Condenser fans use a lot of energy in an air conditioned system. Using variable speed drives (VSDs) on condenser fans to control the head pressure of air-cooled condensers and other HVAC applications improves the energy efficiency of air conditioning systems in part-load conditions. Appropriate head pressure or condensing pressure is of vital importance for the proper operation of AC and refrigeration systems and can be maintained in several ways. The most energy efficient is via VSD controllers on the condenser fan (either with a constant or floating head pressure set point), which varies the amount of heat rejection.
This measure will deliver more savings when carried out in conjunction with the strategy of retrofitting electronic expansion valves (EEVs) as described in Optimisation Opportunity 9.
It is important to seek the advice of the manufacturer of the refrigeration system before embarking on this optimisation measure as failure to consult the manufacturer may void any warranty.
Retrofitting VSDs can result in reduced energy consumption of condenser fans by 10–30 per
cent and improved control and reliability of the operation of air conditioning systems during part-load conditions.
Principle and equipment
During lower ambient air temperatures, the AC load typically decreases, while the capacity of condensers increases. As such, there is the possibility of saving energy by reducing the
condensing pressure of the refrigeration cycle. This pressure can be maintained by VSD-controlled fans, which would save energy, compared to a scenario whereby the fan is simply cycled on
and off. A recent alternative to VSD-controlled fans is the availability of fans with electronically commutated (EC) motors.
Air cooled condensers on the roof of a commercial building
During mild weather conditions, the cooler air can extract more heat from the condenser, thereby over-cooling or over-condensing an AC system. Over-condensing reduces the condensing
temperature and condensing pressure, to an extent where it affects the performance of the system, especially if thermostatic expansion valves (TXVs) are installed.
AC systems should maintain adequate condensing (or head) pressure in order to enable the proper operation of the TXV. Appropriate head pressure is particularly important for:
• the prevention of premature flashing of refrigerant in the refrigerant liquid line (reduced refrigeration capacity)
• the proper operation of expansion devices (reduced refrigerant flow into evaporators) • satisfactory oil return to the compressor.
Minimum required information
The minimum required information for maintaining head pressure includes: • head pressure set point
• control strategy for modulation of the amount of heat rejection • outside temperature (desirable)
• type of refrigerant.
Minimum required equipment
The minimum required equipment includes: • field sensors (temperature, pressure) • controllers and data processors
• condenser fans with VSD-controlled motors or EC motors.
Current practice
Most air-cooled condensers employ one of the following commonly encountered control strategies to maintain head pressure:
• single condenser fan switching on/off
• multi-speed or multiple condenser fan(s) staging and switching on/off.
Variable speed drive (VSD) Outdoor A/C unit Condensor air Indoor AC unit
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VSD maintains condensing pressure more efficiently than switching the condensor fan on and off, reducing energyconsumption of AC unit Figure 18: Condensing
pressure of a split air
conditioning unit maintained by a variable speed drive controller
Opportunity for optimisation
The following control strategy and control parameters are recommended in order to maximise the energy efficiency of refrigeration systems while not compromising their performance:
• Determine the optimal head pressure or calculate for a floating head pressure. Typically, air- cooled condensing units have the condensing temperature 8–12°C above the ambient dry bulb temperature.
• Maintain the optimal head pressure using VSD-controlled motors (or EC motors) on condenser fans to control the amount of heat rejection by controlling the speed of the fans.
Figure 18 outlines the maintenance of optimal condensing pressure via a VSD controller, which minimises energy consumption of an AC unit.
Energy-saving potential, costs, benefits and risks
Variable head pressure control is typically a cost-effective HVAC energy-efficiency improvement. It does require some investment in new drives and/or motors; however, it can immediately reduce the energy consumption of the system by reducing the fan power.
This strategy can save up to 30 per cent of energy consumed by condenser fans. Benefits arising from controlling the head pressure of air-cooled condensers include: • reduced energy costs
• reduced GHG emissions
• quiet operation, especially during night time when ambient temperature and heat rejection is low • improved reliability of refrigeration systems
• reduced maintenance costs.
Problems that can be caused by inappropriate head pressure include: • reduced capacity of AC system
• automated switching off of AC compressors for safety reasons
• insufficient lubricating oil transfer to AC compressors through the AC system • damage to AC compressors.
Application notes
Maintenance of head pressure using condenser fan speed control can be successfully applied to any air-cooled AC system that operates on the vapour compression refrigeration cycle principle.
Getting started
Variable head pressure control strategy optimisation for air-cooled condensers can be implemented by establishing the head pressure set point for the equipment and refrigerant used, selecting and applying a fan control strategy for modulation of the amount of heat rejection, and determining and monitoring the head pressure and outdoor air temperature.