Choosing An HVAC System and Selection Of HVAC

 HVAC System Selection and Selection Guide

    A first step in Central system design involves determination of the individual zones to be conditioned and the type and location of the HVAC equipment.large buildings with variable needs in the different zones can be served well with a central system,in which most of the HVAC equipment is located in one or more mechanical rooms.the energy and moisture addition or removal, the ventilation, and the removal of pollutants can be accomplished by the equipment in the mechanical room. Normally mechanical rooms are outside the conditioned area, in a basement, on the roof, or in a service area at the core of the building. Mechanical rooms reduce the noise, spills, and mechanical maintenance that might otherwise occur in the occupied spaces. Equipment normally found in the central mechanical room includes:

Fans or air handlers for moving air with associated dampers and filters

Pumps for moving heated or Chilled water and appropriate control valves

Heat exchangers for transferring energy from one fluid stream to another

Flow measuring and control devices 

Chillers and furnace or boiler equipment

      Where cooling must be furnished to building spaces there must always be some way to reject the energy to the surroundings. Lakes and rivers are sometimes used for an energy sink. In most cases the energy is discharged to the atmosphere by means of equipment placed outside the building, either on the ground or on the roof. Where the energy exchange is direct from the refrigerant to the air, the outdoor unit is simply called the condensing unit. With no external water evaporation used for cooling,  the unit would sometimes be called a dry condensing unit. Large systems typically transfer energy from the chiller located indoors to circulating water and the energy is carried outside by the water to a cooling tower.

     A zone is a conditioned space under the control of a single thermostat. The thermostat is a control device that senses the space temperature and sends a correcting signal if that temperature is not within some desired range. In some cases the zone humidity may also be controlled by a humidistat. The temperatures within the area conditioned by a central system may not be uniform if a single-zone dust system is used, because air temperature is sensed only at that single location where the thermostat is located. Because conditions vary in  most typical zones, it is important that the thermostat be in a location free from local disturbances or sunlight and where the temperature is most nearly the average over the occupied space.

        Uniform temperatures are more likely to be experienced in spaces with large open areas and small external heat gains or losses, such as in theaters, auditoriums, department stores, and public areas of most building. In Large commercial buildings the interior zones are usually fairly uniform if provisions are made to take care of local heat sources such as large equipment or computers. Variations of temperature within a zone can be reduced by adjusting the distribution of air to various parts of the zone, or by changing local supply air temperatures.

       spaces with stringent requirements for cleanliness, humidity, temperature control, and/or air distribution are usually isolated as separate zones within the larger building and served by separate systems and furnished with precision controls. For applications requiring close aseptic or contamination control of the environment, such as surgical operating rooms,all-air systems generally are used to provide adequate solution of the air in the controlled space

     In spaces such as large office buildings, factories, and large department stories, practical considerations require not only multiple zones but also multiple installation of central systems. In the case of tall buildings, each central system may serve one or more floors.

     Large installations such as college campuses, military bases, and research facilities may best be served by a central station or central plants, where chillers and boilers provide Chilled water and hot water or steam through a piping system to the entire facility, often through underground piping. Since all buildings will probably not be in full use at the same time, the total capacity of the equipment required in the central plant is much less than the sum of the maximum requirements of all of the building. This leads to the concept of a diversity factor, which is the ratio of the actual maximum demand of a facility to the sum of the maximum demands of the individual parts of a facility. For large installation with a low diversity factor, central stations or plants allow designs with much smaller total heating and cooling capacity and therefore much lower capital (first) costs than isolated systems located in each individual building. In addition there is usually greater efficiency, less Maintenance cost, and lower labor costs than with individual central facilities in each building. 

      The choices described above are usually controlled by the economic factors introduced in chapter 1, involving  trade-off between first costs and operating costs for the installation. As the distance over which energy must be transported increases, the cost of moving that energy tends to become more significant in comparison with the costs of operating the chillers and boilers,. As a general rule, the smaller systems tend to be the most economical if they move the energy as directly as possible. For example, in a small heating system the air will most likely be heated directly in a Furnace and transported through ducts to the controlled space. Likewise, in the smaller units the refrigerating system will likely involve a direct exchange between the refrigerant and the supply air (a d-x system). In installations where the energy must be moved over greater distances, a liquid(or steam) transport system will probably be used. Thi is because water, with a hight specific heat and density, and steam, with a hight enthalpy of vaporization, can carry greater quantities of energy per unit volume than air. Not only can pipe sizes be much smaller than ductwork, but the costs of power to move steam or liquid is much less than for air. The required transfer of energy from fluid to air does involve, however, extra heat exchangers and drops in temperature not required in the direct exchange from refrigerant to air or from combustion gases to air.

       Once the user's needs have been appraised and zones have been defined, the cooling and/or heating loads and air requirements can be calculated. With the most suitable type of overall system determined, the designer can star the process of selection and arrangement of the various system components. The equipment should be suitable for the particular application, sized properly, accessible for easy maintenance, and no more complex in arrangement and control than necessary to meet the design criteria. The economic trade-off between initial investment and operating costs must always be kept in mind.

     Consideration of the type of fuel or energy source must be made at the same time as the selection of the energy-consuming equipment to assure the least life- cycle cost for the owner. For example, will the chillers be driven by gas or steam turbines or by electric motors? chapter 17 of the ASHRAE handbook (2) gives the types and properties of fuels and energy sources and guidance in their proper use. This selection is important not only from an economic standpoint but also in making the best use of natural resources.