Cooling; General - Honeywell AUTOMATIC CONTROL Engineering Manual

WARM AIR
RETURN AIR
SUPPLY
RETURN
Fig. 8. Convection Heater.
REFLECTOR
INFRARED
SOURCE
RADIANT HEAT
Fig. 9. Infrared Heater.
In mild climates, heat can be provided by a coil in the central
air handling system or by a heat pump. Heat pumps have the
advantage of switching between heating and cooling modes
as required. Rooftop units provide packaged heating and
cooling. Heating in a rooftop unit is usually by a gas- or oil-
fired furnace or an electric heat coil. Steam and hot water coils
are available as well. Perimeter heat is often required in colder
climates, particularly under large windows.
A heat pump uses standard refrigeration components and a
reversing valve to provide both heating and cooling within the
same unit. In the heating mode, the flow of refrigerant through
the coils is reversed to deliver heat from a heat source to the
conditioned space. When a heat pump is used to exchange heat
from the interior of a building to the perimeter, no additional
heat source is needed.
A heat-recovery system is often used in buildings where a
significant quantity of outdoor air is used. Several types of
heat-recovery systems are available including heat pumps,
runaround systems, rotary heat exchangers, and heat pipes.
In a runaround system, coils are installed in the outdoor air
supply duct and the exhaust air duct. A pump circulates the
medium (water or glycol) between the coils so that medium
heated by the exhaust air preheats the outdoor air entering the
system.
A rotary heat exchanger is a large wheel filled with metal
mesh. One half of the wheel is in the outdoor air intake and
the other half, in the exhaust air duct. As the wheel rotates, the
metal mesh absorbs heat from the exhaust air and dissipates it
in the intake air.
A heat pipe is a long, sealed, finned tube charged with a
refrigerant. The tube is tilted slightly with one end in the
outdoor air intake and the other end in the exhaust air. In a
heating application, the refrigerant vaporizes at the lower end
FINNED TUBE
TO OTHER
FLOOR
HEATING UNITS
FROM OTHER
HEATING UNITS
C2705
C2706
in the warm exhaust air, and the vapor rises toward the higher
end in the cool outdoor air, where it gives up the heat of
vaporization and condenses. A wick carries the liquid
refrigerant back to the warm end, where the cycle repeats. A
heat pipe requires no energy input. For cooling, the process is
reversed by tilting the pipe the other way.
Controls may be pneumatic, electric, electronic, digital, or
a combination. Satisfactory control can be achieved using
independent control loops on each system. Maximum operating
efficiency and comfort levels can be achieved with a control
system which adjusts the central system operation to the
demands of the zones. Such a system can save enough in
operating costs to pay for itself in a short time.
Controls for the air handling system and zones are
specifically designed for a building by the architect, engineer,
or team who designs the building. The controls are usually
installed at the job site. Terminal unit controls are typically
factory installed. Boilers, heat pumps, and rooftop units are
usually sold with a factory-installed control package
specifically designed for that unit.

COOLING

GENERAL

Both sensible and latent heat contribute to the cooling load
of a building. Heat gain is sensible when heat is added to the
conditioned space. Heat gain is latent when moisture is added
to the space (e.g., by vapor emitted by occupants and other
sources). To maintain a constant humidity ratio in the space,
water vapor must be removed at a rate equal to its rate of
addition into the space.
Conduction is the process by which heat moves between
adjoining spaces with unequal space temperatures. Heat may
move through exterior walls and the roof, or through floors,
walls, or ceilings. Solar radiation heats surfaces which then
transfer the heat to the surrounding air. Internal heat gain is
generated by occupants, lighting, and equipment. Warm air
entering a building by infiltration and through ventilation also
contributes to heat gain.
Building orientation, interior and exterior shading, the angle
of the sun, and prevailing winds affect the amount of solar
heat gain, which can be a major source of heat. Solar heat
received through windows causes immediate heat gain. Areas
with large windows may experience more solar gain in winter
than in summer. Building surfaces absorb solar energy, become
heated, and transfer the heat to interior air. The amount of
change in temperature through each layer of a composite
surface depends on the resistance to heat flow and thickness
of each material.
Occupants, lighting, equipment, and outdoor air ventilation
and infiltration requirements contribute to internal heat gain.
For example, an adult sitting at a desk produces about 400 Btu
per hour. Incandescent lighting produces more heat than
fluorescent lighting. Copiers, computers, and other office
machines also contribute significantly to internal heat gain.
11 ENGINEERING MANUAL OF AUTOMATIC CONTROL
CONTROL FUNDAMENTALS