Building Design and Construction Handbook - Merritt - Ventech!

13.2.3 Sensible Heat
HEATING, VENTILATION, AND AIR CONDITIONING 13.9
TABLE 13.1 Specific Heats—Common
Materials
Substance
Specific heat,
Btu/(lb)(�F)
Air at 80�F 0.24
Water vapor 0.49
Water 1.00
Aluminum 0.23
Brick 0.20
Brass 0.09
Bronze 0.10
Gypsum 0.26
Ice 0.48
Limestone 0.22
Marble 0.21
Sand 0.19
Steel 0.12
Wood 0.45–0.65
When heat energy is added to or taken away from a substance, the resulting changes
in temperature can be detected by the sense of touch, or sensibly. Therefore, this
type of heat is called sensible heat. Since sensible heat is associated with a change
in temperature, the quantity of sensible heat energy transferred in a heat exchange
is usually calculated from
Q � Mc (t � t ) (13.3)
p 2 1
where Q � sensible heat, Btu, absorbed or removed
M � mass, lb, of the substance undergoing the temperature change
c p � specific heat of the substance
(t 2 � t 1) � temperature difference of the substance, where t 2 is the final temperature
after the heat exchange and t 1 is the temperature of the material
before the heat exchange
13.2.4 Laws of Thermodynamics
The application of the laws of thermodynamics to HVAC calculations is usually
limited to two well-known laws. These laws can be expressed differently, but in
equivalent ways. A simplification of these laws as follows will permit an easier
understanding.
The first law of thermodynamics states that when work performed produces heat,
the quantity of the heat produced is proportional to the work performed. And conversely,
when heat energy performs work, the quantity of the heat dissipated is
proportional to the work performed. Work, ft-lb, is equal to the product of the force,
lb, acting on the body for a distance, ft, that the body moves in the direction of
the applied force.