Superheat control by pressure ratio

A control method regulates an electronic expansion valve of a chiller to maintain the refrigerant leaving a DX evaporator at a desired or target superheat that is minimally above saturation. The expansion valve is controlled to convey a desired mass flow rate, wherein valve adjustments are based on the actual mass flow rate times a ratio of a desired saturation pressure to the suction pressure of the chiller. The suction temperature helps determine the desired saturation pressure. A temperature-related variable is asymmetrically filtered to provide the expansion valve with appropriate responsiveness depending on whether the chiller is operating in a superheated range, a saturation range, or in a desired range between the two.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The subject invention generally pertains to the control of air conditioners and heat pumps that have a direct-expansion evaporator (DX evaporator), and the invention more specifically pertains to maintaining the refrigerant leaving the evaporator at a desired minimal level of superheat.

2. Description of Related Art

Many refrigerant systems (chillers) have a DX evaporator in which a refrigerant absorbs heat while expanding from a liquid to a gaseous state directly inside the evaporator. The absorbed heat can cool air supplied to a comfort zone or cool an intermediate fluid such as chilled water. If the chiller functions as a heat pump, heat absorbed by the evaporator can be released to the comfort zone by way of a condenser.

The heat transfer coefficient across the tube walls of a DX evaporator is generally greatest when the refrigerant inside the tubes is saturated, partially liquid, rather than superheated to a gas. Liquid refrigerant, unfortunately, can damage a compressor, which draws the refrigerant from the evaporator. So ideally, the refrigerant enters the DX evaporator as a liquid and is not completely vaporized until just prior to leaving for the inlet of the compressor.

To this end, expansion valves, which controllably feed refrigerant from the condenser into the evaporator, are controlled so as to achieve a desired minimal amount of superheat within the evaporator. Examples of superheat-related controllers are disclosed in U.S. Pat. Nos. 4,505,125; 4,523,435; 4,527,399; 5,067,556; 5,187,944; 5,987,907 and 6,032,473. There is a common problem, however, facing perhaps all superheat-related controllers.

During steady state operation near a desired minimal superheat condition, the expansion valve controller preferably has a relatively low gain or response, as a slight adjustment to the opening or closing of the expansion valve can have a dramatic effect on the degree of superheat. The chiller, however, may not always be operating at this optimum steady state condition. Although a slight movement of the expansion valve can produce an appropriate change in superheat when operating just above the desired saturation point, that same amount of movement in opening may be insufficient when operating at greater levels of superheat. Thus, an expansion valve “tuned” for optimum response when operating at slightly above saturation may be too sluggish under conditions of greater superheat or no superheat (in saturation).

One conceivable solution may be to attempt identifying the nonlinear relationship between the amount of superheat and the opening of the expansion valve and adjust the response of the valve accordingly. The nonlinear relationship, however, is not necessarily a static relationship, particularly in cases where the chiller has varying load capability. Many systems vary the load by selectively unloading a compressor, selectively operating multiple compressors, selectively energizing multiple evaporator fans, varying the speed of an evaporator fan, etc. A controller could monitor such load-varying events and try to adjust the expansion valve's response accordingly, but such an approach becomes a daunting challenge, as the effect that each of these events has on the superheat needs to be accurately quantified, not only for when the events occur alone but also when they occur in various combinations with each other.

Consequently, a need exists for a better method of controlling the operation of an expansion valve to maintain a desired minimal level of superheat over widely varying load conditions.

SUMMARY OF THE INVENTION

A primary object of the invention is to maintain the refrigerant leaving an evaporator at a desired level of superheat.

Another object of some embodiments is to achieve the desired superheat by controlling the suction pressure of a chiller.

Another object of some embodiments is to dampen or filter (digitally or otherwise) the reading of the suction temperature to slow down the increase in suction pressure.

Another object of some embodiments is to asymmetrically filter a temperature-related variable to avoid saturation (between the evaporator and the compressor inlet) and to allow rapid response to load reductions, which tend to reduce the superheat.

Another object of some embodiments is to adjust an electronic expansion valve based on a pressure ratio of a desired saturation pressure divided by the suction pressure.

Another object of some embodiments is to determine a desired or target mass flow rate and an actual refrigerant flow rate through an electronic expansion valve, or through a refrigerant-conveying structure connected in series therewith (e.g., evaporator, condenser, compressor, conduit, etc.), and control the expansion valve accordingly.

