5.

Hazards due to heavy electrochemical equipment in the occupied space are avoided.

6. The central system is adaptable to automatic controls.

Typical Central HVAC Systems

The most commonly used central HVAC systems are all-air systems or air-water systems. These systems include: single duct, constant volume; single duct, variable volume; single duct, reheat; double duct; multizone; two-pipe changeover; three pipe; four pipe; and induction. Details of these systems are given in the 1973 Systems Volume of the ASHRAE Handbook (1). Although it is not the intention of this paper to describe all of these central systems, some typical ones will be discussed in the context of energy conservation.

a. Single Duct Constant and Variable Volume Systems

Figure 2 depicts a schematic of a single duct-single zone system which could be either constant volume or variable air volume (VAV), depending upon the air outlet unit in the room's ceiling. In order to respond to the changing space load, the constant volume system regulates its heat delivering capacity by changing the supply air temperature. This is accomplished by regulating the heating coil temperature during the heating season or cooling coil temperature during the cooling season. The variable volume or VAV system on the other hand maintains a constant coil temperature during each season, but regulates the supply air flow rate to meet the changing load. This can be done by either a damper in the VAV box at the ceiling or by dumping part of the supply air into the ceiling plenum space. The air supplied to the space exchanges heat with the air in the space. The space air is then returned to the central plant through the return air duct. Since the occupants of the space that is being conditioned generate excess carbon dioxide, odor, and smoke, these contaminants are diluted by mixing them with outside air. This is the reason that a part of the return air is exhausted and replaced by make-up outdoor air as indicated in Figure 2.

On many days during spring and autumn, the space heat loss could be exactly matched by the heat given off by the lights, equipment, and occupants; no net heating or cooling is required. The single VAV duct systems have difficulty in meeting this zero load condition since the supply air flow cannot be made zero. A minimum of supply air must be fed into the space regardless of the thermal load in order to satisfy the space ventilation requirement.

It is not difficult to control the supply air temperature by regulating the heating or cooling coil temperature if a single duct system is connected to single zone such as shown in Figure 2. In actual practice however, the system supplies air to more than one space. Particularly during the intermediate seasons, it frequently happens that one space calls for heating while others require cooling. It is impossible to satisfy these different space requirements in the single duct systems described above. A common practice employed is to cool the central supply air to the lowest likely required temperature and modulate it up to the desired supply air temperature for the other spaces by the reheat coil such as shown in Figure 3.

It is obvious that the reheating of the air which is once cooled by refrigeration causes a double expenditure of energy. The energy waste is greater in the constant volume system than the VAV system because the latter system requires heating of a smaller quantity of air. In addition, the VAV system that dumps a part of the supply air into the ceiling plenum obviously is not as conserving of energy as the VAV system where the volume control is done with dampers.

Figure 3 shows that the air conditioned space is divided by fictitious dashed line into a perimeter zone and an interior zone. While the heating/cooling requirement for the perimeter zone is constantly affected by the outdoor climate conditions, the interior zone load is usually constant, requiring continuous cooling during the occupied hours. Figure 4 shows how the single zone reheat system can be used in an energy conserving way to meet the requirements of these two different zones. The system has been recommended by the New York consulting firm of Dubin, Mindell, Bloome and Associates for a new federal office building to be built in Manchester, New Hampshire. In this system, the interior zone is supplied with cooled air through VAV boxes, while the perimeter zone has independent fan-coil units which contain separate circuits for the heating coil and the cooling coil. Since two pipes are connected to each coil at the fan-coil unit, this may be considered a four-pipe system. When the space requires cooling, the cooling coil is activated. If heating is required, the heating coil is activated. These two coils will never be activated simultaneously in the same unit, although the unit in one space could be heating while

others are cooling. The four-pipe system requires that hot water as well as chilled water be available throughout the year in the central plant. A unique feature of the federal office building that will use this system is that the hot water will be provided not by the burning of fossil fuels but rather by reclaiming the heat from the condenser of the water chilling unit. This energy is ordinarily rejected to outdoor air. On many days throughout the year, the hot water generated in the condenser of this water chiller, which is used to satisfy the cooling need of the interior zone, is more than enough to satisfy the heating requirements of the perimeter zone. It is customary, however, to install a conventional hot water heater to supplement the deficient heating capacity of the condenser water during a few extremely cold days.

C. Dual Duct System

Another commonly used central all-air system is the dual-duct system which is illustrated in Figure 5. In this system, both heated air and cooled air are provided in two separate supply ducts to all the spaces to be air conditioned. At the ceiling plenum above the space, mixing boxes are used to yield a desired supply air temperature which meets the heating or cooling load at a specific instant and at a specific location. Although the total air flow of the dual duct system after mixing usually remains constant, it could also be varied by a VAV box. The dual duct system is definitely not energy conserving because it requires mixing of heated air with chilled air to obtain the proper degree of heating or cooling. The system could be made energy conserving, however, if the heating coil was provided with hot water from the chilling machine's condenser or reclaimed heat from the lighting fixtures.

