ELECTRONIC EXPANSION VALVES

The electronic expansion valve offers a finer degree of control and system pro­tection. The benefits can be summarized as:

1. Precise flow control over a wide range of capacities.

2. Rapid response to load changes.

3. Better control at low superheats so that less evaporator surface is required for superheat. More surface for evaporation results in higher evaporating temperature and better efficiency.

4. Electrical connection between components offers greater flexibility in system layout, which is important for compact systems.

5. The valve can close when the system shuts down, which eliminates the need for an additional shut off solenoid valve.

Types of electronic valve in use include a continuous flow type in which the orifice size is varied by a stepper motor, and a pulse width modulated (PWM) type. In each case a controller is used in conjunction with the valve. The con­troller is pre-configured for the refrigerant and valve type and it receives the information from sensors, for example, pressure and temperature at the evapor­ator outlet. This enables the superheat to be determined. The output signal to the valve initiates the orifice adjustment. In the case of the PWM valve it is the rela­tionship between the opening and closing which determines the capacity of the valve. The valve is either open or closed and each time interval of a few seconds will include an opening period depending on the signal.

There is a third type of valve that combines both features. A modulating voltage is sent to the actuator, and as the voltage increases the pressure in the actuator’s container increases, resulting in an increased valve opening during an ‘on cycle’ of fixed duration.

In each case the control can be configured so that the valve remains closed in the event of power loss. Under partial load condition or floating condensing pressure, which happens at low ambient temperature, the condensing pressure decreases. Thermostatic expansion valves tend to hunt, but systems with elec­tronic components operate at partial load in exactly the same and stable manner as at full load.

A continuous flow type valve is shown in Figure 8.5 . The valve seat and slider are made out of solid ceramic. The form of the valve slide provides for a highly linear capacity characteristic between 10% and 100%. Depending on the controller and its configuration, a single control valve can be used for different control tasks. Possible uses include: expansion valve for superheat control, suc­tion pressure control for capacity control, liquid injection for de-superheating of compressor, condensing pressure control and hot gas bypass control to com­pensate excess compressor capacity and to ensure evaporating pressure does not go below a set point.

ELECTRONIC EXPANSION VALVES

ELECTRONIC EXPANSION VALVES

1 Stainless steel body

2 Stepper motor

3 Electrical connector

4 Cage assembly

5 Shaft

6 Welding and/or brazing

7 Ceramic inlet port

8 Ceramic slide

9 Ceramic outlet port

10 Brass ball

4

8

9

6

2

3

Open Partially open Closed

(c)

подпись: 
open partially open closed
(c)
(b)

Figure 8.5 (Continued) (b) Sectional view, (c) Sliding orifice (Emerson Climate Technologies)

Posted in Refrigeration and Air Conditioning