Principles - Cooling and Heating - 6 of 80 Problems
Cooling and Heating accounts for approximately 6 questions on the Thermal & Fluids Mechanical PE exam. Cooling and heating is used throughout the various thermodynamic cycles, in many types of equipment like condensers, boilers, cooling towers, etc. Each piece of equipment is discussed in this section, but the concept of cooling/heating is best explained by looking at cooling and heating coils. The important thing to understand is that the heat transfer in cooling and heating is not perfect and that there is efficiency loss. On the exam, this concept is tested with the apparatus dew point term.
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In the Thermal & Fluids field, cooling and heating coils are used to exchange heat between air and a heat exchange fluid. A heat exchange fluid is transmitted through the coil and as air is passed over the coil, the air is either heated or cooled. Coils consist of a metal box framing, which holds a series of copper tubes in staggered rows and columns.
The amount of heat that is transferred is related to the amount of surface area that contacts the air. In order to increase surface area, the size of the tubes may be decreased and more tubes can be provided, the number of rows increased or the amount of fins per inch can be increased. Aluminum or copper fins are provided on each tube to increase the amount of surface area. Coils are rated by the height of the fins and the number of fins per inch.
There are several different types of heat exchange fluids used in cooling/heating coils.
Refrigerant: Hot refrigerant gas or cool refrigerant liquid can be used in a coil to provide either heating or cooling. In a heating-coil, cool air is passed over a coil containing hot gas. Heat is exchanged to the cool air, which warms the air. The heat lost by the refrigerant gas causes it to condense to a liquid. In a cooling-coil, warm air is passed over a coil containing cool refrigerant liquid. Heat is exchanged to the cool refrigerant liquid, causing it to evaporate. The warm air loses heat, thereby decreasing the air temperature.
Water: Chilled water or hot water can be used in a coil to provide either heating or cooling. The air temperature is either raised or lowered as heat is transferred to raise or lower the temperature of the chilled or hot water.
Steam: Steam can be provided to a coil to provide heating. Steam enters the coil and as the air passes over the coil its air temperature increases. As the steam loses heat, it condenses to its liquid form.