- Ensure proper dehumidification
- Cost of conditioning outside air
- Neutral temperature air
- Accurate control
- Effective and verifiable ventilation
Ensure Proper DehumidificationThe first consideration is to ensure proper dehumidification under all load conditions. More than half of the United States is subject to troublesome humidity, that is, a mean dewpoint of 55°F or higher. This is not an isolated problem.
To attain proper space conditions, all the ventilation air must be dehumidified to the indoor design condition as a minimum. A common design is 75° drybulb and 50% rh. This condition is equivalent to 55° dewpoint, which means that all the ventilation air must be chilled to 55°/55° saturated conditions off the cooling coil all the time. This can be accomplished with VAV constant temperature systems but is not achievable in constant volume variable temperature systems. As the supply air temperature rises, the building is humidified and not dehumidified. Further analysis must be made of internal latent loads and infiltration as sources of humidity which may require that lower dewpoint air be introduced.
To determine the dewpoint we can use the example of a school classroom with 30 pupils requiring 450 cfm of ventilation air and use the formula from ASHRAE Handbook - Fundamentals:
ql = 4,840 x Qs x DW, where 4,840 = 60 min/hour x 0.075 lb/ft3 x 1,076 Btuh/lb.
Qs is the cfm, W is humidity ratio in lb of moisture per pound of dry air, and DW is (Wsupply dp - Wdesign dp).
At the indoor design of 75°/50% rh, which corresponds to 55° dewpoint, the humidity ratio is 0.00927 lb/lb.
ql = 30 pupils at 200 Btuh each = 6,000 Btuh latent heat. Substituting in
ql = 4,840 x Qs x DW:
6,000 = 4,840 x 450 cfm x (Wsupply dp - 0.0927)
Solving for DW = 0.00276 lb/lb.
Wsupply dp 0.00651 The humidity ratio 0.00651 is equivalent to 45.7° dewpoint, or 45.7°/45.7° saturated conditions off the coil.
If the classroom envelope has infiltration leakage of 20 cfm, the outside air quantity becomes 450 + 20 = 470 cfm and the additional latent heat is calculated as:
ql = 4,840 x Qs x (Woutside air dp - Wdesign dp)
ql = 4,840 x 20 x (0.01686 - 0.00927)
ql = 735 Btuh
The required dewpoint calculation now becomes:
6,750 = 4,840 x 470 cfm x (Wsupply dp - 0.0927)
Solving for DW = 0.00297 lb/lb.
Wdesign dp 0.00927
Wsupply dp 0.00630
The new humidity ratio of 0.00630 is equivalent to 44.9° dewpoint, or 44.9°/44.9° saturated conditions off the coil.
This means that any time that no less than 44.9° dewpoint air must be introduced to attain design condition.
If outside air is being mixed with recirculated air in a common air handler, all of the air must be chilled to 44.9° saturated in order to attain design condition. In a DOAS, only the outside air must be chilled to this temperature.
Cost of Conditioning Outside AirAnother consideration is the immense cost of conditioning outside air. For typical design parameters of 95°/78° outside air and 75°/50% rh indoor air, it requires 6.8 tons per 1,000 cfm to get the air to the proper humidity level of 55° dewpoint (which equates to 50% rh at 75° drybulb). This requires 55°/55° saturated conditions off the cooling coil.
Some relative examples are as follows: At 11 EER and $0.08/kWh, this amounts to $0.60 per 1,000 cfm/hr. In Washington, using Air Force bin data, this amounts to $887 per 1,000 cfm for dehumidification alone. This cost per 1,000 cfm is $2,930 in Ft. Myers, FL and $614 in New York. Heating cost at 80% gas heating efficiency and $8.00 per MBtuh is $1,812 in Washington; $199 in Ft. Myers; and $1,920 in New York.
