Alfredo Sotolongo, in his usual opinion column, brings to this edition of the magazine ACR LATINOAMÉRICA a reflection on the decrease in energy consumption in an air conditioning system.by: Alfredo Sotolongo
I have just returned from vacation on a Western Caribbean cruise touching ports in Grand Cayman, Cartagena, Panama, Costa Rica, Honduras and Mexico in all those ports I confirmed that our Caribbean is wonderful and that the combination of high temperature and humidity are determining factors to control to maintain not only the desired comfort, but also minimize energy consumption when the air conditioning system to use is of direct expansion. Which has motivated me to share my experience with you in this Opinion column.
Temperature and humidity control is also important for maintaining indoor air quality. Two-position control systems (on and off) have been used for many years, however, the marked fluctuations in these variables create situations where when the temperature is satisfied, the humidity creates discomfort and when the humidity could be satisfied, the conditioned area could be overcooled. This not only creates discomfort but also a waste of energy.
Even when the hot gas bypass is used and the system is operating at partial loads, the two-position compressor shuts down due to low suction pressure and also because the system does not generate enough hot gas to operate continuously at such partial loads.
The technology of the control of direct expansion systems has evolved in recent years and modulating compressors are currently available to meet the needs of temperature and humidity control in the most efficient way possible since they proportionally adjust to the load of the conditioned area.
The modulating compressor, to satisfy both the load by sensitive and latent heat, will not cycle between on and off as do the compressors that their control is of two positions, which cycle constantly to satisfy the load. The modulating compressor will stay on longer, thus maintaining the best possible control of temperature and humidity and minimizing the electrical consumption that is created each time it starts.
Air conditioning systems operate most of the time at part loads and really the problem is created under that condition, especially when there is a great absence of sensitive heat. When comparing a system with a modulating compressor and a two-position compressor using hot gas, the modulant is approximately 35% more efficient.
AAON is the manufacturer that has spent the most time researching and developing direct expansion units using modulating compressors, especially in 100% outdoor air applications. The operation of the modulating compressor is very simple, the motor operates continuously, however, the pumping of refrigerant from the compressor consists of two stages. In the stage called "loaded", the compressor operates as a normal compressor type "Scroll" and supplies 100% of the refrigerant mass, the stage called "discharged" supplies 0% refrigerant.
The time of each stage is controlled through a solenoid valve that allows the supply of refrigerant. This valve allows the "scrolls" inside the compressor to separate and cannot compress the refrigerant, when the solenoid is reactivated the "scrolls" are joined again and discharge the compressed refrigerant. The time the solenoid is at each stage within a cycle is what determines the percentage of capacity.
For example, in cycles of 20 seconds, if the compressor is charged for 4 seconds and unloaded for 16 seconds, then the capacity is 20%. However, if it is charged for 10 seconds and unloaded for 10 seconds, then the capacity is 50%.
A simple and practical example of the application of these units has been a movie theater in Panama where the humidity is extremely high and in the first theaters they built they were presented with problems of humidity control and / or overcooling. They originally used 15-ton direct expansion units with two compressors of 7.5 tons each. Especially when the rooms were with few people, less load than the 7.5 tons of one of the compressors, when the temperature was satisfied and the compressor was turned off, the humidity increased considerably creating an unpleasant environment.
In the newly opened rooms, weatherproof roof units with 2" insulation and two compressors of 7.5 tons each were installed. One of the compressors is modulating to meet partial loads and the other two-position compressor. As the load increases, the modulating compressor increases its capacity, once it reaches 100% of its capacity, the second compressor goes into operation and the modulant is completely discharged. Once the capacity continues to increase, the modulating compressor begins to increase its capacity proportionally to the growth of the load.
These units with modulating compressors, not only satisfy the need to maintain comfort at any partial load, but also their efficiency is higher when compared to the original two-position units (on and off) non-modulating.
These direct expansion units with modulating compressors offer a practical and economical solution since they obtain the desired comfort and the energy savings justify the investment in a very short time.
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