Nortec NHPC, NHTC manual 10-10, Page 20

(5)Using an exhaust air fan:

(a)Formula 5

Assume the print shop has a 1,000 CFM fan to exhaust a drying room. The moisture required to humidify the drying room only is

1,000 x 5.08 x 60 min/hr

_________________________ = 44 lbs/hr

7,000 grains/lb (Refer to Figure 1.)

(6)Using an economizer cycle:

(a)Care should be taken in sizing humidification load when an economizer cycle is incorporated into a building HVAC system. The purpose of an economizer cycle is to provide building cooling using outside air, rather then the building refrigeration system when outside air conditions permit.

(b)The economizer cycle senses and compares outdoor air temperature and return air temperature during the cooling season.

(c)When the HVAC system calls for cooling and the outdoor air temperature is low enough – typically 55°F or lower – the outside air and exhaust air dampers are positioned to provide the required supply air temperature to maintain cooling, and the recirculated air damper is positioned to maintain the required supply air volume. When the outdoor air temperature is higher than the supply air temperature required to maintain cooling but is lower than the return air temperature, the make-up air and exhaust air dampers are 100% open. The recirculation air damper closes, and the building refrigeration system provides the portion of cooling load that cannot be provided by outside air intake.

(d)From this it can be seen that it is possible to introduce 100% outside air into a building during the cooling season.

(e)Formula 6

For example, if out print shop were using 100% outdoor air at 55°F and 40% rh, then the moisture required is

5.19 - (4.89 x 40% rh) = 3.23 gr/ft3

Therefore, on a 15,000 CFM system, the humidification load will be

15,000 x 3.23 x 60 min/hr

__________________________= 415 lbs/hr

7,000 grains/lb

(f)In the above examples, the largest humidification load was due to the economizer cycle at 415 lbs/hr.

(7)Using cooling or refrigeration loads:

(a)As air is cooled, it loses it’s ability to hold moisture. If it is cooled enough, some of the moisture will condense out. This is known as cooling load. Cooling load calculations can be important for process applications or refrigeration applications to product dehydration.

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Page 20 2008-10-01