Central air-conditioning systems

All air or air-water central air-conditioning systems can be catego­rized into two basic groups as follows:

1. Constant volume, variable temperature systems.

2. Variable volume, constant temperature systems.

Constant volume, variable temperature systems supply a constant amount of conditioned air at proper temperatures in response to each thermostat to meet prevailing space loads. Supply air temperature is controlled by:

(1) mixing a cooled air stream with a heated or bypassed air system;

(2) reheating a cooled air stream.

Generally, these systems are very simple to design and reliable to operate. Although any space served by the system can be heated or cooled, energy is wasted under partial load conditions because of mixing or reheat.

Variable volume, constant temperature systems respond to each space thermostat by supplying a varying amount of constant temperature conditioned air to meet the prevailing space load. Since this type is rela­tively new, some adjustment in design philosophy is necessary in the minds of engineers who are used to constant volume system design.

Space temperature control is not dependent on mixing or reheat thereby. Eliminating energy washed, however, only cooling can be pro­vided. Heating is generally supplied by some form of supplementary sys­tem. These systems also have limitations for buildings where fixed mini­mum ventilation rates are required.

Recognizing that both constant volume and variable volume systems have good features and limitations, a group of hybrid systems evolved from these two basic groups to optimize system efficiency, cost and con­venience.

8. Read the text and write down the abstract using the key words:

A commonly used method of saving cooling energy is an economizer cycle. Interior spaces require cooling year round. When outside air tem­perature and humidity drop below the required supply air temperature, the refrigeration plant can be shut off. An economizer cycle can be applied to practically all central air-conditioning systems and it is especially effective on single duct systems. This method of conserving energy is very useful in the cold climates. In general, consideration or using an economizer cycle should include the potential use of waste heat from the refrigeration plant and the balance between the additional first cost of the economizer cycle versus the potential energy savings.