Another object of some embodiments is to determine a target mass flow rate based upon the suction pressure and the suction temperature, wherein the suction temperature helps determine a desired saturation temperature, the desired saturation temperature helps determine a desired saturation pressure, and the desired saturation pressure helps determine the target mass flow rate.

Another object of some embodiments is to determine the actual mass flow rate through an expansion valve by sensing the pressure drop across the valve and multiplying the square root of that times a flow coefficient of the valve, wherein the flow coefficient is based on the physical characteristics of the valve and the degree to which a controller has commanded the valve to open.

Another object of some embodiments is to control an expansion valve more rapidly (higher gain, larger response) during superheated operation than during desired superheat operation, and to control the expansion valve less rapidly during superheated operation than during saturation operation. Saturation operation is when the suction temperature is at the saturation temperature, superheated operation is when the suction temperature is above a target temperature defined as the saturation temperature plus a desired superheat, and desired superheat operation is when the chiller is operating between superheated and saturation operation.

One or more of these and/or other objects of the invention are provided by a method that maintains the refrigerant leaving an evaporator at a desired level of superheat by adjusting an electronic expansion valve in response to sensing a chiller's suction pressure and temperature.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1schematically illustrates a controller10that regulates an electronic expansion valve12of a chiller14to maintain the refrigerant leaving a DX evaporator16at a desired or target superheat that is minimally above saturation. Electronic expansion valve12is schematically illustrated to represent any electrically adjustable flow restriction of which there are many different types well known to those of ordinary skill in the art. Controller10is schematically illustrated to represent any electronic or programmable device capable of performing the steps specified in this description and the claims. Examples of controller10include, but are not limited to, a computer, microprocessor, analog circuit, digital circuit, and various combinations thereof.

Chiller14is schematically illustrated to represent any refrigerant system that includes a compressor, a heat exchanger such as an evaporator for absorbing heat, a heat exchanger such as a condenser for releasing heat, and an expansion valve for providing a controllable flow restriction between the condenser and evaporator. Although in its simplest form chiller14comprises a compressor18, a condenser20, expansion valve12, and evaporator16, chiller14can be much more complicated. Chiller14, for instance, may include multiple compressors for varying load, a variable capacity compressor, multiple or variable speed fans associated with evaporator16or condenser20, reversing capability (heat pump) for switching between heating and cooling modes, etc.

In operation, the compressor18raises the pressure and temperature of gaseous refrigerant and discharges the refrigerant gas into the condenser20. A first external fluid, such as water or air, cools and condenses the refrigerant inside the condenser20. Expansion valve12conveys the condensed refrigerant from the higher-pressure condenser20to the lower-pressure evaporator16. Upon passing through valve12and entering evaporator16, the refrigerant begins expanding and cooling. The cool refrigerant passing through evaporator16absorbs heat from a second external fluid that vaporizes the refrigerant before the refrigerant returns to a suction inlet22of compressor18for recompression. Depending on whether the system is used for heating or cooling, the heat released or absorbed by condenser20and evaporator16can be useful or waste heat.

For maximum efficiency and compressor reliability, chiller14preferably operates where the suction temperature of the refrigerant leaving evaporator16is at a target superheat as indicated by line24ofFIG. 2. Line26ofFIG. 2represents the temperature of the fluid being cooled by evaporator16. The target superheat may be where the suction temperature, for example, is two degrees Fahrenheit above saturation, wherein the saturation threshold is represented by line28. The suction temperature is, for example, preferably at a point30at full load and at a point32at reduced load (e.g., partially unloaded compressor, fewer operating compressors, etc.). Although the actual suction temperature may vary along a curve34under full load, controller10regulates expansion valve12to bring the suction temperature to point30. Likewise, the suction temperature may fluctuate along a curve36during part-load operation.

To sense the suction temperature and provide controller10with suction temperature feedback72, a conventional temperature sensor38can be installed generally between evaporator16and suction inlet22. Sensor38can be attached directly to evaporator16near its outlet, attached to compressor18near its inlet, or attached to a refrigerant line40running between evaporator16and compressor18.

