Patent Description:
Screw compressors are commonly used in air conditioning and refrigeration applications. In such compressors, intermeshed male and female lobed rotors or screws are rotated about their axes to pump a working fluid, such as refrigerant, from a low pressure inlet end to a high pressure outlet end. A screw compressor having fixed inlet and discharge ports built into the housing are optimized for a specific set of suction and discharge conditions and pressures. However, the system in which the compressor is connected rarely operates under constant conditions, especially in an air conditioning application. Nighttime, daytime, and seasonal temperatures can affect the volume ratio of the system and the efficiency with which the compressor operates. Volume ratio or volume index (VI) is the ratio of the volume of vapor inside the compressor as the suction port closes to the volume of vapor inside the compressor as the discharge port opens. Screw compressors, scroll compressors, and other similar machines generally have a fixed volume index based on the geometry of the compressor.

In a system where the load varies, the amount of heat being rejected in the condenser fluctuates causing the high side pressure to rise or fall, and resulting in a volume index different from the compressor's fixed volume index. To improve efficiency, the pressure inside the compressor should be generally equal to the pressure in the discharge line from the compressor. If the inside pressure exceeds the discharge pressure, over-compression of the gas occurs, and if the inside pressure is too low, back flow occurs, both resulting in a system efficiency loss. Therefore, the volume index of the compressor should vary to maximize the efficiency of the compressor at non-uniform operating conditions.

A volume index valve may be employed to selectively open and close at various points in the compression process to obtain better control of the volume index at different operating conditions, such as part load operation. However, the volume index valve does not offer feedback to determine if operational failure has occurred. Therefore, real-time operational monitoring of the volume index valve is unavailable. If a volume index valve is not operating properly with no monitoring, the overall system might undesirably operate at a lower efficiency than otherwise available.

<CIT> discloses a method for determining whether an operational compressor in a multiple-screw compressor water chiller is fully loaded by sending a relatively long duration test pulse to the load solenoid of the compressor.

<CIT> discloses a control system for a variable capacity rotary screw compressor driven by an electric motor and having an adjustable slide valve for varying compressor capacity.

According to a first aspect, a method of monitoring a volume index valve of a screw compressor is provided according to claim <NUM>.

Optionally, further embodiments may include recording a first plurality of readings of the operating condition when the volume index valve is in the first position. Also included is averaging the first plurality of readings. Further included is recording a second plurality of readings of the operating condition when the volume index valve is in the second position. Yet further included is averaging the second plurality of readings, wherein the difference calculated is a difference between the averaged first and second plurality of readings.

Optionally, further embodiments may include initiating an alert if the difference does not exceed the predetermined threshold.

Optionally, further embodiments may include maintaining the alert until the alert is manually reset.

Optionally, further embodiments may include that the compressor continues to operate when the alert is initiated.

Optionally, further embodiments may include that the operating condition is a variable frequency drive power of the compressor.

Optionally, further embodiments may include that the operating condition is a measured current of the compressor.

Optionally, further embodiments may include that the first position of the volume index valve is an open position and the second position of the volume index valve is a closed position.

Optionally, further embodiments may include that the first position of the volume index valve is a closed position and the second position of the volume index valve is an open position.

According to a second aspect, a system is provided according to claim <NUM>.

Optionally, further embodiments may include that the processing device initiates an alert if the difference is less than the predetermined threshold.

The subject matter which is regarded as the disclosure is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification.

Referring to <FIG>, an example of a screw compressor <NUM>, commonly used in air conditioning systems, is illustrated in more detail. The screw compressor <NUM> includes a housing assembly <NUM> containing a motor <NUM> and two or more intermeshing screw rotors <NUM>, <NUM> having respective central longitudinal axes A and B. In the illustrated embodiment, the rotor <NUM> has a male lobed body <NUM> extending between a first end <NUM> and a second end <NUM>. The male lobed body <NUM> is enmeshed with a female lobed body <NUM> of the other rotor <NUM>. The female lobed body <NUM> of the rotor <NUM> has a first end <NUM> and a second end <NUM>. Each rotor <NUM>, <NUM> includes shaft portions <NUM>, <NUM>, <NUM>, <NUM> extending from the first and second ends <NUM>, <NUM>, <NUM>, <NUM> of the associated male lobed body <NUM>, and female lobed body <NUM>. The shaft portions <NUM> and <NUM> are mounted to the housing <NUM> by one or more inlet bearings <NUM>, and the shaft portions <NUM>, <NUM> are mounted to the housing <NUM> by one or more outlet bearings <NUM> for rotation about the associated rotor axis A, B.

