Refrigerant Flow Control
For the vast majority of water chillers, modulation means that control the refrigerant mass flow through the evaporator. Depending on the type of compressor used, several methods are applied:
1. Piston CompressorsMain refrigerant flow is controlled expansion valve, which responds to the temperature of the gas refrigerant from the evaporator to save the set value. Regulation of the activities of the expansion valve causes an increase in pressure in the system and the compressor should then reduce its consumption of the refrigerant to maintain its pressure setpoint discharge.
The most common methods to reduce the refrigerant flow and capacity, reciprocating compressors, opening the suction valves, bypassing the gaseous refrigerant in the compressor, or bypassing the gas refrigerant flow externally compressor.
With the first method, called discharge, external drives can keep the suction valves, on one or more of compressor cylinders, open. Thus, compression can occur in these cylinders and gas flow through them is reduced to zero.
With another approach, called hot gas bypass solenoid valve high-pressure discharge one or more cylinders can open and distract the refrigerant flow back to the suction side of the cylinder.
This effectively reduces the pressure drop or Elevator produced cylinder and reduces the flow rate of a gas-refrigerant from the evaporator.
Unloading occurs in various steps, depending on the number of cylinders in each compressor and the number of compressors in each chiller. During the four-cylinder compressor, there are four stages (or stages) to control the power, 25-50-75-100%. If there are two compressors in the car, this yields a total of eight steps capacity control. Obviously, the greater the number of compressors and a large number of cylinders in each compressor, smooth capacity of the line of control.
2. Screw CompressorsWith screw compressor piston compressor, regulation can reduce suction gas flow of the refrigerant in the compressor. Regulatory control method is desirable to produce essentially endless regulation of power between the minimum and maximum flow rates and capacity.
Slide-valve hot gas bypass control valve with the sliding action parallel to the hole of the rotor is located on a high discharge pressure of the compressor. The valve is then modulated return a variable part of the discharge gas back to the compressor. This valve, in addition to the control of power, and also adjusts the location of the compressor discharge port, load change. This axial port of discharge then provides a good portion of the load without compromising performance at full load efficiency.
3. Centrifugal CompressorsRefrigerant gas flow in a centrifugal compressors can be controlled by adjustable inlet guide vanes, or preswirl or prerotation blades, as in the case of centrifugal fan. These blades are radially at the entrance in the impeller compressor and can be opened and closed from external operator.
Because each vane rotates about the axial shaft, they affect the flow direction of the entrance in the impeller. At the entrance of the blade is completely opened, the gas enters the impeller 90 to the wheel. However, as the inlet vanes begin to close, enters the impeller at increasing the angle in the direction of radial flow along the fan blades. This preswirl condition reduces the ability of the impeller to give the kinetic energy of the refrigerant gas, thus reducing the flow velocity.
Entrance blades causes a drop in pressure or regulation to reduce the refrigerant flow through the centrifugal compressor.
Minimum volume flow rate through the centrifugal compressor is required for stable operation. If the volumetric flow rate falls below that minimum, the compressor will become unstable and overvoltage. When this happens, the refrigerant flows in addition to and fro through the compressor, to generate noise and bad work. Extended operation under the surge conditions will cause mechanical damage to the compressor. Splash envelope will differ from the compressor, the compressor, but, as a rule, occurs when the volumetric flow rate is reduced by 40-60%.
To prevent surge occurs, internal hot-gas bypass can be used to allow the ability to be reduced while maintaining sufficient gas flow through the compressor. (This fact, along with the increasing windage losses and motor inefficiency, is part of the load characteristics shown in Fig. 1.7.)
In recent years, the power regulator centrifugal chillers on the cruise control has been applied. Here a large VFD is used for cooling the motor and the motor speed modulation to control the output. As a rule, cruise control increases efficiency over the inlet vane control down about 55% of rated power, all the inlet vane control more effectively below 55% of nominal capacity.
Speed of rotation of the compressor directly involved, but pressure (Elevator), production of refrigeration compressors is a function of the square of the speed. This can lead to poor performance and a splash of which require the use of hot gas bypass, as the cooler unloads under the control of speed.
Adding cruise control significantly increases the price hmm, this parameter should be carefully analysed to determine whether the concept of cost-effective for the cooling load profile...