Reducing refrigeration plant running costs with variable-speed drives
Refrigeration represents a significant cost for the meat and poultry industry, but huge energy savings are available by installing variable speed inverter drives on compressors, condensers and pumps. Installing ABB VSDs can be a major contributor to energy savings in refrigeration, helping meat processors cut production costs and remain competitive.
Screw and reciprocating compressors and evaporative condensers are widely used throughout industrial refrigeration applications, from cold rooms through to blast freezing. When refrigeration demand is low, compressors are switched off in sequence and restarted when demand increases. Condenser fans use on-off control, sometimes enhanced by two-speed operation. Evaporator fans are stop started as required. A greater awareness of operational costs, particularly energy consumption, and the need to use more environmentally friendly refrigeration systems, has led to greater use of variable-speed inverter drives (VSDs) and high efficiency motors.
VSDs provide an efficient tool to optimise energy consumption of conditioned atmosphere and refrigeration processes. Since cold storages and fridge plants typically run 24 hours, most refrigeration drive applications are potential VSD candidates. Any refrigeration system with a wide variation in operating hours or with a heat load that is less than the peak load can benefit from a VSD.
Using VSDs, the speed of the motor can be slowed down, thereby reducing the amount of energy consumed. A VSD can be retrofitted to many existing systems, with mechanical controls locked open and the VSD controlling the speed of the electric motor.
Traditional systems use compressors that run at fixed speed, usually 3,000 rpm, with capacity control via slide valve operation. Capacity is reduced by recirculating compressed gas back through the compressor. However power consumption does not reduce in proportion with capacity, making fixed speed screw compressors inefficient.
Using variable-speed control, the slide valve can be left in the 100 percent position with capacity lowered by reducing operating speed. No recirculation occurs, saving significant energy, typically 15 to 25 percent. Variable-speed control also enables screw compressor capacity to be increased by over-speeding up to 60 Hz.
Chilled water and glycol pumps
Where chilled water or glycol is the cooling medium, it needs to be circulated around the plant by means of a pump. Chilled water/ glycol supply temperatures typically range from -8 °C to greater than +10 °C with the return temperature some 5 °C higher.
This temperature increase resulting from the heat drawn from a production area or cold store is the key parameter for the control system, referred to as delta T. Once this is selected, all equipment must be designed to operate within that range.
The best form of control is obtained by varying the flow of the cooling medium by using variable flow pumps. In traditional systems, however, flow is often preset with a flow control valve. VSD control allows the control valves to be left in the 100 percent flow position. Electrical power savings are based on the cube of the speed reduction. A five percent reduction in pump speed reduces absorbed power by 15 percent, while a 10 percent reduction cuts power by 30 percent.
A condenser receives superheated compressed gas from the compressor where it is cooled to a liquid. Simple control systems use conventional contactor switching to turn the condenser fans on or off to maintain a fixed system discharge pressure.
Increasingly VSDs are used to drive the condenser fans. The fans may work a little harder and consume more energy but the corresponding reduction in discharge pressure saves a proportionally greater amount of energy by reducing compressor load.
Variable-speed condenser fan control sets the speed of the fans to ensure correct condensing conditions, optimising fan power consumption and overall system efficiency. When the correct operating conditions are established, on off cycling of the fans is minimised, while lower air velocities reduce condenser fouling. Refrigeration efficiency can be improved by up to 15 percent.
See ABB Article Energy saving with VSD controlled compressors
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