MIL-HDBK-1011/2
APPENDIX B (continued)
2.1
Annual Summary--Cooling. Using the annual psychrometric summary
and the overlays, (Figures B-1 to B-3) sum the percentage of time within the
boundary for each strategy and the comfort zone. A 197 fpm (1.0 m/sec)
ventilation rate comprises zones 1 to 3. When summing the percentages, count
four 0.0 percentages as equivalent to one 0.1 percent throughout the method.
This is necessary because percentages less than 0.05 are rounded to 0.0 in the
climate data summaries. The average of such rounded values is assumed to be
0.025.
In general, hot-humid climates require provisions for ventilation
for bodily comfort, and hot-arid climates require either high thermal mass or
evaporative cooling for bodily cooling, with nocturnal ventilation for
structural cooling. Refer to Section 2 for further description of bodily and
structural cooling.
2.2
Monthly Summaries--Cooling. Follow the same procedure using the
monthly psychrometric summaries to observe what periods of the year that the
natural cooling strategy will apply. If more than one cooling strategy is
indicated, then a zoned or seasonally adjustable envelope may be desirable.
Refer to para. 3.2 of the main text for further discussion of zoned and
seasonally adjustable building envelopes.
STEP 3:
DETERMINING NEED FOR MECHANICAL AIR CONDITIONING
3.1
Annual Summary--Air Conditioning. Using the air conditioning
overlay (Figure B-4) and the annual psychrometric chart, sum the percentage
frequency of hours hotter or more humid than the natural cooling strategy(s).
On the overlay, this is the area above the boundary for the strategy (zone 7).
If the total hours above the boundary exceeds 5 percent annually, an air
conditioning system will be needed and the building envelope must be capable
of restricting air infiltration to less than 0.5 air changes per hour. This
eliminates porous wall constructions such as louvered walls and jalousie
windows which cannot be shut tightly.
3.2
Monthly Summaries--Air Conditioning. For seasonal requirements,
repeat the same procedure using the monthly charts to determine which months
will require mechanical air conditioning for more than 10 percent of the time.
If mechanical air conditioning is required for less than 10 percent of the
time during the month, then the natural cooling strategy is viable for that
month and the air conditioning system can be turned off. In zoned buildings,
naturally ventilated spaces will be comfortable for that month.
If mechanical air conditioning or an infiltration-resistant envelope is
required, skip Step 4 and go to Step 5.
STEP 4:
DETERMINING NEED FOR AN INFILTRATION-RESISTANT ENVELOPE
Skip this step if an air conditioning system is to be used. If an air
conditioning system is used, infiltration must be limited to 0.5 air changes
per hour.
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