to the steam at 212°F, the steam is superheated to
TOTAL HEAT is the sum of sensible heat and
latent heat. Since measurements of the total heat in a
certain weight of a substance cannot be started at
absolute zero, a temperature is adopted at which it is
assumed that there is no heat; and tables of data are
constructed on that basis for practical use. Data tables
giving the heat content of the most commonly used
refrigerants start at 40°F below zero as the assumed
point of no heat; tables for water and steam start at 32°F
above zero. Tables of data usually contain a notation
showing the starting point for heat content
DAY-TON OF REFRIGERATION
A day-ton of refrigeration (sometimes incorrectly
called a ton of refrigeration) is the amount of
refrigeration produced by melting 1 ton of ice at a
temperature of 32°F in 24 hours. A day-ton is often
used to express the amount of cooling produced by a
refrigerator or air-conditioner. For example, a 1-ton
air-conditioner can remove as much heat in 24 hours as
1 ton of 32°F ice that melts and becomes water at 32°F.
Figure 6-2.Relationship between temperature and the
amount of heat required per pound (for water at atmospheric
It is a rate of removing heat, rather than a quantity of
heat. A rate can be converted to Btu per day, hour, or
minute. To find the rate, proceed as follows:
Per Day: Multiply 2,000 (number of pounds of
ice in 1 ton) by 144 (latent heat of fusion per
pound) = 288,000 Btu per day
Per Hour: 288,000 (Btu per day) ÷ 24 (hours in a
day) = 12,000
So, a "1-ton" air-conditioner would have a rating
of 12,000 Btu per hour.
PRESSURE is defined as a force per unit area. It is
usually measured in pounds per square inch (psi).
Pressure may be in one direction, several directions, or
in all directions, as shown in figure 6-3. The ice (solid)
exerts pressure downward. The water (fluid) exerts
pressure on all wetted surfaces of the container. Gases
exert pressure on al I inside surfaces of their containers.
Pressure is usually measured on gauges that have
one of two different scales. One scale is read as so
many pounds per square inch gauge (psig) and
indicates the pressure above atmospheric pressure
surrounding the gauge. The other type of scale is read
as so many pounds per square inch absolute (psia) and
indicates the pressure above absolute zero pressure (a
Atmospheric pressure is the pressure of the weight
of air above a point on, above, or under the earth. At
sea level, ATMOSPHERIC PRESSURE is 14.7 psia,
as shown in figure 6-4. As one ascends, the
atmospheric pressure decreases about 1.0 psi for every
2,343 feet. Below sea level in excavations and
depressions, atmospheric pressure increases.
Pressures underwater differ from those under air only
because the weight of the water must be added to the
pressure of the air.
Figure 6-3.Exertion of pressures.