Accuracy, Reliability and Repeatability
By Dave DemmaHVAC Systems Refrigeration
What steps are necessary to achieve optimum valve performance?
Several years ago I submitted an article (see Sept/Oct 2010 in the archives) on electric expansion valves (EEVs), entitled “Repeatability…that is what it is all about.” The foundation for the article was how the expansion valve played a very key role in determining the relative efficiency (or inefficiency, as the case may be) in the total system performance.
As the original title suggested, it is all about repeatability. Repeatability of what? The answer is to repeatedly provide a constant amount of superheat at the evaporator outlet with a minimum of variation and under the extremes of varying ambient and load conditions. And, to do so will allow the system to maintain maximum efficiency and consistency in performance. A good comparison of the EEV’s ability versus that of a thermostatic expansion valve (TEV) is shown in Figure 1. The EEV’s superheat characteristics are contrasted with those of a mechanical TEV, on the same refrigerated display case operated under constant conditions in a laboratory controlled environment. Figure 2 shows the comparative results of what percentage of run-time the EEV and TEV were each within a certain +/- value of set-point. Contrast the EEV being within +/- 1º superheat nearly 90 per cent of the time, whereas the mechanical TEV was within +/- 1º superheat a meager 35 per cent of the time.
There are two keys to the accuracy, reliability and repeatability of the EEV.
1. The most effective variety of EEVs will employ a step motor to drive the valve open or closed. Unlike common induction or commutated motors, which are designed to rotate continuously, a step motor has the ability to rotate a small fraction of a revolution as it receives the step signal sent by its controller. As applied in EEVs, the step motor transforms its rotation into linear movement by employing a digital linear actuator (DLA); a simple gear train, which then turns a threaded shaft, accomplishing valve opening/closing. An added benefit of the DLA is the available increased linear force required to close the valve’s piston against the typical pressure differential present across the valve’s port.
2. While the EEV has the potential to provide a much more stable superheat than its mechanical counterpart, it only operates in response to the controller’s signal. In a sense it is a “stupid” component. It must have a controller outfitted with a well conceived algorithm (program), which has been properly fine tuned in the field for its particular application. It is all about the expertise and experience of the person/persons creating the algorithm, and the field personnel’s ability to fine tune the adjustable parameters.
Given that the function of an expansion valve is to maintain superheat at the evaporator outlet, temperature and pressure from that location is supplied to the EEV controller via a temperature sensor and pressure transducer. The controller is outfitted with a sophisticated PID (proportional, integral, derivative) algorithm, which has been designed to calculate the superheat from the temperature and pressure data supplied to it, and then drive step motor to the position required to maintain the superheat set point. Without going into a lot of technical jargon to describe the function of the PID, let’s simply state this:
The ‘P’ function is to allow the controller to change the output (valve position) in proportion to the input (superheat). The I function will sense the average deviation from the set point and apply an offset to compensate for this deviation, which is continually changing in response to the system load and system condition changes. The ‘D’ function will sense the superheat’s rate of change, and uses this to attempt a prediction of future valve position.
It is this complex set of instructions that will constantly monitor the superheat and minutely drive the EEV open or closed in either small or large increments, to meet constantly changing conditions. For example, a large tonnage EEV applied on a chiller would have approximately 6400 steps of stoke, from fully open to fully closed. That is a linear distance of .0000783 inches per step of resolution.
If an EEV can provide superior performance in maintaining superheat at the evaporator outlet, could the same technology be used in other system applications to provide similar results? Absolutely. The following are some of the more common regulating valve applications that can be replaced by state of the art electric valves/controllers.
Evaporator Discharge Air Temperature Control
1. Discharge Bypass Valve (HGB): used in comfort cooling applications that experience low load conditions, resulting in periods when the SST might fall below 26F. This is normally the range when frost/ice will start to form on the evaporator fin-tube surface. If allowed to operate at this condition, the evaporator’s ability to transfer heat will be reduced, resulting in an inability to maintain the desired space temperature. If the frost/ice buildup becomes severe, it could result in an inability to maintain the superheat setpoint, allowing liquid refrigerant to flow to the compressor and damage it. Bypassing discharge vapour to the system’s low side will artificially raise the low side pressure to the required SST.
