EP3872421A1

EP3872421A1 - Refrigeration circuit and refrigeration unit with microchannel evaporator

Info

Publication number: EP3872421A1
Application number: EP20159559.2A
Authority: EP
Inventors: Gabriela Panzer; Sascha Vinz; Markus Schuster; Christophe Vallée
Original assignee: Carrier Corp
Current assignee: Carrier Corp
Priority date: 2020-02-26
Filing date: 2020-02-26
Publication date: 2021-09-01
Also published as: CN113310231A

Abstract

A refrigeration circuit 200 comprises: a compressor 202; a condenser 203; a expansion valve 208 and sensor 209, wherein the expansion valve sensor 209 is for controlling the degree of opening of the expansion valve 208 in accordance with the temperature and/or pressure of refrigerant fluid at the bulb 209; an evaporator 205, wherein the evaporator 205 is a microchannel or microtube heat exchanger; and a further heat exchanger 211. The further heat exchanger 211 is positioned in the circuit between a refrigerant outlet of the evaporator 205 and the expansion valve sensor 209 and is configured for internal heat exchange between refrigerant in a return flow line leading from the evaporator 205 and refrigerant in a condensed flow line leading to the expansion valve 208.

Description

The present invention relates to refrigeration circuits and refrigeration units and, more particularly, to refrigeration units with an expansion valve for controlling the flow of refrigerant and an evaporator comprising a microchannel or microtube heat exchanger.
In conventional refrigeration circuits, a thermostatic expansion valve is positioned in the refrigeration circuit just before the evaporator and the thermostatic expansion valve bulb is positioned just after the evaporator. The thermostatic expansion valve bulb senses and monitors the temperature of refrigerant leaving the evaporator and the rate and expansion of flow refrigerant into the evaporator is controlled using the thermostatic expansion valve based on this measurement in order to ensure that the refrigerant that leaves the evaporator is a mixture of saturated liquid and vapour. This typically results in superheated vapour refrigerant being present in a latter part of the evaporator. With conventional evaporators this is usually acceptable but when a superheated vapour is present in a microchannel or microtube heat exchanger (e.g. when used as an evaporator in a conventional refrigeration circuit) it results in a reduction in effective surface area for the heat exchange to occur. It has been found that in some applications the required temperature of a refrigerated compartment (e.g. for certain refrigerated products) cannot be maintained in an appropriate range as a result of this effect.
Even if an appropriate range of temperature can be reached, the resultant decrease in efficiency requires the circulation of more refrigerant in order to maintain the required temperature in a refrigerated compartment of a refrigeration unit.
With current environmental trends, improvements in refrigeration units are desirable, particularly toward aspects of environmental impact. Furthermore, microchannel and microtube heat exchangers are desirable in refrigeration circuits as they are more compact compared to conventional evaporators making them useful in applications with limitations on available space.
According to a first aspect, the present invention provides a refrigeration circuit comprising: a compressor; a condenser; an expansion valve and sensor, wherein the expansion valve sensor is for controlling the degree of opening of the expansion valve in accordance with the temperature and/or pressure of refrigerant fluid at the sensor; an evaporator, wherein the evaporator is a microchannel or microtube heat exchanger; and a further heat exchanger, wherein the further heat exchanger is positioned in the circuit between the evaporator and the expansion valve sensor and is configured for internal heat exchange between refrigerant in a return flow line leading from the evaporator and refrigerant in a condensed flow line leading to the expansion valve.
The expansion valve may be a thermostatic expansion valve and the sensor may be a thermostatic expansion bulb. Alternatively, the expansion valve may be an electronic expansion valve and the sensor may be an electronic probe.
By positioning the further (internal) heat exchanger in the circuit between the evaporator and the expansion valve sensor, superheating of the refrigerant in the evaporator may be prevented or reduced during use. As such, the heat exchanger and expansion valve sensor may be configured to prevent or reduce superheating of refrigerant in the evaporator during use.
Furthermore, the location of the expansion valve sensor in the refrigeration circuit after the further heat exchanger may result in control of the degree of opening of the expansion valve in order to reduce or prevent superheated fluid in the evaporator during use.
In other words, the pressure equalization of the expansion valve may be externally equalized.
