Building Design and Construction Handbook - Merritt - Ventech!

13.16 SECTION THIRTEEN
The dew-point temperature can be calculated, when the relative humidity is
known, by use of Eq. (13.13) and Table 13.2. For the temperature of the unsaturated
air, the humidity ratio at saturation is determined from Table 13.2. The product of
the humidity ratio and the relative humidity equals the humidity ratio for the dewpoint
temperature, which also can be determined from Table 13.2. As an example,
to determine the dew-point temperature of air at 90�F and 50% relative humidity,
reference to Table 13.2 indicates a humidity ratio at saturation of 0.0312 at 90�F.
Multiplication by 0.50 yields a humidity ratio of 0.0156. By interpolation in Table
13.2 between humidity ratios at saturation temperatures of 65 and 70�F, the dewpoint
temperature is found to be 69.6�F.
A simpler way to determine the dew-point temperature and many other properties
of air-vapor mixtures is to use a psychrometric chart. This chart graphically
relates dry-bulb, wet-bulb, and dew-point temperatures to relative humidity, humidity
ratio, and specific volume of air. Psychrometric charts are often provided in
books on psychrometrics and HVAC handbooks.
13.2.13 Refrigeration Ton
A ton of refrigeration is a common term used in air conditioning to designate the
cooling rate of air-conditioning equipment. A ton of refrigeration indicates the ability
of an evaporator to remove 200 Btu/min or 12,000 Btu/hr. The concept is a
carry-over from the days of icemaking and was based on the concept that 200 Btu/
min had to be removed from 32�F water to produce 1 ton of ice at 32�F in24hr.
Hence,
lb Btu
200 � 144
day lb
1 ton refrigeration �
hr
24
day
� 288,000 Btu/day (13.15)
� 12,000 Btu/hr
� 200 Btu/min
(‘‘ASHRAE Handbook—Fundamentals,’’ American Society of Heating, Refrigerating
and Air-Conditioning Engineers, 1791 Tully Circle, N. E., Atlanta, GA
30329.)
13.3 MAJOR FACTORS IN HVAC DESIGN
This article presents the necessary concepts for management of heat energy and
aims at development of a better understanding of its effects on human comfort. The
concepts must be well understood if they are to be applied successfully to modification
of the environment in building interiors, computer facilities, and manufacturing
processes.