating medium, which transfers heat to the air - is often glycol - in which case we speak of glycol air heaters, but it can also be water. We'll explain below why glycol, rather than plain water, is often used in air heater circuits in many warehouses or production facilities.
Most often this medium is a mixture of water and glycol, because it provides protection against freezing at low temperatures. This is particularly important in ventilation units, industrial installations, and systems operating outside buildings, where plain water could freeze and damage the air heater or the whole system. Adding glycol lowers the medium's freezing point, increasing the safety and reliability of the whole system, although at the same time it slightly worsens heat-exchange properties compared to pure water - since glycol transfers heat somewhat less effectively.
For such an air heater to work efficiently and provide a warm air supply in a given location, it therefore needs to be fed with a hot medium. But where does that medium get its heat from?
An air heater in a ventilation unit must be fed with a hot medium - which will transfer heat to the air flowing through the heater. For this, some heat source is needed - e.g. a gas or pellet boiler, but it could also be a heat pump. After releasing its heat to the air in the heater, the glycol returns to the heat source cooled down and is heated again, then sent back to the heater - in a continuous circulation. So the system can run smoothly - but why is a plate heat exchanger needed here at all?
In a system with an air heater, the plate heat exchanger acts as an intermediary between the heat source and the glycol circuit feeding the air heater. Its job is to transfer thermal energy from the heating system to the water-glycol mixture without mixing the two media. This solution makes it possible to separate the heat-source circuit from the air-heater circuit, which increases the safety of the installation, makes it easier to operate, and allows different operating parameters to be used on each side of the exchanger.
Note that in the heating system (let's call it, for convenience, the boiler circuit) the heating medium is water - while in the air heater it's usually a water-glycol mixture. A plate heat exchanger is used to separate the glycol from the water-based boiler circuit. It allows heat to be transferred from the boiler circuit to the glycol in the air-heater system - while still mechanically separating the two circuits. The plate exchanger transfers heat without mixing the circuits (it is not a hydraulic separator).
This is particularly important in ventilation and industrial systems, where the air heater may be exposed to low temperatures, and the use of glycol effectively protects it against freezing.
Below we present a, of course, highly simplified diagram illustrating the role of the heat exchanger in a system with an air heater. The diagram omits many important details, such as valves or circulation pumps. Its purpose is to show the placement of the heat exchanger itself as the link between the heat source and the air heater, highlighting that such a system has 2 independent circuits.
The selection rules below are not universal and apply specifically to one brand of heat exchangers, the Polish company Nordic Tec. These rules may not be similar for other manufacturers. The fact that Nordic Tec uses specific plate-area conversion factors that can be used as a guide makes selecting exchangers much easier, however comparing the plate area of models from different manufacturers isn't always meaningful (because the plates themselves can differ - they may have different thickness and profiling, and the spacing between plates can also vary - which means the heat-transfer coefficient can differ despite the same surface area). Put more simply, to illustrate this - a heat exchanger with 1m² of plate area from Nordic Tec can still be more effective at transferring heat than a similar model with the same surface area from a cheaper manufacturer - because Nordic Tec focuses on devices with a higher heat-transfer coefficient. The same plate area doesn't always mean the same effectiveness. In particular, choosing the cheapest exchangers, which are often only slightly cheaper, may not represent much of a saving, and that apparent saving is usually undone anyway by a relatively small difference in price.
If we consider the typical supply temperature used for heating based on air heaters - it usually ranges from 50 to 70 degrees Celsius. Under these conditions, you should select about 0.3-0.35 m² of plates per 10 kW of glycol air heater. If the air heater is larger, you need to scale this accordingly to keep the proportions. Oversizing the exchanger somewhat isn't a problem - however, it's worth avoiding undersizing it, i.e. using too small a unit.
Some people are probably wondering - what's the point of the exchanger then. After all, its job was supposed to be separating the glycol that's in the air-heater circuit. Indeed, many systems with air heaters use water, as long as the investor and designer decide there's no risk of the system freezing - and therefore no need to use anti-freeze fluids. From the perspective of selecting a heat exchanger, that's actually good news - because water absorbs (and releases) heat about 10% more easily than a water-glycol mixture (approximately, since the exact figure depends on the concentration).
This allows for the use of slightly smaller plate exchangers, e.g. 0.25-0.3 m² of plates per 10 kW.
This is a decidedly different matter, but it should be said first that systems with air heaters designed with a supply temperature below 50 degrees Celsius are less common. Air-heater manufacturers generally state that air heaters need higher operating temperatures to achieve relatively high output. Nevertheless - in line with the general trend toward low-temperature systems - heat pumps are also used to heat air-heater circuits. As we know, heat pumps, to operate efficiently, should generally run at lower temperatures, which has its consequences.
In such situations, unfortunately, a larger heat exchanger is needed, and the unit is selected following the rule "the lower the temperature, the larger the exchanger". Even using exchangers in the range of 0.5-0.6 m² of plates per 10 kW makes sense if we want to keep the heat source's operating temperature as low as possible. It's worth considering, however, whether even with correct heat transfer through the exchanger, such an air heater will actually manage to heat the given building. If the building is designed for such low temperatures - there's really nothing standing in the way, but keep in mind that air heaters very often supply buildings that aren't exactly state-of-the-art.
See also:
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Undersizing of a heat exchanger - what does it mean in practice?
Oversizing the heat exchanger - what is it for?
What is the production process of brazed plate heat exchangers?
How to choose a heat exchanger for a gas boiler?
How to properly connect a Nordic Tec plate heat exchanger? Can I reverse the flow directions?
History of plate heat exchangers
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