This cost can be significantly reduced if exhaust air energy recovery can be used. Enthalpic heat exchangers, both rotary and static plate, are so effective (in the range of 60% to 80%) that their use can reduce the first cost of construction by their cost being more than offset by the reduction of the size of the refrigeration plant required. While sensible energy recovery heat exchangers have a sensible effectiveness in the range of 50% to 75%, their total effectiveness might be only 10% to 15%. Because of this, the payback on sensible heat exchangers is in the range of three to five years.
As mentioned previously, a DOAS conditions only the ventilation air to the low dewpoint temperature required to attain space design humidity. In a separate air handler, the recirculated air is cooled only to the degree necessary to offset the sensible load. While this is commonly as low as 55° drybulb at full load, it may be a considerably higher temperature at light loads and thus not dehumidify.
Neutral Temperature AirA DOAS should not be confused with an air conditioner. There usually is an air conditioner for the space, which is used to offset the sensible-only load and is controlled by a drybulb thermostat.
A DOAS is an outside air conditioner that conditions the outside air only to offset the latent load and is controlled by a separate humidistat or dewpoint control. Thus, it is ideal that the DOAS provide the necessary dry air but at a drybulb temperature close to the design drybulb temperature, or neutral air. This requires reheat, which, if not done correctly, is socially unacceptable, an energy hog, and counter to most codes.
Two acceptable methods of reheat are refrigerant reheat and the use of a precool-reheat heat exchanger. Refrigerant reheat utilizes the heat rejected by the compressor performing the dehumidification duty and, thus, requires no outside energy source. The precool/reheat heat exchanger is a heat exchanger in the DOAS that transfers the sensible heat in the outside air from the entering side of the cooling/dehumidification coil to the leaving side. It can add 15° to 20° to the coil leaving air temperature and thus deliver neutral temperature without an additional energy source.
Accurate ControlThe problem with controlling a conventional system with one air handler performing both the sensible and latent cooling functions is that, while it may perform adequately at full load conditions with approximately a 0.70 sensible heat ratio, it cannot cope with part load conditions when the sensible heat ratio may become as low as 0.30. On a cool, damp morning, the supply air temperature will be modulated above the required dewpoint temperature and the building will be humidified.
Accurate control of the building is attained by controlling the drybulb temperature and the humidity separately - the "divide and conquer" approach. The "sensible" air handler is controlled solely by drybulb control and its supply air temperature is modulated as necessary to maintain design drybulb temperature.
The DOAS is controlled solely by humidity controls and its supply air dewpoint temperature is modulated as necessary to maintain design humidity conditions. With the latent and sensible loads controlled independently, a mixing will occur in the space between the ventilation air and the recirculated air that will always fall on the sensible heat ratio line. The load will be absorbed in the correct proportions, and the space design point will be attained under all sensible ratios.
Separate latent treatment has an additional benefit in that all the condensate removal is at the DOAS and none is at the sensible air handler or fancoil. This eliminates potential maintenance and IAQ problems.
Effective and Verifiable VentilationSimply introducing the correct amount of ventilation air to the building is not enough. It must be distributed properly so that the correct amount is introduced to each space. Since the DOAS usually is producing neutral temperature air, it may be separately ducted to a dedicated diffuser in each space. This allows easy verification of ventilation. This ductwork can be low pressure and uninsulated. This overcomes one of the vexing problems of VAV systems, which require complicated minimum outside air controls, high VAV box minimum position settings, and reheat in order to provide effective ventilation.
LEED®It is pretty difficult to attain any level of LEED performance without DOAS. DOAS can contribute to "Water Efficiency," "Energy and Atmosphere," and "Indoor Environmental Quality" categories in the LEED scoring system.
Winter humidification load can be reduced through the use of enthalpic heat exchangers because they reclaim building water vapor. The energy use reduction gained by energy recovery contributes to "Energy and Atmosphere." Carbon dioxide monitoring, verifiable ventilation, controllability of systems, enhanced thermal comfort, and building purge capability contribute to "Indoor Environmental Quality."
The only problem with using DOAS is that there are so many different types of DOAS that it requires a sophisticated analysis to determine just which type to use. This will be addressed in a forthcoming article. ES