iples of Heating Ventilating and Air Conditioning 7th ed. - 10290 0.016 DATA BAS
ID: 2073741 • Letter: I
Question
iples of Heating Ventilating and Air Conditioning 7th ed. - 10290 0.016 DATA BASED ON ISO STANDARD 7730 AND ASHRAE 20 0.014 STANDARD 55 UPPER RECOMMENDED HUMIDITY LIMIT, 0.012 HUMIDITY RATIO 0.012 15 0.010 1.0 Clo 0.5 Clo 0.008 90 10 2 0.0061% 60 0.004 50 40 NO RECOMMENDED 20 -LOWER HUMIDITY LIMI -10 10% RH PMV LIMITS 10 1316 18 21 24 27 29 32 35 38 OPERATIVE TEMPERATURE, Fig. 4-2 SI ASHRAE Summer and Winter Comfort Zones (Acceptable ranges of operative temperature and humidity for people in typical summer and winter clothing during primarily sedentary activity,) (Figure 5, Chapter 9, 2013 ASHRAE Handbook-Fundamentals)Explanation / Answer
The design space temperature and humidity for both heating and cooling seasons should be based on Standard 55 for most applications.The standard establishes a comfort zone (Fig.4-2) for people in winter and summer clothing engaged in primarily sedentary activities (1.2 met). If a designer selects values from the center of the comfort envelopes, temperatures of 77°F [25.0°C] for summer and 72°F [22.2°C] for winter are the likely choices, with RH
values of 45% and 50%, respectively. With respect to energy efficiency, the use of design values that are within, but on the fringes of, the ASHRAE comfort zone makes sense. At the fringes, careful attention must be paid to the effects of the other comfort variables lest discomfort result from draft or MRT.As the comfort chart (Figure 3-1) indicates, RH does not have a significant bearing on thermal comfort in most situations as long as the space dry-bulb temperature is within the comfort range. RH, though, does affect odor perceptibility and respiratory health.Because of these considerations, 40% to 50% RH is a preferreddesign range. Maintaining humidity within this range during winter,however, is complicated by (1) energy use considerations, (2) the risk of condensation on windows and window frames during cold weather, (3) the risk of condensation within the exterior building envelope, and (4) the need to provide and maintain humidifying equipment within the air-conditioning system. The economic value of winter humidity control for occupant well-being is not always appreciated by designers. Significantly reduced absenteeism among children, office workers, and army recruits as a result of winter humidification has been reported (Green 1979, 1982). Where winter humidification is provided for comfort, a minimum RH of 30%
is generally acceptable. This value may need to be reduced during extremely cold outdoor conditions—below 0°F [–17.8°C].If a higher humidity is acceptable under summer conditions,considerable energy savings can be realized, as shown in Figure 4-2. To determine an approximate value of the energy used for dehumidification at a constant 78°F [25.6°C] dry-bulb temperature,enter the annual wet-bulb degree-hours above 66°F [18.9°C] at the bottom left, intersect this value with a chosen indoor RH, then draw a vertical line to the weekly hours of cooling system operation and read the energy used on the upper-left scale. Repeating this procedure for a different value of RH yields the energy savings obtainable by raising RH. Be cautious, however, about choosing excessively high humidities. Computer rooms (particularly their printers), photocopy rooms, and drafting rooms/studios are examples
of spaces where RH in excess of 50%–55% is undesirable or unacceptable. To prevent the growth of mold and mildew, RH should be maintained below 60%.
Related Questions
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.