Summary
In this chapter, we discussed issues of system choice and provided a general description of system control issues. We will return to controls in more depth in Chapter 11.
Examples of Buildings with Single zone Package Air-Conditioning Units For heating and cooling, a packaged unit may require: just an electrical source of power, or a gas or hot water supply, and/or a source of chilled water. The basic operation of the unit stays the same; it is just the source of heating and cooling energy that may change.
Air Handling Unit Components
The overall functions of the air-handler are to draw in outside air and return air, mix them, condition the mixed air, blow the conditioned air into the space and exhaust any excess air to outside. Components of the unit can include: inlet louver screen, the parallel blade damper, opposed blade damper, the relief air damper, actuator, the mixed temperature sensor, filter heating coil, cooling coil, humidifier, fan, return fan. The concept of control logic was introduced as a method to summarize the operation of the compo- nents of the system.
Refrigeration Equipment
The vapor compression refrigeration cycle is generally the basis of mechanical refrigeration. The vapor compression refrigeration system comprises four compo- nents: compressor, condenser, expansion valve, and evaporator. This system can be used directly, to provide cooling to, typically, a local coil. To provide cooling for several coils at greater distances, refrigeration machines are used to chill water that is then pumped around buildings to provide cooling in air-conditioning systems. The heat removed from the water is expelled into the atmosphere through a hot, air-cooled coil, or by evaporating water in a cooling tower.
The components are matched to work together with a specific charge of refrigerant. If you operate the system with too little refrigerant or too little air or water flow over the evaporator or condenser, problems can arise.
While cooling is achieved by pumping heat from a cooled space and rejecting heat to outside, you can reverse the process, in a mild climate, with a heat pump, to obtain heat from ambient air. Similarly, the ground can be used as a source of heat or a sink for waste heat, by using a ground source heat pump.
System Performance Requirements
Before choosing a system, you need an understanding of the types of loads you want the system to manage. Summer cooling loads will be the main determinant of the choice of unit. These summer factors are used to determine the sum- mer load: outside design temperature; outside design humidity; inside design temperature and humidity; inside generation of heat and moisture; ventilation requirements. Once you have determined summer loads, additional decision factors for unit choice are the initial cost to purchase and install, versus the ongoing cost of operation and maintenance; and load versus capacity.
Rooftop Units
In a typical rooftop unit, the return air is drawn up into the base of the unit and the supply air is blown vertically down from the bottom of the unit into the space below. As an alternative, the ducts can come out of the end of the unit to run across the roof before entering the building. Advantages and disadvantages of rooftop units were discussed.
Choice factors to choose a rooftop unit: inside and outside design temperatures, required airflow in cfm, mixed air temperature, and the required sensible and latent cooling loads.
It is important that the airflow is correctly calculated and that the unit is setup and balanced to provide the correct airflow. With direct expansion refrigeration circuits, too little airflow over the evaporator can cause icing problems.
Units come in discrete sizes so a perfect match of unit and calculated loads does not happen. As a result, the design conditions may be jeopardized, and or extra energy costs may arise.
Split Systems
In the split system, the compressor/condenser part of the refrigeration system separate from the evaporator coil and connected by the refrigerant lines to the air system, which includes the evaporator.
Advantages of the split system: It allows the designer a much greater choice of performance; it allows the air handling part of the unit to be indoors, where it is easier to maintain and does not need to be weatherproofed. The noise of the compressor is outside and can be located at some distance from the air-handling unit.
Bibliography
1. ASHRAE Fundamentals of Air System Design
2. ASHRAE Fundamentals of Refrigeration
3. ASHRAE Fundamentals of Heating and Cooling Loads