Useful Information: Water Cooling and Heating Coils

swcoil_superadminCooling, Hot Water Heating, Water Cooling

Water is the most widely used heat transfer medium, because it’s readily available and low in cost. Water can be used as a heat transfer medium, if the temperature range is between 15 degrees F (when mixed with glycol) when cooling and a high of 210 degrees F when heating.

Chilled Water Coils

When maintaining an acceptable living or working condition in a large commercial space, you are more than likely to encounter a chilled water system versus a refrigerant cooling system. Chilled water coils respond to the cooling effect of surfaces when moisture is condensed from the air in most regions of the United States. The sum of sensible and latent heat transferred by a dehumidifying coil represents the total heat values. Sensible heat transfer is the cooling down of the dry bulb temperatures. For example: 80 degrees F dry bulb being cooled to 58 degrees F dry bulb. There is a latent load, and that is the wet bulb temperatures associated with the dry bulb temperatures at design conditions. For example: 67 degrees F wet bulb cooled down to 57 degrees F wet bulb. If the wet bulb (related to relative humidity) is present, then the moisture must be removed to obtain the designed dry bulb temperature. When the amount of moisture needs to be dehumidified, it may require a lower than desired dry bulb temperature since it gets lowered in the dehumidifying process.

Arizona, and other low humidity areas in the country, have dry cooling and rarely have enough moisture to require dehumidification. The operational cost to dehumidify is far more expensive than dry cooling per degree of cooling.

Moisture carry over can be a problem in dehumidifying systems. Air velocity over 550 feet per minute creates condensate that forms on the fin tube surface during the dehumidifying process and can be carried downstream past the unitary drain pan and into fan sections and other parts of the HVAC system. We have seen supply ducts with water in them from carry over. Carry over can also be caused by air stratification and lack of proper safe-offs that allows water in pans to be carried downstream.

Hot Water Heating Coils

Hot water heating coils are used to heat air for specific heating applications. Hot water heating coils, due to the ability of the water to freeze, will generally be found in the reheat position. Air temperatures should never be below 35 degrees F. A clear majority of hot water heating coils are in ductwork and take preheated air from an intermediate temperature to a final zone temperature required by system controls.

Required Selection Information

Air Volume: CFM @ temperature and altitude

Air Temperatures: Air entering and desired air leaving DB and WB temperatures

Water Volume: GPM and percentage and type of glycol, if required

Water Temperatures: Inlet and desired outlet water temperatures

Coil Size: Available finned area or overall area in height and length in inches

Maximum Water Pressure Drop: Stated in “feet of water”

Maximum Air Pressure Drop: Stated in “inches of water”

Coil Construction: Tubes, fins, headers, connections, and casing

Arrangement: Same end or opposite end connections – right or left hand

Water coils are a mainstay in commercial HVAC systems. Modern day coil selection programs provide you with answers, but it’s imperative that the designer, installer, and owner be educated about the selection and operating details for these types of coils.