2. Hot Gas Bypass Valve (HGB): used in air conditioning or refrigeration single compressor applications that require very precise temperature (a tolerance of +/- 1F). Applications such as this might utilize a HGB, with a thermostat controlled solenoid valve connected upstream of the HGB to vary the frequency of the bypassed discharge vapour. While a little cumbersome with the solenoid valve cycling several times per minute and the resulting maintenance issues that can occur with a valve that is so frequently cycled, this can result in very good temperature control.
3. Evaporator Pressure Regulator (EPR): typically used in a refrigeration application with multiple compressors and multiple evaporator systems (common supermarket application). A slightly different application than utilizing discharge bypass to maintain the evaporator discharge air temperature, but practical due to multiple evaporator systems operating at varying temperatures and multiple compressors operating at a relatively constant SST. Whereas the discharge bypass valve will maintain a constant system low side pressure, preventing it from falling below a predetermined setpoint, the evaporator pressure regulator will only maintain a constant evaporator pressure, preventing it from falling below a predetermined set-point. The evaporator pressure regulating valve would be located between the evaporator outlet and the compressor inlet.
The first problem with all of these applications is that the mechanical pressure regulating valve is not actually controlling the system parameter that the specifications are requiring. It is controlling pressure, which in turn has an influence on discharge air temperature. There are other system conditions such as product/space load, amount and condition of infiltrating air, ambient temperature, system operating conditions, which will influence what that discharge air temperature will be. Reacting to those conditions is completely outside of the ability of the mechanical pressure regulating valve. Note: While example two will cycle the hot gas bypass valve in response to temperature, the control strategy of using a thermostat to cycle the solenoid valve supplying discharge gas to the HGB is at best a little archaic.
Using an electric valve in either of these applications will provide a valve that responds to the system parameter whose control is desired–temperature. Pairing the appropriate electric valve with the corresponding electronic controller and
sensors will allow the valve to respond to discharge air temperature.
The electric hot gas bypass valve (EHGB) application would be configured as shown in Figure 3, with discharge air sensor providing the digital signal to the controller, which will determine when the EHGB is required to throttle open or closed.
The mechanical DBV is designed to maintain a constant pressure in the low side of the system, resulting in a fairly constant system SST. The EHGB is an application where the controller/bypass valve is responding to the parameter whose control is desired–evaporator discharge air temperature. As such, this is a temperature responsive application, and not a pressure regulating application.
The mechanical EPR valve is designed to maintain a constant pressure in the evaporator, resulting in a fairly constant evaporator SST. This will yield a fairly constant discharge air temperature. The EEPR application shown in Figure 4 is configured similarly to the EHGB application, with the difference being that discharge air temperature is maintained by modulating the evaporator SST. Again, the conventional pressure regulating valve, in this case a mechanical EPR valve, maintains a constant pressure in the evaporator, which in turn will maintain a somewhat constant evaporator discharge air temperature. The EEPR application will throttle the valve open/closed, varying the evaporator SST as needed, to maintain a constant evaporator discharge air temperature.
While Figure 4 shows a typical stand alone EEPR application, the more common application would be on a multi compressor rack, with multiple evaporator systems operating a varying temperature set-points.
Given the electric valve’s range of steps between fully open and fully closed (somewhere between 2500 and 6386, depending on the valve model) and the fact that as a step motor it is able to throttle in either direction a relatively few number of steps at a time, these applications yield precise control in the range of +/- 0.5F. The beauty of electric valves and electronic controllers is that they are typically able to compensate for changes in system load, operating conditions and other system parameters that will have an effect on temperature control. Because they do not require springs and diaphragms to facilitate valve opening/closing, they are not subject to the inefficiencies in operation that hysteresis causes in mechanical valves.
While computers and their controlling algorithms may be complex in nature, they do exactly what they are designed to do without variation. You can rely on the fact that their control will be reliable and repeatable too.
The final upside is that there is no time consuming mystery in the setup. You simply set the controller for the desired temperature and that is it. <>
Dave Demma holds a degree in refrigeration engineering and worked as a journeyman refrigeration technician before moving into the manufacturing sector where he regularly trains contractor and engineering groups. He can be reached at email@example.com.