Building Design and Construction Handbook - Merritt - Ventech!

13.38 SECTION THIRTEEN
Sensible and Latent Heat. The part of the cooling load that shows up in the form
of a dry-bulb temperature rise is called sensible heat. It includes heat transmitted
through walls, windows, roof, floor, etc.; radiation from the sun; and heat from
lights, people, electrical and gas appliances, and outside air brought into the airconditioned
space.
Cooling required to remove unwanted moisture from the air-conditioned space
is called latent load, and the heat extracted is called latent heat. Usually, the moisture
is condensed out on the cooling coils in the cooling unit.
For every pound of moisture condensed from the air, the air-conditioning equipment
must remove about 1050 Btu. Instead of rating items that give off moisture
in pounds or grains per hour, common practice rates them in Btu per hour of latent
load. These items include gas appliances, which give off moisture in products of
combustion; steam baths, food, beverages, etc., which evaporate moisture; people;
and humid outside air brought into the air-conditioned space.
Design Temperatures for Cooling. Before we can calculate the cooling load, we
must first determine a design outside condition and the conditions we want to
maintain inside.
For comfort cooling, indoor air at 80�F dry-bulb and 50% relative humidity is
usually acceptable.
Table 13.4, p. 13.26, gives recommended design outdoor summer temperatures
for various cities. Note that these temperatures are not the highest ever attained;
for example, in New York City, the highest dry-bulb temperature recorded exceeds
105�F, whereas the design outdoor dry-bulb temperature is 95�F. Similarly, the wetbulb
temperature is sometimes above the 75�F design wet-bulb for that area.
Heat Gain through Enclosures. To obtain the heat gain through walls, windows,
ceilings, floors, etc., when it is warmer outside than in, the heat-transfer coefficient
is multiplied by the surface area and the temperature gradient.
Radiation from the sun through glass is another source of heat. It can amount
to about 200 Btu/(hr)(ft 2 ) for a single sheet of unshaded common window glass
facing east and west, about three-fourths as much for windows facing northeast and
northwest, and one-half as much for windows facing south. For most practical
applications, however, the sun effect on walls can be neglected, since the time lag
is considerable and the peak load is no longer present by the time the radiant heat
starts to work through to the inside surface. Also, if the wall exposed to the sun
contains windows, the peak radiation through the glass also will be gone by the
time the radiant heat on the walls gets through.
Radiation from the sun through roofs may be considerable. For most roofs, total
equivalent temperature differences for calculating solar heat gain through roofs is
about 50�F.
Roof Sprays. Many buildings have been equipped with roof sprays to reduce the
sun load on the roof. Usually the life of a roof is increased by the spray system,
because it prevents swelling, blistering, and vaporization of the volatile components
of the roofing material. It also prevents the thermal shock of thunderstorms during
hot spells. Equivalent temperature differential for computing heat gain on sprayed
roofs is about 18�F.
Water pools 2 to 6 in deep on roofs have been used, but they create structural
difficulties. Furthermore, holdover heat into the late evening after the sun has set
creates a breeding ground for mosquitoes and requires algae-growth control. Equiv-