Honeywell AUTOMATIC CONTROL, Engineering Manual

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ENGINEERING MANULA OF AUTOMATIC CONTROL
INDIVIDUAL ROOM CONTROL APPLICATIONS
412
UNITARY EQUIPMENT CONTROL
GENERAL
Unitary equipment includes natural convection units, radiant
panels, unit heaters, unit ventilators, fan coil units, and heat pumps.
Unitary equipment does not require a central fan. Depending on
design, unitary equipment may perform one or all of the functions
of HVAC–ventilation, filtration, heating, cooling, humidification,
and distribution. Unitary equipment frequently requires a
distribution system for steam or hot and/or chilled water. For
information on distribution systems, refer to Chiller, Boiler, and
Distribution System Control Applications section.
Control of unitary equipment varies with system design and
may be electric, electronic, pneumatic, or digital. Typically, a
room thermostat or sensor provides a signal to a controlled
device to regulate the unit. The unit may use day/night
temperature setpoints for operation at lower setpoints during
unoccupied heating hours. If the unit has a fan, a time clock or
time program may be used to turn the fan off at night and a
night temperature controller may be used to control the
temperature within night limits.
NATURAL CONVECTION UNITS
Natural convection units use steam or circulating hot water
to route heat through a combination of radiation and natural
draft convection units. They can supply total space heat or
supplemental heat to the perimeter of a building to offset heat
loss. The units are classified as follows:
— Radiator: A steel or cast iron unit through which hot water
or steam circulates. Heating is by radiation and convection.
— Convector: A coil, finned tube, or electric heat element
in an enclosure with openings at the top and bottom for
convection circulation of air.
— Baseboard: A unit installed at the base of a wall. Hot water
or steam circulates through a finned tube or a cast iron
enclosure. Air circulates through the unit by convection.
Typically, a natural convection unit is controlled manually
or by a room thermostat controlling a valve or electric heat
coil. Figure 12 shows the control loops for individual room
control. As space temperature falls below the thermostat
setpoint, the valve opens and hot water or steam circulates
through the unit. Natural convection transfers heat to warm the
space and the thermostat resets the valve position as space
temperature reaches setpoint. Depending on space size, the
thermostat may control one valve or several valves.
Fig. 12. Natural Convection Units.
Control of these units can be modulating or two position. On
long sections of steam-fed baseboard or finned-tube radiation,
only two-position control should be used because modulating
control causes steam to condense near the supply end of long
units at light loads.
When used as supplemental heat for perimeter areas, natural
convection units can be connected to central control systems.
As heating requirements change with outdoor conditions and
solar gain, the units can be reset or shut down by central control.
In some applications, the convection units are slaved off a VAV
box controller in sequence with or in lieu of reheat.
Electric resistance heating elements are also used in natural
convection equipment. The thermostat controls electric current
through the elements for heat convection. A modulating
thermostat and a step controller or electronic modulating control
can also control the heating element.
HOT WATER
OR STEAM
SUPPLY
RETURN
STEAM TRAP
(IF STEAM SUPPLY)
RETURN
STEAM TRAP
(IF STEAM SUPPLY)
THERMOSTAT
HOT WATER
OR STEAM
SUPPLY
THERMOSTAT
RADIATOR
CONVECTOR
RETURN
STEAM TRAP
(IF STEAM SUPPLY)
HOT WATER
OR STEAM
SUPPLY
THERMOSTAT
BASEBOARD
C3018