Patent Description:
In multi-stage compressors, the first stage of the compressor may be unloaded by guide vanes governing the mass flow entering the first stage suction. When the first stage is unloaded without corresponding unloading of the second stage, the second stage will continue to draw flow, causing the inter-stage pressure to drop. The lower pressure at the inlet to the second stage impeller reduces mass flow low enough to balance flows. A common problem with many centrifugal compressor designs is the unloading characteristic is not always stable. The reduction in flow and pressure can lead to instability in inter-stage flow and a phenomenon called rotating stall or stall. This effect can be mistaken for surge, but with stall, there is no flow reversal through the compressor. There will be a cyclic variation in mass flow and pressures, but flow direction never reverses as it does in surge. The overall effect may range from not noticeable to highly objectionable noise and vibration. These effects may be particularly pronounced at higher head conditions.

<CIT> is directed to a refrigerant circuit including a multi-stage compressor and an economizer downstream of a condenser, with a flash tank of the economizer connected to a second stage of the multi-stage compressor.

The scope of protection is defined in the attached independent claims to which reference should now be made. Further, optional features are defined in the sub-claims appended thereto.

This disclosure is directed to the unloading of multi-stage compressors, particularly the introduction of a flow into a second stage of the compressor.

Introducing an additional mass flow into the flow into the second stage can stabilize a multi-stage compressor when the first stage is being unloaded. Further, this mass introduction can be used to introduce a swirl into the flow into the second stage that improves the unloading effectiveness at the second stage. Further, the introduction of the mass flow can be used to adjust the velocity vector of the flow, controlling the head capability and volume of flow into the second stage of the compressor.

According to the invention, a heating, ventilation, air conditioning and refrigeration (HVACR) system includes a multi-stage compressor including a first stage discharge and a second stage inlet receiving a fluid from the first stage discharge, a condenser, an expansion device, an evaporator, and a bypass line configured to convey fluid directly from the condenser to the second stage inlet of the multi-stage compressor. The bypass line includes a valve. When the valve is open, the second stage inlet receives a fluid flow. The second stage inlet is configured to direct the fluid flow to join the fluid from the first stage discharge such that a swirl is formed in a combined fluid flow.

In an embodiment, the swirl is in a direction that is the same as a direction of rotation of an impeller in the multi-stage compressor.

In an embodiment, the second stage inlet is further configured to direct the fluid flow in a direction having a component opposite a direction of flow of the fluid from the first stage discharge. The component is a component of a vector of the direction of the fluid flow.

In an embodiment, the valve is a variable flow rate valve. In an embodiment, the valve is opened when the multi-stage compressor is unloaded.

In an embodiment, the second stage inlet does not include movable guide vanes.

In an embodiment, the multi-stage compressor further comprises a first stage suction and a plurality of movable guide vanes at the first stage suction, wherein the plurality of movable guide vanes control a mass flow rate into the multi-stage compressor.

In an embodiment, an inlet duct for a multi-stage compressor includes an inlet opening configured to receive a first fluid flow from a first stage of the multi-stage compressor, and a plurality of channels configured to receive a second fluid flow from a bypass line and introduce the second fluid flow into the first fluid flow such that a swirl is formed in the first fluid flow.

In an embodiment, the channels are configured to introduce the second fluid flow into the first fluid flow in a direction having a component opposite a direction of the first fluid flow.

In an embodiment, the channels are configured to introduce the second fluid flow into the first fluid flow in a direction having a component in a same direction as a direction of the first fluid flow.

In an embodiment, the channels are through holes drilled from an exterior surface of the inlet duct to an interior space of the inlet duct, and wherein the interior space of the inlet duct receives the first fluid flow from the first stage of the multi-stage compressor via the inlet opening.