Effectively, in comparison to a conventional refrigeration circuit, a working superheated vapour section of the refrigeration circuit may be transferred out of the evaporator with use of the additional heat exchanger positioned between the evaporator and the expansions valve sensor.
By reducing or preventing the presence of superheated refrigerant in the microchannel evaporator, the effective surface area for heat exchange in the evaporator is maintained and a reduction in efficiency is avoided.
Superheat is an amount of heat added to a refrigerant after a change in state and so superheated vapour is a vapour with heat added after the phase change to vapour has occurred. In this state, a loss of heat from the superheated vapour will not result in condensation of the vapour.
A saturated liquid is a liquid with a temperature and pressure such that the absorption of any more heat or any decrease in pressure without a change in temperature will cause it to vaporise. A saturated vapour is a vapour with a temperature and pressure such that any loss of heat or any increase in pressure without change in temperature will cause it to condense.
The further heat exchanger may be configured to ensure that during use the evaporator is flooded along its entire length.
The term flooded is used in relation to the evaporator to mean that the refrigerant in the evaporator remains substantially in a saturated liquid-vapour state (i.e. a mixture comprising saturated liquid and vapour, where both phases coexist). The heat exchange carried out by the evaporator is most efficient and effective when it is flooded along its entire length (i.e. along the entire refrigerant path).
The refrigerant may be heated (e.g. to a superheated vapour) in the further heat exchanger as a result of the heat exchange with refrigerant in the condensed flow line leading to the expansion valve. The expansion valve sensor may therefore detect a superheated vapour.
The evaporator may be a single-pass microchannel heat exchanger. As such, the microchannel heat exchanger may comprise a plurality of parallel passages. The present invention is particularly advantageous when used with a single-pass microchannel heat exchanger because when a superheated vapour is present in any passage of a microchannel heat exchanger it cannot function effectively in the surface area of the region where the superheated vapour is present; this reduces the effective surface area for heat exchange to take place. It is particular troublesome for a single-pass microchannel heat exchanger as these typically comprise a multitude of parallel tubes/passages extending from a manifold at one end of the heat exchanger (at the inlet) to a manifold at the other end (at the outlet) and if the vapour towards one end is superheated (e.g. toward the outlet manifold in an evaporator) all of the microchannel tubes may be affected at that superheated end, resulting in a large reduction in effective surface area for the heat exchange to take place. The evaporator may be a single-slab microchannel heat exchanger. Similar to the advantage described above, the present invention is particularly advantageous for single-slab microchannel heat exchangers because the presence of any superheated vapour at one end of the (one and only) slab affects a relatively large proportion of the total heat exchanging area of the evaporator (compared to multiple slabs connected in series).
In other words, a single-pass microchannel heat exchanger typically has one flow path that goes from the inlet to the outlet such that the refrigerant only passes in a flow path along the length of the heat exchanger once. On this flow path, the inlet and outlet are located opposite one another (e.g. the inlet on the left and outlet on the right side of a display cabinet). In a single-slab microchannel heat exchanger, there is not only one passage in the flow path but a plurality of parallel passages where the refrigerant flows (allowing for more effective heat transfer). The control of heat exchange in a single-slab, single-pass microchannel heat exchanger with a expansion valve is more difficult than with a conventional (e.g. round-tube, plate-fin) evaporator. With the conventional arrangement where the expansion valve sensor is placed directly after the evaporator, all of the parallel passages of the single-slab, single-pass microchannel heat exchanger end up containing at least some superheated refrigerant in the last section. Consequently, the cooling capacity of the microchannel evaporator is substantially reduced. In the present invention, in order to avoid losing cooling capacity by operating a microchannel evaporator with standard control by an expansion valve, the position of the expansion valve sensor is changed (to be after the further heat exchanger). In this way, the microchannel may be operated in a flooded condition (as the refrigerant is superheated in the further heat exchanger) and no decrease of the cooling capacity of the evaporator takes place.
