The importance of solving this kind of problem allows to offer an installation with an optimal air quality. For this there are ideal strategies and technologies.
by Alfredo Sotolongo*
The most economical energy to produce is the one that is saved!
In recent years we have had in our industry a tendency to improve indoor air quality and this movement has pointed to humidity as one of the main causes affecting air quality in the conditioned area. Many colleagues have contacted me to once again concentrate on solutions to this very harmful and common problem.
In our countries, mainly between the tropics of Cancer and Capricorn, the outside air we use for ventilation contains high enthalpy values. If this air is not properly controlled it causes mold, condensation on surfaces, allergic reactions, etc.
When selecting units dedicated to handling 100% outdoor air, the purpose is to control humidity without using additional energy, only the energy needed to cool and compensate for the thermal load. It is that the air entering the cooling coil must be cooled to the saturation point where both the dry bulb and the wet bulb are as close to 55°F as possible.
This is the ideal point in the psychometric table so that when applying sensitive heat it brings the air conditions to those of neutral air. The air is discharged from the cooling coil with as much water removed as possible, but saturated at 100% relative humidity. When reheating the air, applying pure sensitive heat, in the psychometric table follows the horizontal line until it reaches the point of neutral air 75°F and 50% relative humidity. Under these conditions is that the ventilation air that is delivered to the rest of the air units of the building must be, which prevents them from requiring greater thermal capacity.
In addition, it is about using energy recovery units to exchange the energy of the exhaust air with the ventilation air, with the purpose of reducing the capacity of the units of 100% outdoor air. With this system, the outside ventilation air enters the cooling coil at less critical conditions is achieved.
In our experience, the most effective way to reheat the air out of the cooling coil, without expending additional energy, is by using hot gas reheating. Originally, manufacturers of 100% outdoor air units designed their units using a two-way, two-position solenoid valve that directed the hot gas-shaped refrigerant into the reheating coil installed at the air outlet of the cooling coil instead of directing it to the condenser. In this design, the thermostat controlled the cooling coil while a humidistate controlled the solenoid valve.
As this solenoid valve was not modulating, the humidity problem was partially controlled, but many other problems continued, such as:
a) Poor control of the amount of overheating.
b) Sudden changes in the temperature of the supply air.
c) Temperature and humidity values that are not acceptable especially in units of 100% outdoor air.
Companies like AAON have designed the units with the components and controls that allow them to continue removing moisture from the conditioned area even when the dry bulb has been satisfied, but supplying the exact amount of overheating to compensate for the dehumidification requirements (See diagrams that appear next to this article).
How has AAON managed to achieve the ideal conditions to allow the balance between temperature and humidity? It has achieved this with a one-of-a-kind design, adding to the reheating coil a modulating valve to proportionally control the hot gas and another modulating valve in the condenser and electronic control.
For example, if one valve opens 20%, the other closes 20% and both receive the signal from the control system that uses thermostat that controls the compressor and humidistat that controls the hot gas valves.
The most important advantages of this design are the following:
a) If the temperature control value of the dry bulb is satisfied and the humidistate continues to ask for relative humidity to be reduced, the valves would modulate the maximum amount of hot gas that would pass through the reheating coil at any given time.
b) The control system also consists of a temperature sensor to be installed in the supply air that modulates the temperature of the overheating.
c) When the hot gas passing through the reheating coil is modulated, the system supplies the exact amount of reheating necessary to meet the conditions of the conditioned area.
d) The occupants of the conditioned area will be satisfied with the comfort achieved because there are no sudden changes in the conditions as in the case of two-position solenoid valves.
The main purpose of the unit dedicated to handling 100% of the ventilation outside air is to contribute to improving indoor air quality and at the same time achieve it without spending any additional energy.
Very often we find units that handle 100% outside air without having the necessary thermal capacity to lower the outdoor air conditions to as close as possible to 55°F/55°F saturated air and thus, remove as much water as possible (grains) and also, without any overheating, which creates serious problems of relative humidity in the conditioned area and therefore, unacceptable indoor air quality. Whenever 100% of outdoor air is handled, it is imperative that the thermal capacity of the unit is sufficient to bring the air as close as possible to 55°F/55°F and it is necessary that there is reheating that is applied without additional energy consumption.
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.
