Variable Frequency Drive to control HVAC systems

Heating, ventilation, and air conditioning (HVAC) are the second largest energy consumers after propulsion in passenger vessels. The HVAC system has three elements functioning separately.
Onboard HVAC systems are generally divided into three elements: the cooling system (chiller plant), chilled water circuit, as well as air handling units (AHU). Chiller plants play an important role in air conditioning systems, since they supply the air handling unit with chilled water. Onboard ships, the most commonly used chiller systems are vapor compressed refrigeration cycle chillers, comprising compressor, condenser, expansion or flow control devices, as well as evaporators.

The most commonly used compressors are screw or centrifugal types, with an indirect central cooling system design. This means that secondary circuits are also installed, where the condenser is cooled via a seawater cooling circuit and the chilled water circuit (evaporator side) uses cold water to cool the air handling units.

Consideration can be given to using a variable frequency drive (VFD) for the chilled water circuit in both variable flow and constant pressure systems. In practice, energy efficiency improvements are always achieved in cases where the existing system uses balancing valves to adjust either the flow or pressure. If the operational profile of the ship varies significantly during the year, it is worth considering whether to install VFD on seawater cooling pumps on the condenser side.

Several methods are available for capacity control of the centrifugal compressor, each of which has its advantages and disadvantages. Two of the most common capacity control methods are speed variation and pre rotation vanes, also known as inlet guide vanes. Pre rotation vanes modulate capacity by altering the direction of the refrigerant flow entering the impeller. Capacity control based on variable speed (with VFD) is more economical than pre rotation vanes, in applications where the pressure requirements vary under a part load. 



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