Example 3: Controlled Humidity and Temperature Areas

Many air conditioned manufacturing areas often have a required air flow to handle a sensible load in that space. This air quantity requirement and the accompanying dehumidifier size are usually greater than those needed to handle a latent load. By designing a system for the sensible load situation and then determining the appropriate dehumidifier to handle the moisture load, the desired conditions for the space can be maintained.

Physical Facts

1. Area to be conditioned - 62.5' x 55' x 14'
2. Outside design conditions- 95°F db; 7]°F wb
3. Controlled space requirement - 55°F db; 30 percent rh; 20 gr/lb.
4. Doors - 1 (6' X 8'), 6 openings/hr; 1 (3' x 7'), 40penings/hr
5. Other (fixed) openings - 2.8 sq ft, w/tunnel 10' deep
6. Number of people working in area - 10
7. Air required for sensible temperature control - 24,715 cfm, 42°F
8. Construction - Block walls; drywall ceiling with vapor proofing; concrete floor on grade
9. Make-up air required - 2400 cfm
10. Air available for make-up - 50°F db/49°F wb; 50 gr/lb.

Problem

To determine the size dehumidifier needed in a controlled humidity and temperature area.

Moisture Load to be Computed

1. Permeation
2. Load through doors
3. Load through fixed openings
4. Population load.

= grs/hr

Door Load

Fixed Openings

Population Load

At a db of 55° and working at a "light physical exertion" - 1100 gr/hr/person

10 people x 1100 gr = 11,000 gr/hr

Total Room Moisture Load

450,846 gr/hr Permeation
12,491 gr/hr Door Load
3,643 gr/hr Door Load
1,822 gr/hr Fixed Opening Load
11,000 gr/hr Population Load
479,802 gr/hr Total Room Load

The total room latent moisture load is 479,802 gr/hr, which is added into the calculation below to find the entering grain condition needed for the space.

G = 15.4 gr/lb.

Thus the air to the room must be 15.4 gr/lb. and the air mixture (return from the room plus the dehumidifier discharge) entering the main system fan should be 15 gr/lb. to allow for possible leakage into the system duct work. Here one must resort to trial and error techniques to select the dehumidifier size.

Note that the make-up air of 2400 cfm must mix with 5100 cfm of return air before entering the dehumidifier.

Recommendation

The VFB-75 Dehumidifier will satisfy the room load conditions when mixed with the remaining 17,215 cfm of return air and delivered into the conditioned space.

Bry-Air Dehumidifier Calculation Sheet

Project: Example IV – Controlled Humidity and Temperature Areas

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