Fundamentals of HVAC Controls .
The application of Heating, Ventilating, and Air-Conditioning (HVAC) controls starts with an
understanding of the building and the use of the spaces to be conditioned and controlled. All control
systems operate in accordance with few basic principles but before we discuss these, let’s address
few fundamentals of the HVAC system first.
Why Automatic Controls.
The capacity of the HVAC system is typically designed for the extreme conditions. Most operation is
part load/off design as variables such as solar loads, occupancy, ambient temperatures, equipment &
lighting loads etc keep on changing through out the day. Deviation from design shall result in drastic
swings or imbalance since design capacity is greater than the actual load in most operating scenarios.
Without control system, the system will become unstable and HVAC would overheat or over cool spaces.
HVAC systems .
HVAC systems are classified as either self-contained unit packages or as central systems. Unit
package describes a single unit that converts a primary energy source (electricity or gas) and
provides final heating and cooling to the space to be conditioned. Examples of self-contained unit
packages are rooftop HVAC systems, air conditioning units for rooms, and air-to-air heat pumps.
With central systems, the primary conversion from fuel such as gas or electricity takes place in a
central location, with some form of thermal energy distributed throughout the building or facility.
Central systems are a combination of central supply subsystem and multiple end use subsystems.
There are many variations of combined central supply and end use zone systems. The most
frequently used combination is central hot and chilled water distributed to multiple fan systems. The
fan systems use water-to-air heat exchangers called coils to provide hot and/or cold air for the
controlled spaces. End-use subsystems can be fan systems or terminal units. If the end use
subsystems are fan systems, they can be single or multiple zone type. The multiple end use zone
systems are mixing boxes, usually called VAV boxes.
Another combination central supply and end use zone system is a central chiller and boiler for the
conversion of primary energy, as well as a central fan system to delivery hot and/or cold air. The
typical uses of central systems are in larger, multi storied buildings where access to outside air is more
restricted. Typically central systems have lower operating costs but have a complex control sequence.
How does central air-conditioning system work.
Cooling Cycle (chilled water system): The supply air, which is approximately 20° F cooler than the
air in the conditioned space, leaves the cooling coil through the supply air fan, down to the ductwork
and into the conditioned space. The cool supply air picks up heat in the conditioned space and the
warmer air makes its way into the return air duct back to the air handling unit. The return air mixes
with outside air in a mixing chamber and goes through the filters and cooling coil. The mixed air gives
up its heat into the chilled water tubes in the cooling coil, which has fins attached to the tubes to
facilitate heat transfer. The cooled supply air leaves the cooling coil and the air cycle repeats.
The chilled water circulating through the cooling coil tubes, after picking up heat from the mixed air,
leaves the cooling coil and goes through the chilled water return (CHWR) pipe to the chiller's
evaporator. Here it gives up the heat into the refrigeration system. The newly "chilled" water leaves
the evaporator and is pumped through the chilled water supply (CHWS) piping into the cooling coil
continuously and the water cycle repeats.
The evaporator is a heat exchanger that allows heat from the CHWR to flow by conduction into the
refrigerant tubes. The liquid refrigerant in the tubes "boils off" to a vapor removing heat from the water
and conveying the heat to the compressor and then to the condenser. The heat from the condenser is
conveyed to the cooling tower by the condenser water. Finally, outside air is drawn across the cooling
tower, removing the heat from the water through the process of evaporation.
The figure below provides a conceptual view of chilled water air-conditioning system with water cooled
condenser.
The main equipment used in the chilled water system is a chiller package that includes
1) A refrigeration compressor (reciprocating, scroll, screw or centrifugal type),
2) Shell and tube heat exchanger (evaporator) for chilled water production
3) Shell and tube heat exchanger (condenser) for heat rejection in water cooled configuration
(alternatively, air cooled condenser can be used, where water is scarce or its use is prohibited)
4) A cooling tower to reject the heat of condenser water
5) An expansion valve between condenser and the evaporator
The chilled water system is also called central air conditioning system. This is because the chilled
water system can be networked to have multiple cooling coils distributed through out a large or
distributed buildings with the refrigeration equipment (chiller) placed at one base central location.
What Parameters are controlled.
A proper environment is described with four variables: temperature, humidity, pressure and
ventilation.