Another important result of having no superheated section in the evaporator is that in a single-pass microchannel exchanger, the air flowing across the evaporator (and so the air temperature inside of the display cabinet) is stable and even across the entire length of the evaporator (and the same in the whole length of the cabinet) which helps to maintain a stable temperature.
The further heat exchanger may be a tube-in-tube or tube-and-tube heat exchanger and it may be a heat exchanger with a known length (i.e. refrigeration path length) and/or perform a known amount of heat exchange. The skilled person will appreciate that the further heat exchanger may take the form of any other suitable heat exchanger as needed for a particular application.
The further heat exchanger may be a plate heat exchanger, such as a brazed plate heat exchanger, or any other form of internal heat exchanger (e.g. two tubes soldered together in the longitudinal direction; this provides a simple arrangement and is cheap to produce).
Furthermore, the skilled person will appreciate that the geometry/form/length of the further heat exchanger may be adjusted as need for a particular application.
The further heat exchanger may be arranged for sufficient heat transfer between the two refrigerant paths. By sufficient, it is meant that the required amount of internal heat exchange is performed by the further heat exchanger for a particular purpose (e.g. to prevent superheating in the evaporator).
The further heat exchanger may have refrigerant paths of sufficient length to prevent superheating in the evaporator.
The further heat exchanger may have refrigerant paths of sufficient length to ensure the evaporator is flooded along its entire length.
The heat exchange relationship of the further heat exchanger (i.e. the amount of heat transfer carried out by the further heat exchanger) may be known or determined. This may be based at least in part on the refrigeration path lengths of the further heat exchanger. This may also be based on any one or a combination of: the speed at which the compressor operates; the operating temperature of the refrigerant and/or evaporator; the degree of opening of the expansion valve; or the type of refrigerant. With a known/determined heat exchange relationship, it can be ensured that the further heat exchanger will carry out sufficient heat exchange in order to reduce or prevent superheated fluid in the evaporator during use.
The compressor may be a variable speed compressor.
The further heat exchanger may be configured to prevent superheating of refrigerant in the evaporator for the full range of speeds that the variable speed compressor can perform. The further heat exchanger may have refrigerant paths of sufficient length to do so.
The further heat exchanger may be a first internal heat exchanger and the refrigeration circuit may comprise an additional internal heat exchanger positioned in the circuit between the expansion valve sensor and the compressor.
The additional internal heat exchanger may be configured to facilitate heat exchange between refrigerant in a return flow line from the first internal heat exchanger and refrigerant in a condensed flow line leading toward the first internal heat exchanger. In this way, the refrigerant in the condensed flow line leading toward the first internal heat exchanger can be subcooled before later entering the evaporator so that a greater cooling capacity can be achieved by the refrigeration circuit.
The additional internal heat exchanger may prevent damage to the compressor. This is because the additional internal heat exchanger helps to ensure that the refrigerant on the suction side of the compressor suction is in the superheated region. The refrigerant in this region should be completely superheated (i.e. in the gaseous state). The greater cooling capacity also results in an improved coefficient of performance (COP).
By internal heat exchanger it is meant that the heat exchange performed by the heat exchanger is between parts of the refrigeration circuit itself rather than with an external heat source or heat sink. Thus, the internal heat exchanger(s) are for transferring heat between refrigerant fluid that is at two different points within the refrigerant circuit.
The additional internal heat exchanger may be located in the bottom of the display cabinet in order to protect it from damage, this also allows the space within the top of the display cabinet to be used for other purposes (e.g. for housing products).
The expansion valve and sensor may control the rate of flow of refrigerant through the further heat exchanger and the evaporator. By controlling the flow in this way, no additional control may be used (or required) in order to prevent superheating in the evaporator during use. This provides a simplified arrangement from preventing superheating in the evaporator.
According to a second aspect, the present invention provides a refrigeration unit comprising any of the refrigeration circuits described above in relation to the first aspect and a refrigerated compartment that is cooled via heat absorption at the evaporator.
