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When selecting cooling coils for air handling units, it is essential to use relevant input data to ensure that the cooling power is appropriately sized. Oversized cooling coils lead to oversized chillers, which can significantly increase the investment cost. This blog post written by William Lawrance, Senior Product Manager Special Projects, explains this issue in detail with the help of a practical example and by using Mollier diagrams for temperature and humidity calculations.
Energy is removed from the air when it is cooled and the amount of energy in the air depends on both temperature and humidity. So, when selecting a cooling coil it is important to ensure that the correct temperature and humidity of the air as it enters the cooling coil is used in the calculation. The relationship between temperature and humidity is called enthalpy, and the change in enthalpy represents the cooling energy.
The outdoor air temperature and humidity fluctuate depending on the weather conditions, but they do so independently of one another. A typical scenario is the morning, when there is a change in temperature and humidity – the temperature rises and humidity levels decrease.
The graphs below illustrate this relationship, showing temperature, humidity and enthalpy on the warmest day of the test period.
During midday, it is apparent that the highest temperature roughly coincides with the lowest relative humidity. This is logical because warmer air can hold more moisture than colder air. As a result it is obvious that the maximum enthalpy occurs approximately at the maximum temperature. Knowing this, it is evident that temperature and humidity measures should be selected from the same point in time.
When designing an AHU with a cooling coil, the selected temperature and humidity levels are normally slightly lower than the maximum levels. There are different methodologies for making this selection but what is essential is that the temperature and humidity are coincidental.
The unfavourable consequence of selecting the maximum temperature and humidity independently is explained in the following example:
First of all, cooling coils in comfort applications are generally sized for a supply air temperature of 17 degrees Celsius (AHU Design statistical data).
Calculations are made for Norrköping, Sweden, using the actual outdoor air conditions given in the table below. Valencia and Madrid, Spain, as well as London, UK, are used to demonstrate the concept.
When the Norrköping, Sweden, case is plotted in the Mollier diagram, the results show that the calculated cooling power based on maximum humidity rather than the correct values of humidity, results in a cooling power that is more than three times higher.
The findings confirm that it is important to use the correct humidity data when selecting cooling coils.
Energy recovery
The above example is calculated without considerations made to cooling energy recovery. When taking that parameter into account, the cooling coil power can be substantially reduced when a sorption rotary exchanger is used to recover cooling energy.
To exemplify, using the Norrköping case again, assuming a supply air temperature of 17°C and an extract air temperature of 23°C with 55% relative humidity. The rotary heat exchanger has a temperature efficiency of 81.1% and a humidity efficiency of 80.9%.
A sorption rotary heat exchanger removes moisture from the outdoor air and transfers it to the exhaust air. It also reduces the temperature by 2-3 degrees Celsius, which replaces about one-third of the needed cooling power. In moderate cooling scenarios like this, the sorption rotor provides the additional benefit of a significantly drier supply air. To reach the same moisture content in the supply air using only a cooling coil would require cooling the air to 13°C and that in turn would result in a cooling power demand some 70% higher than that of the setup with a sorption rotor. So, the sorption rotary exchanger reduces the cooling power and allow for a smaller chiller, hence a lower refrigerant charge. In addition, the chiller energy consumption is significantly reduced.
For more information on this topic, see our blog post about sorption technology, learn more about dry cooling and get familiar with AHU Design – our product selection software that makes the selection of cooling coils both easy and correct.
