The correct management of outdoor air in air conditioning and ventilation units offers much more appropriate conditions for users of a certain space such as a commercial building, hospitals, hotels, among others.
by Alfredo Sotolongo*
The most economical energy to produce is the one that is saved!
In recent months I have had the opportunity to select multiple units to handle 100% outside air. This is an extremely important issue when it comes to energy saving, especially in places where the outside air has high humidity and temperature.
As you know, the most important instrument to understand the behavior of air is the Psychrometric Table that presents each air condition such as Enthalpy, Dry Bulb, Wet Bulb, Relative Humidity, etc., these four being the most important in terms of outdoor ventilation air.
- Enthalpy: it is the calorific content of the air measured in BTU per pound of dry air and that is very practical, because with the amount of air in cubic feet per minute (CFM) and the Differential of Enthalpia, the total heat can be calculated. The equation is as follows:
Qt = (4.5)(CFM)(Enthalpy change). Where the constant 4.5 comes out of dividing 60 minutes in an hour by the specific volume of air, in this case 13.4 cubic feet per pound. The value of the specific volume is subject to pressure and temperature.
- Dry Bulb: is the air temperature without change of state, measured with a simple thermometer with the dry bulb.
- Wet Bulb: is the temperature of the air when evaporating, measured with water in the bulb of the thermometer.
- Relative Humidity: is the ratio that exists, in percentage, when dividing the density of water vapor in the air expressed in pounds per cubic foot by the density of saturated water vapor at a particular dry bulb temperature, also expressed in pounds per cubic foot.
Among the reasons why units designed to handle 100% of the outside air are needed are the following:
- In order not to recharge the units inside the building with the thermal load that the outside air has.
- Minimize moisture within the conditioned areas which can create mold situations in the environment resulting in damage to the decoration and what is worse, affecting the health of the people who occupy those conditioned areas.
These units can also be selected with air-to-air energy exchangers, which take advantage of the low enthalpy of the previously conditioned air that is extracted from the building to reduce the enthalpy to the outside air that is introduced for ventilation before entering the coil of the unit, decreasing the capacity of the same.
Many responsible hotel chains and of course hospitals, take this situation very seriously and try to avoid the deterioration of indoor air quality. To ensure that this is achieved, they resort to the use of units that handle 100% outside air and that these units come prepared with all the necessary components to achieve humidity control and optimal energy savings.
In areas near the sea, where the air also contains abrasives such as sand and saltpeter, it is advisable to protect the cooling and condensation coils with special anticorrosive paints, to achieve a service life as long as possible.
In our countries, especially among the Tropics of Cancer and Capricorn, not only in areas near the sea, the enthalpy of the outside air is extremely high and can fluctuate between 43 and 48 BTU per pound of dry air. While the condition of the ventilation outside air that is recommended for injection, after being processed by such units, should be as close as possible to what is called Neutral Air, with a dry bulb of 75F and relative humidity of 50% representing an Enthalpy of approximately 28.4 BTU per pound of dry air.
For example: the unit must be able to remove the following tons of total heat per 1,000 CFM from the outside air and also reheat the air to inject it as Neutral Air:
Qt = (4.5) (1000) (48 – 28.4) = 88,200 BTU-HR = 7.35 tons per 1,000 CFM
There are several applications of 100% outdoor air units to achieve Neutral Air conditions, the most recommended being direct expansion, using modulating hot gas reheating.
In the case of cold water coils being used, heat pipes are recommended, which contributes to lowering the thermal capacity of the coil of the unit and reheats the air to the conditions of Neutral Air. In both cases, using modulating hot gas reheating or using heat pipes, reheating is achieved without adding any additional energy.
All established standards recommend following the ASHRAE tables to consider the value of the Enthalpy of the outside air in each country. In my opinion, we should select units that are truly designed taking into consideration the following:
- Cooling and reheating coils with the necessary capacity to bring air to Air Neutral conditions minimizing energy consumption.
- The controls supplied with the unit to make temperature and humidity control as efficient as possible.
- If located outside, the unit cabinet must be constructed with the thermal resistance of at least R-13 and external protection against abrasives such as saltpeter and sand.
- Corrosion protection for all coils in contact with outside air.
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.
