Conditioning outside air in a humid climate: proper dehumidification and control of outside air is a tough task in stickier climates. Deploying a DOAS is often a good first step toward effective ventilation in these situations. Consider the benefits, the LEED advantages, and how to clear potential hurdles like effective reheat to achieve neutral temperature air

Engineered Systems, Oct, 2005 by John E. Bergman

When a large amount of ventilation air is required, engineers are wise to consider the specialized nature of properly conditioning outside air. This process is greatly simplified if a DOAS is used. There are several reasons to use a DOAS:

* Ensure proper dehumidification

* Cost of conditioning outside air

* Neutral temperature air

* Accurate control

* Effective and verifiable ventilation

* LEED[R]

ENSURE PROPER DEHUMIDIFICATION

The 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[degrees]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[degrees] drybulb and 50% rh. This condition is equivalent to 55[degrees] dewpoint, which means that all the ventilation air must be chilled to 55[degrees]/55[degrees] saturated conditions off the cooling coil all the time. This can 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:

[q.sub.l] = 4,840 x [Q.sub.s] x [DELTA]W, where 4,840 = 60 min/hour x 0.075 lb/[ft.sup.3] x 1,076 Btuh/lb.

[Q.sub.s] is the cfm, W is humidity ratio in lb of moisture per pound of dry air, and [DELTA]W is ([W.sub.supply dp] - [W.sub.design dp]).

At the indoor design of 75[degrees]/50% rh, which corresponds to 55[degrees] dewpoint, the humidity ratio is 0.00927 lb/lb.

[q.sub.1] = 30 pupils at 200 Btuh each = 6,000 Btuh latent heat. Substituting in [q.sub.1] = 4,840 x [Q.sub.s] x [DELTA]W:

6,000 = 4,840 x 450 cfm x ([W.sub.supply dp] - 0.0927) Solving for [DELTA]W = 0.00276 lb/lb.

[W.sub.design dp] 0'00927

[DELTA]W/[W.sub.supply dp] -0.00276/0.00651

[FIGURE 1 OMITTED]

The humidity ratio 0.00651 is equivalent to 45.7[degrees] dewpoint, or 45.7[degrees]/45.7[degrees] 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:

[q.sub.l] = 4,840 x Q x ([W.sub.outside air dp] - [W.sub.design dp])

[q.sub.l] = 4,840 x 20 X (0.01686 - 0.00927)

[q.sub.l] = 735 Btuh

The required dewpoint calculation now becomes:

6,750 = 4,840 x 470 cfm x ([W.sub.supply dp] - 0.0927)

Solving for [DELTA]W = 0.00297 lb/lb.

[W.sub.design dp] 0.00927

[DELTA]W/[W.sub.supply dp] -0.00297/0.00630

The new humidity ratio of 0.00630 is equivalent to 44.9[degrees] dewpoint, or 44.9[degrees]/44.9[degrees] saturated conditions off the coil.

This means that any time that no less than 44.9[degrees] 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[degrees] saturated in order to attain design condition. In a DOAS, only the outside air must be chilled to this temperature.

COST OF CONDITIONING OUTSIDE AIR

Another consideration is the immense cost of conditioning outside air. For typical design parameters of 95[degrees]/78[degrees] outside air and 75[degrees]/50% rh indoor air, it requires 6.8 tons per 1,000 cfm to get the air to the proper humidity level of 55[degrees] dewpoint (which equates to 50% rh at 75[degrees] drybulb). This requires 55[degrees]/55[degrees] 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[degrees] drybulb at full load, it may be a considerably higher temperature at light loads and thus not dehumidify.