Determining the correct size of a chiller unit first requires the interior of the vessel to be properly sized for air handler capacity in each section of the vessel and an allowance for a fresh air make up unit if used. Proper balancing of a system regarding the chiller plant and air handlers will ensure the correct water flow is achieved and maintained and the system as a whole should function at optimal levels.
Each ton of air conditioning for a chiller system requires a pre determined gallon per minute (GPM) of water flow. For marine applications the rule of thumb is 3 GPM per ton of air conditioning. Therefore a 24 ton system would require 72 GPM of water. Consideration should also be taken into the flow rate of the water also known as the velocity at which the water flows in feet per second (FPS). FPS is important because the length of time the chilled water is in contact with the area or substance being cooled (thermal contact) is relevant for the cooling process. In other terms, chilled water flowing through an air handler at a rate of 4 FPS will remove heat from the room more efficiently than if it flowed at 20 FPS. Water flowing at too high of a velocity can also wear out pipes and air handler coils.
The velocity in which water flows through a pipe will be determined by the GPM and pipe diameter. Therefore a matched system would have the same size chiller unit to match the air handlers throughout the vessel to ensure pipe sizes are matched for the correct water flow. An installed chiller systems that is smaller in tonnage than air handler tonnage generally has pipe sizes designed to match the chiller unit. Air handlers will suffer from a lack of water flow in this type of situation. In a case like this a bypass line can be installed from the outlet of the circulation pump to the outlet of the chiller unit to ensure the vessel’s air handlers get the proper amount of water. This solution is not ideal because untreated water that has not been re-chilled will return to the air handlers. But slight variations and compromise may have to be made.
One of the biggest problems is when a vessel adds additional cooling due to an extension of the vessel. In a case like this, if at all possible take the new pipe lines feeding the new air handlers all the way back to the chiller unit.
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