Efficiency in variable volume systems

The technical developments and access to computer programs that simulate and analyze the behavior of variable volume systems, allow us to design optimal systems in terms of energy savings.

by Alfredo Sotolongo *

The most economical energy to produce is the one that saves!

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At the beginning of the 2016 I concentrated on writing articles related to the new trends in the design of efficient air conditioning systems, where all the fluids: air, water and refrigerant vary in volume to compensate the thermal load at each moment of the day. It is established that the thermal load present in many structures represents around 60% to 70% most of the time.
Therefore, by varying the volume of the three fluids in coordination with each other, the efficiency of the system is maximized, reflecting savings never before seen in air conditioning systems. This is achieved by using frequency variators where the equation reflects the relationship between horsepower and revolutions per minute, engine RPM, which we call affinity laws:

  • HP1 / HP2 = (RPM1 / RPM2) ᶾ
  • Where:
  • Horses HP1 = original strength.
  • Horses HP2 = resulting force.
  • RPM1 = revolutions per minute originals directly proportional to the original volume.
  • RPM2 = Revolutions per minute reduced directly proportional to the reduced volume.
  • Therefore HP2 = HP1 / (RPM1 / RPM2) ᶾ

Again we clarify that in the case of air and water, centrifugal fans and centrifugal water pumps, there may be limitations in the minimum speed of rotation since there are resistances such as friction in the bearings. However, whenever the rotation and therefore the volume is reduced, the saving in energy is considerable. In the case of the variable refrigerant this limitation does not apply and the reduction in rotation of the centrifugal compressor may be greater. This is because there is no additional friction because the bearings are magnetic.

In the past two years, manufacturers such as Acutherm have developed combinations of controls to maximize the efficiency of the variable volume system. When variable air volume is applied, the purpose is to subdivide the conditioned area into as many areas as possible and this is achieved when each diffuser is a control point with its integrated thermostat.

Acutherm has invested considerable human and financial resources to develop variable air volume systems that operate at a low sound level, minimizing pressure drop and avoiding over cooling and overheating. In addition, these systems contribute to reduce the electrical consumption of the fan motor of the air conditioning unit.

Through computer programs that analyze the design of variable volume systems, such as the so-called EnergyPro, the energy savings are demonstrated when using Acutherm's intelligent diffusers. Below I list the benefits of the variable volume system that highlight these savings:
1) Low pressure systems. All diffusers are terminals that operate at low pressure. Far below the boxes of variable volume.
2) Reduction of cold air below the minimum of the boxes. They allow to lower the air volume to below 10%. This means that cold air can be reduced to the point where overcooling is avoided, provided local standards allow.
3) The program identifies areas of high thermal load concentration. Areas that require greater capacity due to their use and occupation must be zoned separately.
4) Develop a realistic model. The averages-based energy analysis simplifies the calculation but does not accurately estimate the diversity that exists in real buildings.
5) Individual temperature control for each diffuser. Each Acutherm smart diffuser is an independent control zone with its integrated thermostat and gate. This prevents that in very small areas, when they are unoccupied, it is present on cooling or on heating.
6) It is avoided to use reheating in the diffusers. First because many municipal governments forbid it and secondly because it is a waste of unnecessary energy.

The EnergyPro program, developed by an independent company, has integrated models from several manufacturers including Acutherm. When entering the data to do the analysis, in the case of a system using Acutherm, it is only necessary to select the model of the diffuser. It is an easy to use program in which the result of the energy analysis is based on the information of the US Department of Energy DOE 2.1 and is based on the ASHRAE 90.1 standard. It also includes tools to calculate the mechanical equipment and the life cycle cost.

Technical developments and access to computer programs that simulate and analyze the behavior of variable volume systems, even when they are only an idea, allow us to design optimal systems in terms of energy savings.

If you need more information on any of the topics covered in this column, please contact me at: asotolongo@protecinc.com

* President of Protec, Inc., is certified as a professional engineer in Puerto Rico and in the state of Florida; He has over 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 the association.

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