In the illustrated embodiment, the motor <NUM> and the shaft portion <NUM> of the rotor <NUM> may be coupled so that the motor <NUM> drives the rotor <NUM> about axis A. When so driven in an operative first direction, the rotor <NUM> drives the other rotor <NUM> in an opposite second direction. The housing assembly <NUM> includes a rotor housing <NUM> having an upstream/inlet end face <NUM> and a downstream/discharge end face <NUM> essentially coplanar with the rotor second ends <NUM>, <NUM>. Although a particular compressor type and configuration is illustrated and described herein, other compressors, such as having three rotors, for example, are within the scope of the invention.

The housing assembly <NUM> further comprises a motor/inlet housing <NUM> having a compressor inlet/suction port <NUM> at an upstream end and having a downstream face <NUM> mounted to the rotor housing upstream face <NUM> (e.g., by bolts through both housing pieces). The assembly <NUM> further includes an outlet/discharge housing <NUM> having an upstream face <NUM> mounted to the rotor housing downstream face <NUM> and having an outlet/discharge port <NUM>. The rotor housing <NUM>, the motor/inlet housing <NUM>, and outlet housing <NUM> may each be formed as castings subject to further finish machining. The refrigerant vapor enters into the inlet or suction port <NUM> with a suction pressure and exits the discharge port <NUM> of the compressor <NUM> with a discharge pressure. The refrigerant vapor within the compression mechanism of the two or more rotors <NUM>, <NUM>, between the inlet port <NUM> and the discharge port <NUM> has an intermediate pressure.

Referring now to <FIG>, with continued reference to <FIG>, a volume index valve <NUM> is positioned within the rotor housing <NUM>, adjacent to the discharge end <NUM>, <NUM> of the rotors <NUM>, <NUM>. The volume index valve provides a flow path for vapor from an intermediate point of the rotors <NUM>, <NUM> to the discharge port <NUM>, bypassing the last portion of the compression. The valve <NUM> moves automatically between a closed position and an open position in response to the operating pressure of the refrigerant vapor within the compressor <NUM> to control the bypass flow and thus the volume index of the compressor <NUM>. The valve <NUM> is controlled by an actuator. In some embodiments, the actuator is a solenoid actuator. Proper operation of the volume index valve <NUM> enables increased efficiency of the compressor <NUM> by actively controlling the fluid flow therethrough. This is particularly beneficial when the compressor is operated at part load, for example.

Referring now to <FIG>, a flow diagram illustrates a method <NUM> and system of monitoring operation of the volume index valve in the form of a diagnostic routine. Failure to ensure that the volume index valve <NUM> is opening and closing properly results in compressor operation at an efficiency that is lower than otherwise available with proper valve operation. The method and system advantageously provide verification that the volume index valve is opening and closing in a desired manner.

Automatic initiation <NUM> of the method is provided and based on a periodic timer to cause the method to be performed at a specified time interval. Upon initiation, the method includes waiting for normal and stable operation conditions of the compressor to be met <NUM> and/or stable operation conditions of the system that the compressor operates within. This may include ensuring that one or more operating modes are present and that stability has been satisfied for a specified period of time. For example, compressor temperature and/or pressure within a specified range over a minimum time period may be required to perform the method. Regarding stable operating conditions of the system that the compressor operates within, an example of a system that the compressor operates within is an air conditioning application. In such embodiments, a refrigerant flow rate, system pressure, system temperature, and system efficiency are examples of operating conditions that may be required to be within a specified range to perform the method. If the stability conditions are not met, the method is aborted.

Subsequent to the conditions for stability being met, detection and recordation of an operating condition of the compressor is made <NUM> with the volume index valve in a first state that corresponds to a first position. In some embodiments, a plurality of recordings are made over a given time interval with the volume index valve in the first position, with the recordings averaged to provide a single operating condition reading, referred to herein as a first reading. Alternatively, or in combination with averaging the recordings, the first reading may be determined by analysis, trending, filtering, etc. The preceding list is merely illustrative and is not intended to be limiting of analysis techniques that may be employed to determine the first reading. In some embodiments, the first state of the volume index valve corresponds to an energized (i.e., ON) state that provides a closed position of the volume index valve. Once sufficient data is recorded with the volume index valve in the first state (i.e., first position), the volume index valve is switched with a controller <NUM> (<FIG>) that is in operative communication with the volume index valve to a second state that corresponds to a second position. As with the first position, one or more readings are detected and recorded <NUM> with the volume index valve in the second position. In embodiments where a plurality of recordings is made, the recordings are averaged to provide a single compressor operating condition reading, referred to herein as a second reading. In some embodiments, the second state of the volume index valve corresponds to a non-energized (i.e., OFF) state that provides an open position of the volume index valve. Although the method is described being carried out by switching the volume index valve from the first (i.e., closed) position to the second (i.e., open) position, it is to be appreciated that the reverse may be true in some embodiments.

