Author: Don Buckler, CFM
Facilities managers often are business majors, real estate majors or other non-technical people. HVAC and other highly technical aspects of their responsibility are profound mysteries that they entrust to other qualified staff. This does not totally isolate them from having to confront technical decisions or technical situations. This course is designed for you. It will be presented in layman's terms. It will try not to overwhelm you with large doses of technical information. It takes the mystery out of an otherwise complicated subject. You will not be able to design or troubleshoot HVAC systems, but you will understand what engineers and sales people are talking about when confronted with HVAC concerns.
This article will discuss HVAC concepts, systems and equipment using the most generic model possible. The model is a multi-story office building with a simple air handling unit, a chilled water chiller and a cooling tower. There are many types of buildings with various uses and HVAC requirements. Air handling units can be as involved as necessary to satisfy the desired results for the building and its occupants. Cooling can be accomplished using several cooling methods, but most buildings of any appreciable size uses water chillers and cooling towers that serve the air handling units.
It will help you to understand HVAC concepts if you accept the fact that we are dealing only with heat....not cold. Cold is the absence of heat. You warm a building by adding heat or allowing the internal heat load (lights, people, equipment, etc.) to bring up the temperature (more on this concept later). You cool a building by removing heat from the occupied space and transferring the heat load elsewhere.
In the summer when a building is calling for cooling the following takes place: An office gets warm by heat that is generated by people, lighting and various electrical equipment used in the space i.e.: computers, copiers, coffee makers, etc. An office also gains a great deal of heat through the outer walls and windows. people get uncomfortable because all this heat gain causes the temperature to rise. Obviously the solution is to get rid of the heat that keeps building up. Cool air (usually about 50 to 55 degrees) is delivered to the office in a metered amount to absorb the heat. Thermostats regulate (meters) the proper volume of air needed to absorb the proper amount of heat to maintain the desired office temperature. After this cool air has absorbed enough heat , this now warm air is removed from the room by the return air system. The air (about 75 degrees) is returned to the fan unit (air handling unit). The fans force the warm air through a cooling coil. It enters the coil at about 75 degrees and as it passes through the coil the heat is removed from the air and the air is cooled down to about 50 to 55 degrees. The cooled air is sent back to the office to absorb more heat.
The cooling coil is a system of copper pipes that has aluminum fins on the outside of the pipes. Chilled water (42 to 44 degrees) is circulated through the pipes. As the air passes over the fins, the heat in the air is transferred from the air through the fins and copper pipes to the chilled water. The chilled water absorbs the heat from the air and the heated water is pumped back to the chillers.
At the chillers, the heat in the chilled water is transferred to the freon and the freon transfers the heat to the condenser water. the warm condenser water is pumped up to the cooling tower which usually is located on the roof of most large buildings. At the cooling tower the heat is transferred to the outside air. At this time we will not discuss what takes place in the chiller or the cooling tower to effect the heat transfer.
This may seem like a lengthy and involved discussion, but I just want you to get a feel for the path the heat removed from an office travels before it is finally discharged to the outside. In this case, every Btu of heat removed from the office ultimately ends up being discharged above the roof to the outside air.