Editing Heat exchanger (section)

==HVAC and refrigeration air coils==
One of the widest uses of heat exchangers is for refrigeration and [[air conditioning]]. This class of heat exchangers is commonly called ''air coils'', or just ''coils'' due to their often-serpentine internal tubing, or [[Condenser (heat transfer)|condensers]] in the case of [[Vapor-compression refrigeration|refrigeration]], and are typically of the finned tube type. Liquid-to-air, or air-to-liquid [[HVAC]] coils are typically of modified crossflow arrangement. In vehicles, heat coils are often called [[heater core]]s.

On the liquid side of these heat exchangers, the common fluids are water, a water-glycol solution, steam, or a [[refrigerant]]. For ''heating coils'', hot water and steam are the most common, and this heated fluid is supplied by [[boiler]]s, for example. For ''cooling coils'', chilled water and refrigerant are most common. Chilled water is supplied from a [[chiller]] that is potentially located very far away, but refrigerant must come from a nearby condensing unit. When a refrigerant is used, the cooling coil is the [[evaporator]], and the heating coil is the [[condenser (heat transfer)|condenser]] in the [[vapor-compression refrigeration]] cycle. HVAC coils that use this direct-expansion of refrigerants are commonly called ''DX coils''.  Some ''DX coils'' are "microchannel" type.<ref name=":1">{{cite web|url=http://www.carrier.co.th/knowledge/download/Knowledge_4_MCHX_Technology.pdf|archive-url=https://web.archive.org/web/20130604202141/http://www.carrier.co.th/knowledge/download/Knowledge_4_MCHX_Technology.pdf|url-status=dead|title=MICROCHANNEL TECHNOLOGY|archive-date=June 4, 2013}}</ref>

On the air side of HVAC coils a significant difference exists between those used for heating, and those for cooling. Due to [[psychrometrics]], air that is cooled often has moisture condensing out of it, except with extremely dry air flows. Heating some air increases that airflow's capacity to hold water. So heating coils need not consider moisture condensation on their air-side, but cooling coils ''must'' be adequately designed and selected to handle their particular ''latent'' (moisture) as well as the ''sensible'' (cooling) loads. The water that is removed is called ''condensate''.

For many climates, water or steam HVAC coils can be exposed to freezing conditions. Because water expands upon freezing, these somewhat expensive and difficult to replace thin-walled heat exchangers can easily be damaged or destroyed by just one freeze. As such, freeze protection of coils is a major concern of HVAC designers, installers, and operators.

The introduction of indentations placed within the heat exchange fins controlled condensation, allowing water molecules to remain in the cooled air.<ref>[http://www.google.com/patents/pdf/_RAISLEY_X.pdf?id=zwxKAAAAEBAJ&output=pdf&sig=63-9nepnkjAp6ngTtM6iTQCAca4 Patent 2,046,968 John C Raisley]{{dead link|date=June 2024|bot=medic}}{{cbignore|bot=medic}} issued July 7, 1936; filed Jan. 8, 1934 [https://patents.google.com/patent/US2046968]</ref>

The heat exchangers in direct-combustion [[Furnace (house heating)|furnace]]s, typical in many residences, are not 'coils'. They are, instead, gas-to-air heat exchangers that are typically made of stamped steel sheet metal. The combustion products pass on one side of these heat exchangers, and air to heat on the other. A ''cracked heat exchanger'' is therefore a dangerous situation that requires immediate attention because combustion products may enter living space.