The operating condition of the compressor described above refers to a power reading in some embodiments. In particular, a variable frequency drive power reading of the compressor is taken at the two above-described states/positions of the volume index valve. In other embodiments, the operating current of the compressor may be utilized as the operating condition readings. The readings are obtained with a processor <NUM> that is in operative communication with the volume index valve <NUM> and the compressor <NUM> generally (<FIG>). The processor <NUM> may be part of the controller <NUM> or a separate module. Although a variable speed compressor is noted above, it is to be appreciated that a fixed speed compressor benefits from the embodiments described herein.

The first and second readings are processed by the processor <NUM> and a difference between the two readings is calculated. As shown in <FIG>, when the volume index valve is in the first state/position, a first operating condition reading <NUM> is detected. A step-like falloff of the operating condition is observed in certain areas of the compressor map when the volume index valve is switched to the second state/position, as represented with numeral <NUM>. In particular, there are overlapping areas or "dead zones" in the operating envelope where running with or without the volume index valve does not result in much difference. The compressor could be either a fixed or variable speed compressor. Due to the availability of power reading in the variable frequency drive, that can be used to perform the volume index valve operational determination. Otherwise, the current reading may be employed for the determination for both variable and fixed speed compressors.

In the second state/position, a second operating condition reading <NUM> is detected. The method includes utilizing the processor <NUM> to determine the difference between the operating condition readings and to compare that difference to a predetermined threshold stored in memory of the processor <NUM>. A correctly operating system will produce a measurable difference that exceeds the predetermined threshold. As described above, the operating condition measured is power in some embodiments. If the measured power difference fails to exceed the predetermined threshold, this is indicative of a hardware problem with the volume index valve itself and that it is not opening and closing properly. In the case of current as the measured operating condition, a failure to exceed the predetermined threshold is indicative of an electrical failure of the volume index valve. Additionally, installation or mechanical failure may lead to a failure to exceed the predetermined threshold.

If the predetermined threshold is not exceeded, the method includes initiating an alert <NUM> that prompts an operator to take a corrective action. As described above, a failure of the volume index valve impacts efficiency, but does not warrant a complete shutdown of the compressor so the system continues to operate while the alert is on <NUM>. The alert is maintained until it is manually reset, thereby ensuring that an operator has addressed the problem. Once manually reset, a timer may be reset <NUM> to determine when the diagnostic routine is again initiated.

Advantageously, the method and system described herein provides a form of failure detection of the volume index valve. The volume index valve is primarily responsible for providing efficiency benefits. Therefore, a failed valve would reduce unit efficiency. Without the method and system described herein, a volume index valve failure could go unnoticed and impair operating efficiency.

The use of the terms "a" and "an" and "the" and similar referents in the context of the present disclosure (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. Further, it should further be noted that the terms "first," "second," and the like herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. The modifier "about" used in connection with a quantity is inclusive of the stated value and has the meaning dictated by the context (e.g., it includes the degree of error associated with measurement of the particular quantity).

Claim 1:
A method (<NUM>) of monitoring a volume index valve (<NUM>) of a screw compressor comprising:
recording (<NUM>) a first reading (<NUM>) of an operating condition of the screw compressor when the volume index valve is in a first state that corresponds to a first position;
switching the volume index valve to a second state that corresponds to a second position;
recording (<NUM>) a second reading (<NUM>) of the operating condition of the screw compressor when the volume index valve is in the second state;
calculating a difference between the first reading and the second reading; and
comparing (<NUM>) the difference to a predetermined threshold difference to determine if the volume index valve is moving between the first position and the second position in a desired manner;
the method further comprising:
verifying that the volume index valve is opening and closing in the desired manner if the difference exceeds the predetermined threshold;
determining that the volume index valve has failed and is not opening and closing in the desired manner such that it is impairing operating efficiency if the difference does not exceed the predetermined threshold;
wherein the method is automatically initiated based on a periodic timer to cause the method to be performed at a specified time interval,
wherein upon initiation of the method, the method comprises waiting for stable operation conditions (<NUM>) of the screw compressor to be met and/or stable operation conditions of the system that the screw compressor operates within.