The refrigeration unit may be a display cabinet. Utilising a compact microchannel heat exchanger in a display cabinet allows more room to be provided for housing the refrigerated compartment and so more refrigerated products can be displayed in the refrigeration unit.
The further heat exchanger may be configured to prevent superheating of refrigerant in the evaporator for the full range of operating temperatures of the refrigerated compartment and/or the refrigerant. The further heat exchanger may have refrigerant paths of sufficient length to do so.
The refrigeration unit may be configured to operate as a frost-free refrigeration unit. The refrigeration unit may comprise a heater around part of the evaporator in order to periodically melt away ice in a defrost cycle (e.g. using a sensor/thermistor for detecting ice or a timer). Alternatively, the refrigeration unit may operate with the evaporator above 0 °C so that a heater is not necessary; this operation may occur periodically to melt away ice buildup. The refrigeration unit may comprise a fan to circulate air in the refrigerated compartment and/or around the evaporator.
According to a third aspect, the present invention provides a method of manufacturing any of the refrigeration units described above in relation to the second aspect, the method comprising: determining the length of the further heat exchanger required in order to meet the cooling capacity of the refrigeration unit whilst preventing superheating in the evaporator and ensuring the evaporator is flooded along its entire length; and providing the further heat exchanger for internal heat exchange between refrigerant in a return flow line leading from the evaporator and refrigerant in a condensed flow line leading to the expansion valve.
The method may comprise determining the length of the heat exchanger required to do any one or a combination of: prevent superheating of refrigerant in the evaporator for the full range of operating temperatures of the refrigerated compartment and/or the refrigerant; prevent superheating of refrigerant in the evaporator for the full range of speeds that the (variable speed) compressor can perform; or ensure the evaporator is flooded along its entire length,
This determination may also take into account any heat exchange performed by the additional internal heat exchanger described above.
The method may comprise positioning the further heat exchanger in the circuit between the evaporator and the expansion valve sensor.
The method may comprise determining the optimum location in the refrigeration circuit for the further heat exchanger and/or expansion valve sensor in order to prevent superheating in the evaporator and/or ensure the evaporator is flooded along its entire length. The method may comprise positioning the further heat exchanger and/or expansion valve sensor at these locations.
According to a fourth aspect, the present invention provides a method of operating any of the refrigeration circuits described above in relation to the first aspect and/or any of the refrigeration units described above in relation to the second aspect, the method comprising controlling the degree of opening of the expansion valve via the expansion valve sensor in order to reduce or prevent superheated refrigerant in the evaporator.
The method may comprise controlling the degree of opening of the expansion valve via the expansion valve sensor in order to ensure that during use the evaporator is flooded along its entire length.
The method may comprise ensuring that substantially all of the refrigerant leaving the further heat exchanger is vapour, superheated vapour, and/or that it is of a predetermined temperature.
The method may comprise detecting the state of the refrigerant leaving the further heat exchanger and/or a temperature state of the refrigerant leaving the further heat exchanger.
The method may comprise superheating the refrigerant in the further heat exchanger by heat exchange with refrigerant in the condensed flow line leading to the expansion valve.
The method may comprise varying the speed of the compressor.
The further heat exchanger may be a first internal heat exchanger and the method may comprise carrying out heat exchange between refrigerant in a return flow line from the first internal heat exchanger and refrigerant in a condensed flow line leading toward the first internal heat exchanger using an additional internal heat exchanger. In this way, the further heat exchanger is located in the refrigeration circuit between the evaporator and the expansion valve sensor and the additional internal heat exchanger is located in the refrigeration circuit between the expansion valve sensor and the compressor.
The method may comprise using the expansion valve and sensor to control the rate of flow of refrigerant through the further heat exchanger and the evaporator, and the method may be performed using no additional control to prevent or reduce superheating in the evaporator during use.
Certain example embodiments will now be described by way of example only and with reference to the accompanying drawings, in which:

