Evaporator Coils (DX) are made for heat absorption and generally function at a lower pressure. Coils made with 3/8-inch diameter tubing is rated for 400 PSIG and 300°F maximum operating temperature. Coils made with 1/2- or 5/8-inch diameter tubing are rated for 250 PSIG and a maximum operating temperature of 300°F. All evaporator coils are factory tested at 600 PSIG. These coils are generally used for spot cooling or as part of an air handling system.
Most evaporator coil are designed with either 1/2” or 3/8” O.D. tubes. However, there are certain applications where it is desirable to use 5/8” tube coils. Perhaps the most critical factor in designing a refrigerant evaporator coil is the need to keep the velocity of the refrigerant flow up high enough that the oil that is present remains entrained within the refrigerant. This is one of the primary reasons that so many dx coil are manufactured with 3/8” tubes. The smaller tube diameter facilitates the mixing of the oil within the refrigerant. However, on coils with longer tube lengths, the smaller diameter can lead to excessive refrigerant pressure drops so larger tubes may be used.
The most commonly used tube material is copper, and there are a variety of tube wall thickness options to handle different applications. Aluminum is the most frequently used material for the fins on a coil, because of good heat transfer characteristics and low cost. There are many different fin designs that will either enhance heat transfer or reduce air pressure drop, depending on the requirement. Coil frames (or casings, flanges, whatever you might call them) are normally galvanized but stainless steel is a good consideration to increase the longevity of the coil. Coil connections are almost universally copper.
Direct Expansion (DX) Coils are part of a refrigerant filled system consisting of a condenser coil, evaporator coil and a refrigerant compressor. The evaporator coil must be paired with a thermal expansion valve (TXV) intended for the specific capacity and refrigerant type. When used in conjunction with a heat pump and reversing valves, a coil serves for both heating and cooling.
Heat pump coils in their most basic form are simply evaporator coils that become condenser coils when the system is operating in the heating mode (also known as the reverse cycle). Compressor-driven air conditioners and freezers are examples of heat pumps. However, the term "heat pump" is more general and applies to devices which are used for space heating, or space cooling. When a heat pump is used for heating, it uses the same basic refrigeration-type cycle employed by an air conditioner or a refrigerator, but releasing heat into the conditioned-space rather than into the surrounding environment. In this use, heat pumps generally draw heat from the cooler external air or from the ground.
Heat pumps are used to provide heating because less high-grade (i.e., low-entropy) energy is required for their operation, than appears in the released heat. Most of the energy for heating comes from the external environment, and only a fraction comes from electricity (or some other high-grade energy source). In electrically powered heat pumps, the heat transferred can be three or four times larger than the electrical power consumed, giving the system a Coefficient of Performance (COP) of 3 or 4, as opposed to a COP of 1 of a conventional electrical resistance heater, in which all heat is produced from input electrical energy.
Heat Pump Diagram
A simple stylized diagram of a heat pump's vapor-compression refrigeration cycle: 1) condenser, 2) expansion valve, 3) evaporator, 4) compressor.
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Evaporator DX Coils are used in a wide variety of applications from comfort cooling and dehumidification to industrial and commercial processes and have been designed to meet a wide range of temperatures, from HVAC to sub-zero freezing applications.
Evaporator DX coils are basically the same as chilled coils, but they use refrigerant as the working fluid and require a different valve arrangement. The coils can be used for central systems or duct applications.
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