To sense the suction pressure and provide controller10with suction pressure feedback74corresponding to saturated suction temperature for the calculation of superheat, a conventional pressure sensor60can be installed somewhere downstream of valve12and upstream of compressor inlet22. Pressure sensor60is preferably installed downstream of evaporator16to avoid having to consider the pressure drop across evaporator16although the pressure sensor60could be installed elsewhere if the pressure drop was accounted for.

The challenge of maintaining the operation of chiller14on target superheat line24may be better understood with reference toFIG. 3. InFIG. 3, curves42and44, lines24′ and28′, and points30′ and32′ respectively correspond to curves36and34, lines24and28, and points30and32ofFIG. 2. A relatively steep slope46or tangent of curve44at point30′ indicates that a small change48in the opening of expansion valve12causes a significant change50in the level of superheat. Thus, the rate in which controller10adjusts the opening of valve12is preferably rather slow to avoid overshooting point30or30′. If this slow responsiveness is maintained when the superheat rises to a point52, which is on a more level portion of curve44, controller10and valve12may bring the suction temperature back to point30or30′ at an unnecessarily slow rate. With a slope54or tangent of curve44at point52being more level than slope46, it is clear that even a small change56in superheat requires a substantial change58in the opening of valve12.

When operating in the saturated range, such as at a point51, it may take an even larger, more drastic change in the opening of valve12to return to the target superheat because the slope of curve44and42at point51is essentially zero.

Although conceivably the gain or responsiveness could be adjusted depending on what point along curve44that chiller14is operating, in reality that may be impractical, as the shape of the curve can change. The shape, for instance, can change from curve44to curve42depending on the load and numerous other factors.

Rather than regulating valve12directly in response to the superheat, controller10regulates valve12in response to suction pressure feedback74from pressure sensor60and suction temperature feedback72from temperature sensor38. In response to suction pressure feedback74and suction temperature feedback72, controller10provides an output signal62that commands expansion valve12to convey a target mass flow rate, which will drive the suction temperature at an appropriate rate toward a desired saturation temperature that achieves the target superheat.

Controller10generates output signal62upon comparing a target mass flow rate64to the actual mass flow rate66through valve12. Although the actual mass flow rate66can be measured directly using a flow meter, in a currently preferred embodiment, controller10calculates the actual flow rate as being the product of the known flow coefficient of valve12times the square root of a pressure differential across valve12. Determining the pressure differential across valve12may involve sensing a discharge pressure (discharge pressure feedback68) via a pressure sensor70installed somewhere downstream of compressor18and upstream of valve12. The pressure drop across valve12would then be approximated by the difference between the discharge pressure (signal68) and the suction pressure (signal74). The actual flow coefficient of valve12would of course be a function of the degree to which valve12is open, however, controller10is aware of the valve's degree of opening, as it is controller10that commands the operation of valve12.

Controller10calculates the target mass flow rate64as being the product of the actual mass flow rate66times a pressure ratio, wherein the pressure ratio is a function of the suction pressure (signal74) and the suction temperature (signal72). More specifically, the ratio can be considered as a desired saturation pressure divided by the sensed suction pressure. Since refrigerants have a known relationship between their saturation temperature and their saturation pressure, the desired saturation pressure is determined based on its corresponding desired saturation temperature, wherein the desired saturation temperature is calculated. The desired saturation temperature equals the suction temperature (sensed by temperature sensor38) minus a predetermined desired target superheat (e.g., 2-degrees Fahrenheit).

An alternative to the use of a pressure ratio is the use of a density ratio, such that the target mass flow rate is the product of the actual mass flow rate times the density ratio. Specifically, the density ratio can be considered as the density of the desired suction refrigerant state divided by the density of the measured suction refrigerant state. The density ratio is an “ideal” alternative because the density ratio is related directly and linearly to the mass flow rate through a compressor operating at a constant volumetric flow rate. The density of the measured suction refrigerant state can be determined from the pressure and temperature of a vapor measured in the suction line, while the density of the desired suction refrigerant state can be determined from the suction pressure, the suction temperature and the superheat setpoint. Compressors in chillers with DX evaporators typically operate on the principle of a fixed suction volumetric flow rate corresponding to any particular load adjustment. For a single refrigerant circuit with non-branched flow, the mass flow rate through the compressor must equal the mass flow rate through the expansion valve over time. The pressure ratio can be computed without performing refrigerant density computations and is an adequate approximation of the density ratio.