One of the most interesting energy conservation HVAC central systems is the McFarland 3-pipe system (2) which is depicted in Figure 6. In this system the air coils in the central plant, as well as in the individual spaces are fed with two supply pipes and one return pipe.

The coils in the supply and return air ducts are fed with chilled water from the chiller and room temperature water from the run-around loop. These are mixed to provide the desired coil temperatures. The perimeter reheat coils are supplied with the hot condenser water and the room temperature run-around water. With a delicate balancing and mixing of the chilled water, run-around water and condenser water, it is possible to attain a very high degree of energy conservation. Details of this system are given in reference (1).

When the outdoor temperature and humidity conditions are favorable, it is possible to shut off the cooling plant and use the cool outdoor air to cool the interior zone. This operation is called using an economizer cycle. The dampers indicated in Figures 2 through 6 for the outdoor air intake, the return air duct and the exhaust air duct are controlled to regulate the mixing rate of outdoor and return air to achieve economizer cycle operation. The economizer cycle can obviously be a very energy conserving feature of a HVAC system and its use should be very much encouraged. Ross F. Meriwether used a computer simulation (3) to predict the energy savings by the use of an economizer cycle with a dual duct and VAV system for an office building in San Francisco. The results were as follows:

As can be seen from this table, the energy savings resulting from employing the economizer cycle are significant indeed.

The Heat Pump and Energy Conservation

A heat pump is a device which utilizes work to extract heat from a source at one temperature and reject this heat plus the heat equivalent of the work done to a higher temperature sink. It may provide a heating function by extracting heat from an outside source and delivering heat to the inside space or it may provide for the cooling of the space by removing heat from it and rejecting it to the surroundings. Simultaneous heating and cooling can be done if the space is both the source and sink of all or part of the heat transferred.

There are two types of heat pumps in common use today. One type uses the vapor compression refrigeration cycle and the other uses the absorption refrigeration cycle. The basic components of the vapor compression machines are a condenser, a throttling valve, an evaporator, and a compressor. As illustrated in Figure 7, high pressure liquid refrigerant undergoes a constant enthalpy expansion through the throttling valve and is then fed into the evaporator where it picks up heat. It leaves the evaporator as either a saturated vapor or a slightly superheated vapor and passes on to the compressor where it is compressed to the condenser pressure. In the condenser it gives up heat and leaves as a high pressure liquid to complete the cycle.

The four major components of an absorption machine are a generator, a condenser, an evaporator and an absorber. In the generator, heat is applied to a solution of refrigerant and absorbent. The refrigerant is driven out of solution by this heat and passes into the condenser while the absorbent travels to the absorber. In the condenser the refrigerant vapor becomes a liquid by giving up its heat of condensation. The liquid refrigerant then undergoes a reduction in pressure and flows into the evaporator where it absorbs heat and evaporates. The resulting vapor continues on to the absorber where it dissolves in the absorbent. The solution is then returned to the generator, completing the cycle as shown in Figure 8.

a. The Vapor Compression Refrigeration Cycle

The vapor compression heat pumps are classified according to the type of compressor employed. The compressor may be either the positive displacement type, including the reciprocating, rotary and helical rotary (screw) compressors or the centrifugal type (4). While the different compressors have different operating characteristics, in general the positive displacement compressors tend to be capable of operating over a wide range of temperature lifts or heads, while the centrifugal compressors do not have a high lift capability with low volumes of gas during part load operation (4, 5). On the other hand, the centrifugal compressors have a greater volumetric capacity, size for size. This makes the positive displacement compressor more suitable for handling small heating loads and the centrifugal compressor more suitable for large cooling loads. In applications involving both large heating and cooling loads, the practice appears to be to use either a centrifugal compressor with a higher compressor speed and/or larger impeller or to use a cascade system employing two or more centrifugal compressors (6). The compressors may be driven with either an electric motor or an engine. Although a heat engine would probably be the most suitable engine drive if a reliable one was available, the most common types in use are the gas engine, the diesel engine and the gas turbine.

The potential of the heat pump for energy conservation can be illustrated by considering the coefficient of performance, COP for a small, unitary, electric driven, air-source heat pump operated in a heating mode. The COP, which in this case is defined as the ratio of heat delivered to the work done, is presented in Table 2 for a typical unit.*

It can be seen from this table that a typical COP will vary from about 3.9 to 1.4 as the outside temperature ranges from 60 F to -10 F. Thus, at 45 F, this heat pump will deliver 2.9 units of heat for each heat equivalent unit of work done. While this might at first seem a tremendously efficient way of heating a building, the picture is less bright when one takes into account the fact that only about 30%** of the heat energy of the fuel burned at the power plant in making electricity will reach the heat pump. Thus what will be called in this paper the effective COP (EFF. COP) of the power plant-heat pump combination is really (2.9 x .3) or .87 at an outside temperature of 45°F. Other values of the EFF. COP at different outside temperatures are given

*The COP's in Table 2 are taken from Carrier Catalog #38AC and are for a unit having an instantaneous heating of 112,000 Btu per hour at an outdoor air temperature of 40°F.

** 30% is intended to be representative of the efficiency of converting fuel into electricity and distributing it to the point of use.