Objective: What is your system to achieve?
Before starting to design a system, it is critical that you know what your system is to achieve.
Often, the objective is to provide a comfortable environment for the human occupants, but there are many other possible objectives: creating a suitable environment for farm animals; regulating a hospital operating room; maintaining cold temperatures for frozen food storage; or maintaining temperature and humidity to preserve wood and fiber works of art. Whatever the situation, it is important that the objective criteria for system success are clearly identified at the start of the project, because different requirements need different design considerations.
Let us very briefly consider some specific design situations and the types of performance requirements for HVAC systems.
Example 1: Farm animals. The design issues are economics, the health and well being of both animals and workers, plus any regulations. Farm animal spaces are always ventilated. Depending on the climate, cooling and/or heating may be provided, controlled by a simple thermostat. The ventilation rate may be varied to:
e Maintain indoor air quality (removal of body and excrement fumes.)
e Maintain inside design temperature (bring in cool air and exhaust hot air.)
e Remove moisture (bring in drier air and exhaust moist air.)
e Change the air movement over the animals (higher air speed provides
cooling.)
A complex control of ventilation to meet the four design requirements may well be very cost effective. However, humidification and cleaning are not required.
Example 2: Hospital operating room. This is a critical environment, often served by a dedicated air-conditioning system. The design objectives include:
e Heating, to avoid the patient from becoming too cold.
e Cooling, to prevent the members of the operating team from becoming too hot.
e Control adjustment by the operating team for temperatures between 65°F (Fahrenheit) and 80°F.
e Humidifying, to avoid low humidity and the possibility of static electricity sparks.
e Dehumidifying, to minimize any possibility of mold and to minimize operating team discomfort.
e Cleaning the incoming air with very high efficiency filters, to remove any airborne organisms that could infect the patient.
e Ventilating, to remove airborne contaminants and to keep the theatre fresh.
e Providing steady air movement from ceiling supply air outlets down over the patient for exhaust near the floor, to minimize contamination of the operating site.
This situation requires a very comprehensive air-conditioning system.
Example 3: Frozen food storage. The ideal temperature for long storage varies: i.e. ice cream requires temperatures below -12°F and meat requires temperatures below -5°F. The design challenge is to ensure that the temperature is accurately maintained and that the temperature is as even as possible through- out the storage facility. Here, accurate cooling and good air movement are the prime issues. Although cooling and air movement are required, we refer to this system as a “freezer,” not as an air-conditioning system, because heating, ventilation, humidification and dehumidification are not controlled.
Example 4: Preserving wood and fiber works of art. The objectives in this environment are to minimize any possibility of mold, by keeping the humidity low, and to minimize drying out, by keeping the humidity up. In addition, it is important to minimize the expansion and contraction of specimens that can occur as the moisture content changes. As a result the design challenge is to maintain a very steady humidity, reasonably steady temperature, and to minimize required ventilation, from a system that runs continuously. For this situation, the humidity control is the primary issue and temperature control is secondary. Typically, this situation will require all seven of the air-conditioning features and we will describe the space as fully “air-conditioned.”
Now let us go on to consider the more complex subject of human comfort in a space.