Condensation and frosting in rotary heat exchangers

Air-to-air heat exchangers used for energy recovery in ventilation units sometimes need to operate in very low outdoor temperatures. During operation the surfaces in the heat exchanger are cooled by the outdoor air and warmed by the extract air. This means that the heat exchanger has a cold side and a warm side, no matter the type of exchanger. However, temperature distribution inside a rotary heat exchanger is complex, to predict condensation and frost is quite difficult. Good thing, William Lawrance, Senior Product Manager , will explain more in this blog post.

When the outdoor air temperature is cold enough, the extract air is cooled below the dew point and moisture is deposited on the heat exchanger surfaces. When this happens at a temperature below freezing, we can expect the moisture to be deposited either in the form of frost or in the form of a thin layer of ice. The frost or ice in the rotor cause a problem because it restricts the flow of air and significantly increases the pressure drop through the heat exchanger, the result is increased power consumption of the fans. Further, since a part of the heat exchanger is blocked, the heat transfer is reduced, the heat recovery performance is impaired.

In fixed heat exchangers, such as plate heat exchangers, frost starts to build up as soon as the outdoor air entering the heat exchanger falls below freezing point, but in rotary heat exchangers the frost build-up starts at substantially lower temperatures. This, thanks to the rotation of the rotor’s matrix between the two air streams which allows the condensate or frost to occur on the extract side to evaporate on the supply side of the rotor. To explain, at an extract air temperature of 21°C and relative humidity <30%, freezing can start at outdoor temperatures below -12C. However, there will only be a continuous build-up of condensate or frost on the matrix when there is an excess in water, where more water condenses or freezes on the extract side than can be taken up on the supply side.

Hygroscopic coating or sorption rotors

In most buildings, the indoor humidity reduces in cold weather conditions and indoor humidities as low as 10% are not uncommon, unless humidification occurs. With such low humidity in the extract air, we would not normally expect to see excess water or frost on a traditional untreated aluminium rotor. However, such low humidity levels are uncomfortable and unhealthy. Rotors with a hygroscopic coating, generally called sorption rotors, are beneficial in those climates because the sorption treatment absorbs moisture from the extract air and transfers it to the supply air, as long as the extract air contains less moisture than the outdoor air. The transfer of moisture occurs before it can condense in the form of water or frost. In other words, the dew point temperature is continuously dropping as the extract side temperature drops. This gives the major advantage mentioned above, frosting will occur at much lower outdoor temperature.

If the above is compared to a plain aluminium rotor, moisture transfer can only occur when the extract air is cooled below the dew point so that moisture condenses or freezes on the surface of the rotor.

Let the Mollier-diagram explain

This is a non-hygroscopic rotor where the extract air is cooled at constant moisture content to a temperature about 4°C above the dew point. A line is drawn between that point and the outdoor air condition. If that line intersects the saturation line then there will be condensation as can be seen in Figure 1. This method has been shown to correlate well with testing in our laboratory. In this diagram the mean condition of the exhaust air is shown for a temperature efficiency of 80%. Because there is condensation occurring at a temperature significantly under freezing point, frost is to be expected in the rotor.

For the hygroscopic rotor with a humidity efficiency similar to the temperature efficiency, excess water will form in the rotor when a line drawn between the entering condition of the extract and supply air flows intersects the saturation line.

This Mollier-diagram technique can be used to create a diagram with the limiting outdoor temperature at any extract air humidity with the other parameters fixed. Testing in our climate chamber has shown that this approach agrees with reality.

Using the above method, a diagram showing the excess water and frosting boundaries can be created. The extract air temperature does not normally vary much so that is fixed at 21°C. The temperature efficiency of the rotor is 80% and the humidity efficiency of the hygroscopic, or sorption, rotor is near 80%. The relative humidity of the outdoor air is 80%.

The diagram shows that excess water can occur in non-hygroscopic rotors from 0°C and below at normal room condition but that can only occur in a sorption rotor if the humidity in the room is very high. Freezing will start at about -8°C if there is excess water. The diagram also shows that the sorption rotor can operate at significantly lower outdoor temperatures and higher indoor humidity without problem of excess water compared with the plain aluminium rotor.

The most significant factors that affect the condensation and freezing are the extract air humidity and the outdoor air temperature, but the extract air temperature and the efficiency of the rotor also play a vital role which is why it is complex to determine the risk of freezing.

Defrosting

Excess water in rotary heat exchangers is normally not a problem in most building applications because the small amount of water created, is usually evaporated when the conditions return above the limit. Also, ice and frost takes several hours to build up in rotary heat exchangers. However, there is a problem in prolonged periods of cold weather and there are then two ways to deal with it. One way is to heat the outdoor air, the other is to control the efficiency of the rotor so that no frost occurs. In regards to the latter, it is possible to measure the pressure drop and defrost the rotor when it gets too high, which is done by reducing the rotational speed so that the efficiency is reduced. The exhaust air temperature is then raised and the mean temperature gets above freezing.

In either of the two alternatives, heat has to be added but the operating cost is considered to be about the same. Worth to notice, with the preheating method it is important to ensure that the outdoor air is not heated too, an unnecessary energy spend. Further, the air temperature after the coil needs to be controlled carefully so that the condition is kept on the condensation boundary.

Temperature efficiency as a function of the rotational speed of the rotor typically follows the form shown in the diagram to the left. What is shown is that defrosting control functions usually reduce the rotor speed when frost is detected. When the control system detects that the frost has cleared, the rotor speed is ramped up again. The temperature efficiency is significantly reduced when running at slow speed but it is important the rotor does not actually stop, because then only half of the rotor would be defrosted.

AHU Design

As discussed above, the occurrence of moisture and frost depends on several factors and it is not an easy task to figure out exactly where the limits are for a given set of conditions. Fortunately, our selection program for GOLD units, AHU Design, is armed with a powerful algorithm based on our research and testing. It calculates the limits automatically and gives warnings where there is a risk of excess moisture and frosting. It also advises when a preheater is needed and gives the optimum power output for that heater.

A preheater can be used with good effect, especially in extreme conditions with low outdoor temperature together with humid extract air, then the rotor can be overwhelmed with excess water. At this point it can be difficult to defrost using only the speed of the rotor. The preheater will raise the mean temperature of the rotor but it will also reduce the relative humidity of the supply air so that it can absorb more moisture. This means that excess water and frost is avoided. This is illustrated in figure 5.

It is shown in the Mollier diagram that the air does not need to be heated that much to avoid excess water and frost. That means that it is possible to use a low temperature heat source such as a borehole with the advantage that it can be used for cooling in the summer. Of course, the liquid needs to be protected from freezing!

Since the heater would only be used for a small number of hours per year, an electric heater might be a more attractive solution because of a lower installation cost. Lets us come back to this at a later stage.

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