Below we will highlight some of the benefits of the units that act as a single zone of variable air volume.
by Alfredo Sotolongo*
The most economical energy to produce is the one that is saved!
In this article I want to emphasize the importance of energy saving accompanied by the comfort of users.
This is the application in department stores or retail stores, where changes in thermal load can change considerably several times a day. Comfort is vital for customers who in many cases are susceptible to temperature and humidity extremes. Comfort through the control of the combination of temperature and humidity, low sound levels and savings in energy consumption are the parameters to consider in this type of application.
This is achieved using air conditioning units that operate as a zone of variable air volume, accompanied by a modulating compressor and also modulating hot gas reheating.
Until now, most common units have been used, without taking into account the effect of humidity, varying the temperature and keeping the air flow constant. Which operate by turning the compressors on and off to meet the temperature and keeping the supply fan always operating at maximum speed.
Depending on the capacity and quantity of the compressors and the variations in thermal load, it can occur over cooling or total lack of humidity control. In both cases, comfort is affected and the energy consumption that is pursued in this type of application increases.
What are air conditioning units that operate as a variable air volume zone about? These units, by changing the thermal load, vary the amount of air while keeping the discharge temperature constant. By adapting directly proportional to the thermal load at all times and keeping the temperature and humidity constant, the desired comfort is satisfied and at the same time only the minimum energy required to maintain the design conditions is consumed.
How is it possible to reduce energy consumption achieving the desired comfort? The first thing that happens is that the variable frequency drive, which governs the centrifugal fan motor, constantly modulates the fan revolutions, so that if the air flow drops by 50%, the energy consumption of the motor drops by 88%. For example: a 20HP motor that has its revs reduced to 50%, the power consumption is reduced to 2.4HP.
Second, the compressor modulates its capacity and only compresses the percentage of refrigerant needed to exactly satisfy the existing condition. Third, it modulates the hot gas and deflects it proportionally from the condenser to the reheating coil to maintain moisture control. In this way, overheating is obtained without the need to consume additional energy.
As in almost all air conditioning designs, the thermal load is maintained most of the time between 25% and 75% of the total. At partial load, in this type of application the variations are much more marked. Therefore, the units that act as a single zone of variable air volume have many benefits for the benefit of the user:
1) They maintain the desired comfort.
2) As it will definitely operate at partial loads most of the time, the fan motor will exponentially reduce its electricity consumption to the cube.
3)With the ability to modulate both the supply fan and the compressor, it allows precise temperature control and additionally passive dehumidification.
4) The modular capacity of the compressor considerably reduces the start and stop cycles which reduces wear, extending the useful life of the compressor.
5)When operating most of the time at lower revs, the sound level is further reduced.
6) The modulating controls supplied internally in the unit, programmed to optimize its operation, ensure that commissioning is simplified and maintenance is facilitated.
7)Because all components are located within the same cabinet, they are easy to install by the contractor.
In addition to shops, where the thermal load changes several times a day, there are many other applications such as classrooms and conference rooms, auditoriums, libraries, hospitals, supermarkets, churches, restaurants, etc.
The important thing is that the behavior of the air conditioning system is analyzed to determine in which areas there are frequent variations in the thermal load and apply the unit of a single zone of variable volume in the correct way to achieve comfort and optimal energy savings.
In the next Opinion article I will explain in detail how this type of unit complies with the standards set by ASHRAE and all the benefits they represent for the user.
If you need more information on any of the topics covered in this column, please contact me at the email [email protected]
* President of Protec, Inc., is certified as a professional engineer in Puerto Rico and the state of Florida; has more than 40 years of experience in the application and sale of systems and equipment for energy conservation. He is a member of ASME (American Society of Mechanical Engineers), AEE (Association of Energy Engineers), ASHRAE and was president of the Miami chapter of that association.
