WattMaster WCC III part 5 - Manual (Page 6)

WCC III Technical Guide 4-4
4. SCUSCR.EXE SCREENS
VAV Overview
Variable Air volume (VAV) is a technique for controlling the
capacity of a heating, ventilating, and/or air-conditioning (HVAC)
system. The simplest VAV system incorporates one supply duct
that, when in cooling mode, distributes approximately 55°F (13°C)
supply air. Because the supply air temperature, in this simplest of
VAV systems, is constant, the airfl ow rate must vary to meet the
rising and falling heat gains or losses within the thermal zone
served.
There are two primary advantages to VAV systems. The fan
capacity control, especially with modern electronic variable
speed drives, reduces the energy consumed by fans which can
be a substantial part of the total cooling energy requirements of
a building. Dehumidifi cation demands are greater with a VAV
system than with a constant volume system which modulates the
discharge air temperature to attain part load cooling capacity.
The air blower’s airfl ow rate must be variable. For a single VAV air
handler that serves multiple thermal zones, the airfl ow rate to each
zone must be varied as well.
A VAV terminal unit
controller, often called a VAV BOX, is
the zone-level airfl ow control device. It is basically a quality,
calibrated air damper with an electronically controlled actuator.
The VAV terminal unit is connected to a local SAT 3d controller
which then connects to a central control system (WCC III - MCD).
Historically, pneumatic control was commonplace, but electronic
direct digital control systems are popular especially for mid-to-large
size applications. Hybrid control, for example, having pneumatic
actuators with digital data collection, is popular as well.
Control of the system’s main fan capacity is critical in VAV
systems. Without proper and rapid airfl ow rate control, the
system’s ductwork, or its sealing, can easily be damaged by over
pressurization. This system airfl ow control is usually done with a
SAT III controller with the feedback from the zones in a global
format.
While invented earlier, TempMaster (WattMaster) Corporation of
Kansas City, Missouri is generally credited with perfecting early
VAV technology. The VAV BOX hardware and software (Actuator,
TUC Controller, Temperature sensor, differential pressure sensor,
and proprietary software) is the heart of the technology.
Heat Pump Overview
The most common type of heat pump is an “air-source” system.
“Split” air-source systems have an outdoor unit which includes
a compressor, outdoor coil, fan, and reversing valve. The unit is
connected with refrigerant-fi lled tubing to an indoor component.
The indoor unit contains a fan, indoor coil, and a supplemental
resistance heating element. “Package” systems combine both
components in a single unit that’s typically placed on the roof.
Variable Air Volume, Heat Pump, & Constant Volume Overview
Depending on whether the heat pump is in a cooling or heating
mode, the refrigerant moving through the system makes the indoor
coils either hot or cold. A blower draws room air in through a fi lter
and pulls it across the indoor coil. An optional electric-resistance
heating element can kick on when needed to supplement the heat.
As the air passes by the coils, it either gathers or gives off heat
depending on whether the coils are hot or cold. Warm or cool air
travels through ductwork and registers into the building’s rooms.
Heat pumps give off less heat at one time than do conventional
gas furnaces. This means that they offer a mellower type of heat,
do not turn off and on with the same frequency as a gas furnace,
and therefore circulate more air throughout the building. They are
controlled by the same type of basic controls used for forced-air
systems. On really cold days, a heat pump must work especially
hard to collect heat. That’s when the supplemental heater switches
on.
Constant Volume Overview
Constant Volume (CV) is a type of heating, ventilating, and air-
conditioning (HVAC) system. In a very basic simple CV system,
the supply air fl ow rate is constant, but the supply air temperature
is varied to meet the thermal loads of the space.
Most CV systems are small and serve a single thermal zone.
However, variations such as CV with reheat, CV multizone, and
CV primary-secondary systems can serve multiple zones and
larger buildings.
In mid to large size buildings, new central CV systems are not very
common. Due to fan energy savings potential, variable air volume
(VAV) systems are more common. However, in small buildings
and residences, CAV systems are often the system of choice due to
simplicity, low cost, and reliability. Such small CAV systems often
have on/off control rather than supply air temperature modulation
to vary their heating or cooling capacities.
There are two types of CV systems that are commonly in use to
modify the supply air temperature—the terminal reheat system and
the mixed air system.
The terminal reheat system cools the air in the air handling unit
down to the lowest possible needed temperature within its zone of
spaces. This supplies a comfortable quality to the space but wastes
energy.
The mixed air system has two air streams, typically one for the
coldest and one for the hottest needed air temperature in the zone.
The two air streams are strategically combined to offset the space’s
load. The mixed air system option is not as profi cient at controlling
the humidity, yet it does do well at controlling the temperature.