According to the invention, a method for unloading a multi-stage compressor in a heating, ventilation, air conditioning, and refrigeration system includes receiving a first fluid flow from a first stage discharge of the multi-stage compressor at a second-stage inlet of the multi-stage compressor at a flow straightener of an inlet duct; opening a bypass valve in a bypass line, the bypass line directly connecting a condenser to the second-stage inlet, and directing a second fluid flow from the bypass line to join the first fluid flow through one or more channels in a duct of the second-stage inlet, such that a combined fluid flow has a swirl and a head pressure of a combined fluid flow is boosted.

In an embodiment, the second fluid flow travels in a direction having a component opposite a direction of the first fluid flow when the second fluid flow is directed to join the first fluid flow.

In an embodiment, the second fluid flow travels in a direction having a component in a direction that is the same as a direction of the first fluid flow when the second fluid flow is directed to join the first fluid flow.

In an embodiment, the method further includes reducing a flow rate into a first stage of the multi-stage compressor using a plurality of movable guide vanes.

<FIG> shows a schematic of a heating, ventilation, air conditioning and refrigeration (HVACR) circuit <NUM> according to an embodiment.

HVACR circuit <NUM> includes compressor <NUM>, condenser <NUM>, expansion device <NUM>, and an evaporator <NUM>.

The compressor <NUM>, the condenser <NUM>, the expansion device <NUM>, and the evaporator <NUM> may be fluidly connected to form the HVACR circuit <NUM>. The HVACR circuit <NUM> can alternatively be configured to heat or cool a gaseous process fluid (e.g., a heat transfer medium or fluid such as, but not limited to, air or the like), in which case the HVACR circuit <NUM> may be generally representative of an air conditioner or a heat pump.

Compressor <NUM> compresses a working fluid (e.g., a heat transfer fluid such as a refrigerant or the like) from a relatively lower pressure gas to a relatively higher pressure gas. The relatively higher pressure gas is also at a relatively higher temperature, which is discharged from the compressor <NUM> and flows through the condenser <NUM>. Compressor <NUM> is a multi-stage compressor. Compressor <NUM> includes first stage suction <NUM>. Compressor <NUM> further includes line <NUM> connecting the first stage to the second stage inlet <NUM>. Line <NUM> may be, for example, a pipe. In compressor <NUM>, the working fluid is received at the first stage suction <NUM>, compressed a first time, then discharged from the first stage to the line <NUM>. The working fluid compressed by the first stage is then received at the second stage inlet <NUM>, and compressed a second time, then discharged to condenser <NUM>.

Condenser <NUM> is directly fluidly connected to gas bypass line <NUM>. Gas bypass line <NUM> receives hot gas from within condenser <NUM> and conveys the hot gas from condenser <NUM> to the second stage inlet <NUM> of the compressor <NUM>.

Gas bypass line <NUM> includes a valve <NUM>. Valve <NUM> regulates flow through the gas bypass line <NUM>. In an embodiment, valve <NUM> is a valve having an open position and a closed position. In an embodiment, valve <NUM> is a variable flow rate valve, such as a valve having multiple discrete flow rates or a continuously variable flow rate. Valve <NUM> may be controlled according to the unloading of the first stage of compressor <NUM>, for example increasing flow through gas bypass line <NUM> when unloading the first stage of the compressor <NUM>.

Channels <NUM> allow a bypass flow of fluid from the gas bypass line <NUM> to join the first stage discharge flow from line <NUM> in second stage inlet <NUM> and enter the second stage of compressor <NUM>. The channels <NUM> are oriented such that a swirl is induced into the combined flow of the first stage discharge flow from line <NUM> and the bypass flow from channels <NUM>. In an embodiment, the swirl is in a direction that is the same as a direction of rotation of a rotating component within the second stage of compressor <NUM>. In an embodiment, the combined flow may be a mass flow having a velocity that is less than the velocity of the first stage discharge flow when it is received from line <NUM>. An example embodiment of channels <NUM> is shown in <FIG> and discussed below.

HVACR circuit <NUM> further includes expansion device <NUM>. Expansion device <NUM> is a device configured to reduce the pressure of the working fluid. As a result, a portion of the working fluid is converted to a gaseous form. Expansion device <NUM> may be, for example, an expansion valve, orifice, or other suitable expander to reduce pressure of a refrigerant such as the working fluid.

Evaporator <NUM> is an evaporator where the working fluid absorbs heat from a process fluid (e.g., water, glycol, air, or the like), heating the working fluid. This at least partially evaporates the working fluid. The working fluid then flows from evaporator <NUM> to the first stage suction <NUM> of compressor <NUM>. The circulation of the working fluid through HVACR circuit <NUM> continues while the refrigerant circuit is operating, for example, in a cooling mode (e.g., while the compressor <NUM> is enabled).

HVACR system <NUM> may further include an economizer <NUM>. Economizer <NUM> may direct some working fluid from at or near the condenser into line <NUM> conveying fluid to the second stage inlet <NUM>. Economizer <NUM> may be any standard economizer included in HVACR circuits. In an embodiment, economizer <NUM> includes a brazed plate heat exchanger.

<FIG> shows a perspective view of an impeller inlet duct <NUM> according to an embodiment. Impeller inlet duct <NUM> may be located at an intake of a second stage of a multi-stage compressor, such as second stage inlet <NUM> of compressor <NUM> shown in <FIG>. Impeller inlet duct includes flow straightener <NUM>, an internal space <NUM> defined by outer wall <NUM>, a plurality of channels <NUM>, and outlet <NUM>.

Flow straightener <NUM> receives a fluid flow and is configured to smooth and straighten the received fluid flow. Flow straightener <NUM> may include multiple concentric circular openings, connected by a plurality of vanes to define a plurality of openings. Flow straightener <NUM> may direct fluid flow entering the flow straightener <NUM> through to internal space <NUM> of the inlet impeller duct <NUM>. The flow straightener <NUM> may be connected to a fluid line such as line <NUM> shown in <FIG> and described above that conveys a flow from the first stage discharge of a multi-stage compressor to the flow straightener <NUM>. In an embodiment, the fluid line may further receive fluid from an economizer such as economizer <NUM> shown in <FIG> and described above.

Internal space <NUM> is a hollow space within the impeller inlet duct <NUM>. Internal space <NUM> may be defined by outer wall <NUM> of the impeller inlet duct. Internal space <NUM> may receive fluid flow from flow straightener <NUM> and from channels <NUM>. The fluid flow from flow straightener <NUM> and from channels <NUM> may be combined and mixed within the internal space <NUM>. The internal space <NUM> may continue to outlet <NUM>, which allows fluid flow from the internal space <NUM> to the second stage compression of the multi-stage compressor.

Channels <NUM> are one or more channels by which fluid flows may be introduced into internal space <NUM>. In an embodiment, channels <NUM> are straight-drilled through holes in the outer wall <NUM> of the impeller inlet duct <NUM>. Non-limiting examples of channels <NUM> include holes, slots, or nozzles. Channels <NUM> may be provided in one or more rows. The channels are oriented such that fluid flow entering the internal space <NUM> through the channels <NUM> introduces a swirl into a fluid flow passing from flow straightener <NUM> through internal space <NUM> to outlet <NUM>. The number of channels may be varied based on, for example, the size of the channels <NUM> and flow rates through the channels <NUM>, the orientation of the channels with respect to internal space <NUM>, and the properties of the compressor including impeller inlet duct <NUM>. In an embodiment, the channels <NUM> are oriented such that the direction L of flow through the channels <NUM> into internal space <NUM> includes a component that is tangential to the direction F of the fluid flow from flow straightener <NUM>. The tangential component may induce the swirl in the combined flow within internal space <NUM>, and may also be referred to as a circumferential component to the direction F. The direction F may define a central axis of the tangential or circumferential component.

In an embodiment, the channels <NUM> are further oriented such that the direction L of flow through the channels <NUM> into the internal space <NUM> includes a component opposite to the direction F of the fluid flow from flow straightener <NUM>. This velocity component reduces the velocity of the fluid flow in direction F as it passes through internal space <NUM>. Reducing the velocity of flow may assist unloading, for example by reducing the volume of flow into the second stage compression. In an embodiment, the channels are oriented such that the direction L of flow through the channels <NUM> into the internal space <NUM> includes a component that is in the same direction as the direction F of the fluid flow from flow straightener <NUM>. In this embodiment, head pressure may be boosted by the component of fluid flow through channels <NUM> that is in the same direction as the direction F of the fluid flow from flow straightener <NUM>.

Outlet <NUM> allows the fluid from internal space <NUM>, including fluid received at flow straightener <NUM> and fluid received via channels <NUM>, to continue through the second stage of the compressor to be compressed.

<FIG> is a schematic view of an inlet housing <NUM> of a compressor according to an embodiment. Inlet housing <NUM> may surround an impeller inlet duct such as impeller inlet duct <NUM> shown in <FIG> and described above. Inlet housing <NUM> may include second stage intake aperture <NUM> and bypass intake aperture <NUM>. Inlet housing <NUM> may be installed in a compressor having a direction of rotation R as shown in <FIG>.

Second stage intake aperture <NUM> is an aperture to which a fluid line from a first stage discharge of the multi-stage compressor may be connected. The fluid line may be, for example, line <NUM> shown in <FIG> and described above. The second stage intake aperture may provide fluid communication between the fluid line from the first stage discharge and a flow straightener of an inlet impeller duct, such as flow straighter <NUM> of inlet impeller duct <NUM> shown in <FIG> and described above.

Bypass intake aperture <NUM> may receive fluid from a gas bypass from a condenser of an HVACR circuit such as condenser <NUM> of HVACR circuit <NUM> shown in <FIG> and described above. The gas from the gas bypass may be conveyed to the bypass intake <NUM> by gas bypass line <NUM>. In an embodiment, bypass gas may be sourced to gas bypass line <NUM> from compressor discharge of the compressor including inlet housing <NUM>. Flow through gas bypass line <NUM> may be controlled by valve <NUM>. In an embodiment, valve <NUM> is a valve having an open position and a closed position. In an embodiment, valve <NUM> is a variable flow rate valve, such as a valve having multiple discrete flow rates or a continuously variable flow rate. Valve <NUM> may be controlled according to the unloading of the first stage of a compressor including inlet housing <NUM>, for example increasing flow through gas bypass line <NUM> when the first stage of the compressor is unloaded. In an embodiment, valve <NUM> may be controlled in response to a measurement of stall occurring in the compressor.

Flow into inlet housing <NUM> through bypass intake aperture <NUM> enters a space between the inlet housing and an impeller inlet duct of the compressor, such as the inlet duct <NUM> shown in <FIG> and described above. This space may be separate from the path from second stage intake aperture <NUM> provides from the fluid line to the flow straightener of the inlet impeller duct. The flow then may proceed through channels, such as channels <NUM> and <NUM> shown in <FIG> and <FIG>, respectively, and then the inlet duct, such as inlet duct <NUM> to impart a swirl into the flow that passes through second stage intake aperture <NUM> into the second stage of the compressor. The swirl may be in a direction that is the same as direction of rotation R of rotating components of the second stage of the compressor.

<FIG> is a flow chart of a method <NUM> of unloading a multi-stage compressor according to an embodiment. Method <NUM> optionally includes unloading a first stage of the multi-stage compressor <NUM>. Method <NUM> includes receiving a first stage discharge flow <NUM>, opening a bypass valve <NUM>, directing a bypass flow directly from a condenser to one or more channels <NUM>, directing the bypass flow using the one or more channels <NUM>, and combining the first stage discharge flow and the bypass flow to form a combined flow having a swirl <NUM>.

Method <NUM> may optionally include unloading a first stage of the multi-stage compressor <NUM>. Unloading the first stage of the compressor at <NUM> may include using guide vanes to regulate the flow of fluid into the first stage of the compressor, for example by deploying the guide vanes to limit this flow.

Method <NUM> includes receiving, at the second stage of the multi-stage compressor, a first stage discharge flow. The first stage discharge flow is a flow of fluid that has been compressed by the first stage of the multi-stage compressor. In an embodiment, the first stage of the multi-stage compressor may be operated while unloading the first stage, for example unloading via guide vanes at <NUM>. In an embodiment, the first stage discharge flow may further include fluid from an economizer in the circuit including the multi-stage compressor, such as economizer <NUM> in <FIG> and described above. In an embodiment, the first stage discharge flow is received at a flow straightener of an impeller inlet duct, such as flow straightener <NUM> of inlet impeller duct <NUM> shown in <FIG> and described above. The flow straightener may condition the first stage discharge flow, such that it flows smoothly in a consistent direction through the inlet impeller duct. The first stage discharge flow received at <NUM> may continue through the inlet impeller duct into a space within the inlet impeller duct such as internal spaces <NUM> and <NUM> shown in <FIG> and <FIG>, respectively.

Method <NUM> also includes opening a bypass valve <NUM>. The bypass valve opened at <NUM> may be a valve such as valve <NUM> shown in <FIG> and described above or valve <NUM> shown in <FIG> and described above. The valve may be along a bypass line, such as bypass line <NUM> or bypass line <NUM>. Opening the bypass valve <NUM> allows fluid to flow through the bypass valve. In an embodiment, opening the bypass valve includes moving the bypass valve from a closed position to an open position. In an embodiment, opening the bypass valve includes increasing an amount of fluid flow through the bypass valve, where the bypass valve is a variable flow rate valve, such as a valve having multiple discrete flow rates or a continuously variable flow rate. In an embodiment, the extent of opening the bypass valve at <NUM> may be based on the extent of unloading of the multi-stage compressor, such as increasing the fluid flow by a larger amount when the unloading of the compressor is at a higher value and/or when stalling or instability in compressor flow is detected or determined to be occurring.

When the bypass valve is opened at <NUM>, a bypass flow is directed from the bypass valve to one or more channels <NUM>. The bypass flow may be directed to the one or more channels by, for example, a portion of the bypass line downstream of the bypass valve, and/or by a housing around an impeller inlet duct that receives the bypass flow. The housing and impeller inlet duct together may provide a space between the housing and impeller inlet duct that allows fluid within the space to reach and enter openings of channels through the impeller duct, such as channels <NUM> shown in <FIG> and described above or channels <NUM> and <NUM> shown in <FIG> and <FIG>, respectively.

At the one or more channels, the bypass flow is directed at <NUM>. At <NUM>, the bypass flow is directed towards the first stage discharge flow received at <NUM> within an internal space of the impeller duct, such as internal space <NUM> shown in <FIG> and described above. The flow is directed via channels formed in the impeller duct. The channels may orient the direction of flow into the internal space of the impeller duct such that flow into the impeller duct enters the internal space at a position and angle that induces a swirl when combined with the first stage discharge flow received at <NUM>. In an embodiment, the channels further orient the direction of the bypass flow into the internal space such that the bypass flow is introduced at an injection angle I as shown in <FIG> or an injection angle J as shown in <FIG>. In this embodiment, a vector representing the direction of the bypass flow includes a component in a direction opposite the direction of the first stage discharge flow that is received at <NUM>.

The bypass flow directed by the one or more channels at <NUM> and the first stage discharge flow received from the first stage of the compressor at <NUM> are combined to form a flow having a swirl at <NUM>. The respective directions of each of the bypass and first stage discharge flows results in a combined flow having a swirl due to the directions of the flow directed by the one or more channels. In an embodiment, the swirl is in a direction that is the same as a direction of rotation of at least one rotating part of the second stage compression of the multi-stage compressor. In an embodiment, the combination of flows also has a linear velocity that is less than the linear velocity of the flow received from the first stage of the compressor at <NUM>. This combined flow may then enter second stage compression in the multi-stage compressor, where it is compressed and discharged from the multi-stage compressor.

<FIG> is a diagram <NUM> of the velocity vectors of the flow from the first stage discharge and the bypass flow within an impeller duct in a multi-stage compressor according to an embodiment. The velocity vectors represent the velocities of fluid flows within a second-stage impeller duct according to an embodiment during unloading of the compressor, such as impeller duct <NUM> shown in <FIG> and described above.

First stage discharge flow velocity vector <NUM> represents the velocity of fluid flow received from the first stage discharge of a multi-stage compressor. The first stage discharge flow is the flow received by the second stage at an impeller duct such as impeller duct <NUM>. The flow may have a consistent direction provided by travelling through a flow straightener such as flow straightener <NUM>. The flow travels in a direction from the entry into the impeller duct from the first stage discharge towards second stage compression in the multi-stage compressor.

A gas bypass flow is provided at entry point <NUM>. Entry point <NUM> is, for example, an opening where a channel such as channel <NUM> shown in <FIG> and described above or channel <NUM> that introduce fluid flow from a gas bypass to a fluid flow within the inlet duct. The gas bypass flow has a velocity represented by gas bypass flow velocity vector <NUM>.

The gas bypass flow may be provided at an injection angle I with respect to first stage discharge flow velocity vector <NUM>. In an embodiment, the injection angle I is <NUM> degrees. In an embodiment, the injection angle I is an acute angle. When injection angle I is an acute angle, a component of the gas bypass flow velocity opposes the first stage discharge flow velocity, thus reducing the total velocity of the fluid flow entering the second stage compression of the multi-stage compressor.

The total velocity of the combined first stage discharge flow and the gas bypass flow is represented by total velocity vector <NUM>. Total velocity vector <NUM> includes a swirl in a direction. In an embodiment, the swirl is in a direction corresponding to a direction of rotation of a component in the second stage compression of the multi-stage compressor. In an embodiment, the velocity represented by total velocity vector <NUM> has a velocity that is reduced in comparison with the first stage discharge flow. The combined first stage discharge flow and the gas bypass flow travels into the second stage compression of the multi-stage compressor with the velocity represented by total velocity vector <NUM>.

The gas bypass flow may be provided at an injection angle J with respect to first stage discharge flow velocity vector <NUM>. In the embodiment shown in <FIG>, the injection angle J is an obtuse angle. When injection angle J is an acute angle, a component of the gas bypass flow velocity is in the same direction as the first stage discharge flow velocity, thus increasing the total velocity of the fluid flow entering the second stage compression of the multi-stage compressor. This may provide a boost to head pressure for the second stage of the compressor.

<FIG> is a sectional view of an impeller duct and an inlet housing assembled together <NUM> according to an embodiment. The assembled impeller duct and inlet housing <NUM> receives fluid flow from a prior stage of a multi-stage compressor at stage inlet <NUM>, and directs this fluid flow to impeller <NUM> and the second stage of the multi-stage compressor. The fluid flow is combined with a bypass flow that is received at bypass intake aperture <NUM> and travels into space <NUM> defined by inlet housing <NUM>, where it enters channels <NUM> in impeller inlet duct body <NUM>. The combined fluid flow and bypass flow continue to impeller <NUM>.

Stage inlet <NUM> is defined by the inlet housing <NUM>. The stage inlet <NUM> receives fluid discharged from the prior stage of a compressor including the assembled impeller duct and inlet housing <NUM> and directs it to flow straightener <NUM> of the impeller inlet duct. Flow straightener <NUM> may include a plurality of vanes to condition the flow of fluid passing through it. Flow straightener <NUM> may be flow straightener <NUM> shown in <FIG> and described above. The fluid flow through flow straightener <NUM> may enter an internal space defined by impeller inlet duct body <NUM>. The internal space <NUM> can be seen in the sectional view provided in <FIG>.

Inlet housing <NUM> also includes a body that forms a space <NUM> between the inner side of inlet housing <NUM> and the impeller inlet duct body <NUM>. Inlet housing <NUM> may be the inlet housing <NUM> shown in <FIG> and described above. Inlet housing <NUM> includes a bypass intake aperture <NUM> that allows fluid from a bypass line to be introduced into space <NUM> within the inlet housing <NUM>. In an embodiment, bypass intake aperture <NUM> receives fluid from a bypass line such as bypass line <NUM> shown in <FIG> and described above. In an embodiment, bypass intake aperture <NUM> receives fluid from a bypass line connected to compressor discharge ducting. In an embodiment, the fluid received at bypass intake aperture <NUM> may be controlled by a valve, such as valves <NUM> and <NUM> described above and shown in <FIG> and <FIG>, respectively. In an embodiment, the valve may be controlled based on unloading of the compressor and/or a detected or determined instability or stall in the compressor.

Channels <NUM> may allow flow of fluid from space <NUM> into impeller inlet duct body <NUM>. Bypass flow may enter impeller inlet duct body <NUM> to join fluid from the prior stage of the multi-stage compressor that has passed through flow straightener <NUM>. The channels <NUM> may be oriented to induce a swirl in the combined fluid flow as it continues to pass through the multi-stage compressor including the assembled impeller duct and inlet housing <NUM>. The internal space <NUM> within impeller inlet duct body <NUM> and the orientation of channels <NUM> is shown in <FIG> and described below.

The combined fluid flow from the prior stage and the bypass then passes from within impeller inlet duct body <NUM> to impeller <NUM> and continues through the multi-stage compressor including the assembled impeller duct and inlet housing <NUM>.

<FIG> is a sectional view taken across line A-A in <FIG>. In the sectional view of <FIG>, internal space <NUM> is visible, defined by impeller inlet duct body <NUM>. Internal space <NUM> receives fluid from prior stage discharge of the compressor via flow straightener <NUM>. The direction of channels <NUM> as they pass through impeller inlet duct body <NUM> is visible. The direction of rotation of a compressor receiving fluid from the internal space <NUM> is shown by arrow C. Channels <NUM> are oriented such that the velocity of the fluid flow introduced by those channels <NUM> has a component in a direction tangential to the direction of flow of the fluid from the flow straightener <NUM>, which may be flowing into the page in the sectional view of <FIG>. The tangential component of the velocity of the fluid flow introduced by channels <NUM> may induce a swirl in the combined fluid flow through internal space <NUM>. The swirl induced in the combined flows through internal space <NUM> may be in the same direction as the direction C of the rotation of the compressor receiving the combined fluid flow.

Claim 1:
A heating, ventilation, air conditioning and refrigeration (HVACR) system, comprising:
a multi-stage compressor (<NUM>) including a first stage discharge and a second stage inlet (<NUM>) receiving fluid from the first stage discharge;
a condenser (<NUM>);
an expansion device (<NUM>);
an evaporator (<NUM>);
a bypass line (<NUM>) from the condenser to the second stage inlet of the multi-stage compressor, the bypass line including a valve (<NUM>),
wherein when the valve is open, the second stage inlet receives a fluid flow, and the second stage inlet is configured to direct the fluid flow to join the fluid from the first stage discharge in a direction having a component that is the same as a direction of flow of the fluid from the first stage discharge, and
when the fluid flow joins the fluid from the first stage discharge, a head pressure is boosted in a combined fluid flow prior to entering the second stage;
characterised in that
the bypass line (<NUM>) is configured to convey fluid directly from the condenser (<NUM>) to the second stage inlet (<NUM>) of the multi-stage compressor.