Temperature.
While no single environment can be judged satisfactory by everybody, it varies between people,
regions and countries. Uniformity of temperature is important to comfort. The temperatures should not
vary within single zone or change suddenly or drastically.
Humidity
Humidity is the presence of water vapor in air and it affects human comfort. ASHRAE 55-1992
recommends the relative humidity (RH) to be maintained between 25 and 60%. Usually air is
humidified to between 25 -45% during winter and dehumidified to below 60% during summer. Any
figure outside this range shall produce discomfort and indoor air quality (IAQ) problems.
Ventilation .
ASHRAE Standard 62-1999: “Ventilation for Acceptable Indoor Air Quality” recommends minimum
ventilation rates per person in the occupied spaces. In many situations, local building codes stipulate
the amount of ventilation required for commercial buildings and work environments. The
recommended value of outside air is typically 20 CFM for each occupant.
The ventilation rates specified by ASHRAE effectively dilutes the carbon dioxide and other
contaminants created by respiration and other activities; it supplies adequate oxygen to the
occupants; and it removes contaminants from the space. The ventilation rates greater than
recommended by ASHRAE criteria are sometime required controlling odors and where cooling is not
provided to offset heat gains.
Pressure .
Air moves from areas of higher pressure to areas of lower pressure through any available openings. A
small crack or hole can admit significant amounts of air, if the pressure differentials are high enough
(which may be very difficult to assess). The rooms and buildings typically have a slightly positive
pressure to reduce outside air infiltration. This helps in keeping the building clean. Typically the stable
positive pressure of .01-.05” is recommended. Pressure is an issue that comes in to play in buildings
where air quality is strictly watched for example hospitals.
Special Control Requirements
The special requirements pertain to the interlocking with fire protection systems, smoke removal
systems, clean air systems, hazardous or noxious effluent control etc.
Control Strategies .
The simplest control in HVAC system is cycling or on/off control to meet part load conditions. If
building only needs half the energy that the system is designed to deliver, the system runs for about
ten minutes, turns off for ten minutes, and then cycles on again. As the building load increases, the
system runs longer and its off period is shorter.
One problem faced by this type of control is short-cycling which keeps the system operating at the
inefficient condition and wears the component quickly. A furnace or air-conditioner takes several
minutes before reaching "steady-state" performance. "Well," you say, "why not just lengthen the time
between starts to avoid short-cycling?" This is possible but at a cost of some discomfort for short time.
The longer the time between cycles, the wider the temperature swings in the space. Trying to find a
compromise that allows adequate comfort without excessive wear on the equipment is modulation or
proportional control. Under this concept, if a building is calling for half the rated capacity of the chiller,
the chilled water is supplied at half the rate or in case of heating furnace; fuel is fed to the furnace at
half the design rate: the energy delivery is proportional to the energy demand. While this system is
better than cycling, it also has its problems. Equipment has a limited turn-down ratio. A furnace with a
5:1 turn-down ratio can only be operated above 20% of rated capacity. If the building demand is lower
than that, cycling would still have to be used.
An alternate method of control under part-load conditions is staging. Several small units (e.g., four
units at 25% each) are installed instead of one large unit. When conditions call for half the design
capacity, only two units operate. At 60% load, two units are base-loaded (run continuously), and a
third unit swings (is either cycled or modulated) as needed. To prevent excessive wear, sequencing is
often used to periodically change the unit being cycled. To continue our example at 60% load:
assume Units 1 and 2 are base-loaded, and Unit 3 has just cycled on. When the cycling portion of the
load is satisfied, Unit 1 cycle off, and Units 2 and 3 become base loaded. When more capacity is
needed, Unit 4 cycles on, and so on.
Where are HVAC controls required.
The HVAC control system is typically distributed across three areas:
1) The HVAC equipment and their controls located in the main mechanical room. Equipment
includes chillers, boiler, hot water generator, heat exchangers, pumps etc.
2) The weather maker or the “Air Handling Units (AHUs)” may heat, cool, humidify, dehumidify,
ventilate, or filter the air and then distribute that air to a section of the building. AHUs are available
in various configurations and can be placed in a dedicated room called secondary equipment
room or may be located in an open area such as roof top air-handling units.