To ensure that valve12responds at an appropriate rate regardless if chiller14is operating in a saturated range78(on line28ofFIG. 2), in a superheated range80(appreciably above line24), or within a desired superheat range82(substantially on line24), controller10determines the desired saturation temperature (for ultimately determining the target mass flow rate) by asymmetrically filtering (i.e., asymmetrically dampening) a temperature-related variable. Controller10, for instance, asymmetrically filters the sensed suction temperature (to generate the filtered suction temperature) or asymmetrically filters the desired saturation temperature (to generate the desired filtered saturation temperature).FIGS. 4 and 5show how the change in the filtered value varies as an asymmetric and nonlinear function of suction temperature. Regardless of whether the filtering or dampening is applied to the sensed suction temperature or the target saturated temperature, the end result is the same. Controller10adjusts expansion valve12more rapidly when chiller10operates in superheated range80than when operating in the desired superheat range82, and controller10adjusts expansion valve12less rapidly when chiller10operates in the superheated range80than when operating in the saturated range78.

The above-described operational steps performed physically or carried out logically according to a control algorithm of controller10are illustrated inFIG. 6. The steps are not necessarily performed in discrete, independent steps; the steps are not necessarily done in the order in which they are shown; and not all of the illustrated steps are necessarily required to accomplish the invention.

A block84represents the step of sensing the suction pressure via pressure sensor60. A block86represents the step of sensing the suction temperature via temperature sensor38. A block88illustrates pressure sensor70sensing the discharge pressure. The actual mass flow rate through valve12(or an equivalent mass flow through evaporator16, condenser20, or compressor18) can be measured in various ways including, but not limited to, as discussed previously, by using a flow meter or by referring to certain known performance characteristics of compressor18. In block90, the actual mass flow rate is calculated generally as the square root of the pressure drop across valve12(approximated by the square root of the difference between the discharge pressure and the suction pressure) times a known operating characteristic of valve12. A block92illustrates the step of determining a target superheat, which can be a predetermined value permanently stored in controller10, or the superheat value can be a user-selected value.

A block98represents the step of determining a desired saturation temperature (Tsat sp) based upon the suction temperature (Tsuc) decreased by the target superheat (S/Hsp), and a block94illustrates asymmetrically filtering the desired saturation temperature to achieve a desired filtered saturation temperature (filtered Tsat sp). Alternatively, a block96illustrates asymmetrically filtering a sensed reading of the suction temperature to achieve a filtered suction temperature (filtered Tsuc), and a block97represents the step of determining a desired filtered saturation temperature (filtered Tsat sp) based upon the filtered suction temperature (filtered Tsuc) decreased by the target superheat (S/Hsp).

Either blocks98and94or blocks96and97can be used for selectively dampening the response of valve12so that the expansion valve is more responsive under certain conditions, such as when the refrigerant is excessively superheated and even more responsive when the refrigerant is saturated or nearly so.

A block100illustrates the desired saturation pressure (Psp) being determined based on its known relationship to its corresponding desired filtered saturation temperature (filtered Tsat sp). A block102shows the step of determining the target mass flow rate (msp=mact(Psp/Psuc)) through expansion valve12that could achieve the target superheat, wherein the target mass flow rate is at least partially determined based on the suction pressure (Psuc). An alternative implementation of block102determines the target mass flow rate (msp=mact(ρsp/ρsuc)) through expansion valve12that could achieve the target superheat, wherein the target mass flow rate is at least partially determined based on the suction density (ρsuc) A block104shows the step of adjusting or controlling expansion valve12to help maintain the actual mass flow rate at the target mass flow rate.

Blocks102and104are shown as separate steps in order to disclose the pressure ratio (alternatively density ratio) basis for determining the ratio of mass flow rate through the evaporator. For implementation, these blocks may be combined into one step of adjusting or controlling expansion valve12to maintain the actual suction pressure at the desired saturation pressure (Psp). In such an implementation, the ratio of actual mass flow rate to suction pressure (mact/ρsuc) serves as a conversion factor from pressure units of the feedback signal to mass flow rate units of the expansion valve determining output.

Although the invention is described with reference to a preferred embodiment, it should be appreciated by those of ordinary skill in the art that other variations are well within the scope of the invention. Therefore, the scope of the invention is to be determined by reference to the following claims: