The Equipment Design Process
351
The thickness selected normally results in a heat gain of 9 to 10
Btu/h/ft 2 and these thicknesses are normally satisfactory for most
applications. It is usually impractical to design for prevention of condensation for all conditions. Insulation vendors can usually provide
an analysis of weather conditions for most parts of the United States,
and they can provide an estimate of the percentage of the time that
condensation can be expected at various insulation thicknesses for
the operating conditions of the piping or equipment.
Antisweat insulation may also be used on nearby piping and
equipment that is not part of a refrigeration system or does not contain fluids that are covered by a refrigeration system. Antisweat
insulation is also used to control surface condensation at locations
where the dripping of condensate may cause an electrical hazard to
equipment or discomfort to personnel.
2. Cold service:
The design criteria for cold service insulation is to provide for the
reduction in heat gain/loss or control excessive moisture condensation. Normal cold insulation is applied on piping and equipment
that operates from ambient temperature to –100ºF.
Insulation thickness for cold service is generally selected on the
basis of a heat gain of 9 to 10 Btu/h/ft 2 .
The service temperature of the insulation is affected by several
factors: orientation, size, shape, and emissivity of the outside surface,
and air movement. A shiny jacketed aluminum surface will require
from two to three times the insulation thickness as will a dull blackcoated surface to achieve the same surface temperature.
Choosing thicknesses from most vendors’ charts for 90ºF dry bulb
and 90% relative humidity will result in heat gains of 4 to 5 Btu/h/
ft 2 . The increased thickness for zero heat gain is an additional 4 in.
for a flat insulation or from 3.5 to 7.5 in. for pipe. The increase in cost
is never justifiable. It must be emphasized that emissivity and wind
have a substantial effect on heat gain and surface temperature, and
these factors must be considered in any thickness calculation.
The control of moisture entering the insulation system is the single
most critical aspect of cold insulation design. The insulating properties of wet insulation, water or ice, are much less than the insulation
properties of dry insulation. The difference in the rate of heat transmission between dry insulation with a thermal conductivity of 0.35
Btu/h/ft 2 and ice with a thermal conductivity of 15.5 Btu/h/ft 2 is
approximately 50 times. Although no insulation system reaches the
conductivity of ice, this illustrates the need to keep cold insulation
systems dry.
Metal jacketing is not normally used over insulation for cold service.
However, in areas where severe abuse is expected, metal jacketing may
351
The thickness selected normally results in a heat gain of 9 to 10
Btu/h/ft 2 and these thicknesses are normally satisfactory for most
applications. It is usually impractical to design for prevention of condensation for all conditions. Insulation vendors can usually provide
an analysis of weather conditions for most parts of the United States,
and they can provide an estimate of the percentage of the time that
condensation can be expected at various insulation thicknesses for
the operating conditions of the piping or equipment.
Antisweat insulation may also be used on nearby piping and
equipment that is not part of a refrigeration system or does not contain fluids that are covered by a refrigeration system. Antisweat
insulation is also used to control surface condensation at locations
where the dripping of condensate may cause an electrical hazard to
equipment or discomfort to personnel.
2. Cold service:
The design criteria for cold service insulation is to provide for the
reduction in heat gain/loss or control excessive moisture condensation. Normal cold insulation is applied on piping and equipment
that operates from ambient temperature to –100ºF.
Insulation thickness for cold service is generally selected on the
basis of a heat gain of 9 to 10 Btu/h/ft 2 .
The service temperature of the insulation is affected by several
factors: orientation, size, shape, and emissivity of the outside surface,
and air movement. A shiny jacketed aluminum surface will require
from two to three times the insulation thickness as will a dull blackcoated surface to achieve the same surface temperature.
Choosing thicknesses from most vendors’ charts for 90ºF dry bulb
and 90% relative humidity will result in heat gains of 4 to 5 Btu/h/
ft 2 . The increased thickness for zero heat gain is an additional 4 in.
for a flat insulation or from 3.5 to 7.5 in. for pipe. The increase in cost
is never justifiable. It must be emphasized that emissivity and wind
have a substantial effect on heat gain and surface temperature, and
these factors must be considered in any thickness calculation.
The control of moisture entering the insulation system is the single
most critical aspect of cold insulation design. The insulating properties of wet insulation, water or ice, are much less than the insulation
properties of dry insulation. The difference in the rate of heat transmission between dry insulation with a thermal conductivity of 0.35
Btu/h/ft 2 and ice with a thermal conductivity of 15.5 Btu/h/ft 2 is
approximately 50 times. Although no insulation system reaches the
conductivity of ice, this illustrates the need to keep cold insulation
systems dry.
Metal jacketing is not normally used over insulation for cold service.
However, in areas where severe abuse is expected, metal jacketing may