Figure 1 shows a schematic view of a conventional refrigeration unit comprising a conventional evaporator; and
Figure 2 shows a schematic view of a refrigeration unit comprising a microchannel heat exchanger evaporator and an additional heat exchanger.

A conventional refrigeration unit 100 for a display cabinet 101 is shown in Figure 1. The refrigeration circuit comprises a compressor 102, a condenser 103, an internal heat exchanger 104, and an evaporator 105. The evaporator 105 has an inlet 106 and an outlet 107 for refrigerant and is a round-tube plate-fin (RTPF) heat exchanger with a length L₁. The refrigeration unit also comprises a thermostatic expansion valve 108 and a thermostatic bulb 109 for measuring the temperature of the refrigerant as it leaves the evaporator outlet 107.
Compressed refrigerant flows from the compressor 102, through the condenser 103 and to the internal heat exchanger 104. In the internal heat exchanger 104, the refrigerant leaving the evaporator 105 subcools the refrigerant in the main line 112 after it leaves the condenser 103 and before later entering the evaporator 105 so that a greater cooling capacity can be achieved.
The subcooled refrigerant then passes from the internal heat exchanger 104 into the display cabinet 101 and to the thermostatic expansion valve 108. The thermostatic expansion valve 108 regulates the flow and expansion of refrigerant into the inlet 106 of the evaporator 105. The flow and expansion of refrigerant is based on a temperature measurement made by the thermostatic bulb 109 at the evaporator outlet 107. The amount of refrigerant passed into the evaporator 105 by the thermostatic expansion valve 108 is controlled based on this measured temperature in order to ensure that the refrigerant that leaves the evaporator 105 is in the saturated liquid-vapour state (i.e. a mixture of saturated liquid and vapour), this typically results in the mixture also comprising some superheated vapour and/or to ensure that the refrigerant leaving the evaporator is of a predetermined temperature.
This results in a transition point 110 in the evaporator 105 where the refrigerant transitions from a saturated liquid state in a first region 109 of the evaporator 105 to a mixture of saturated liquid and vapour and subsequently a superheated vapour in a second region 111 of the evaporator 105 as it absorbs heat from a (warmer) refrigerated compartment of the display cabinet 101.
The superheated temperature of the refrigerant is controlled by the expansion bulb; there is typically a set static superheat of around 4 °C (i.e. a minimum temperature increase above the transition point) and an opening superheat which cannot be altered in the system. The total superheat is typically around 8 °Cor more.
In an RTPF heat exchanger evaporator 105 this is not usually an issue. However, when the evaporator 105 is replaced with a single-pass, single slab microchannel heat exchanger (MCHX) evaporator the passage of superheated vapour in the reasonably large region 111 of the evaporator results in a drastic reduction in efficiency of the evaporator. This is because a microchannel heat exchanger comprises a multitude of small passages, and when a superheated vapour is present in these passages the tube cannot function properly as a heat exchanger and it reduces the effective surface area for heat exchange to take place. It is particular troublesome for a single-pass microchannel heat exchanger as these comprise a multitude of parallel tubes or passages extending from a manifold at one end of the heat exchanger (at the inlet) to a manifold at the other end (at the outlet) and if the vapour towards one end is superheated (i.e.. toward the outlet manifold in an evaporator) all of the microchannel tubes may be affected at that superheated end, resulting in a large reduction in effective surface area for the heat exchange to take place.
This is also particularly troublesome in single-slab microchannel heat exchanger because the presence of superheated vapour at one end of the (only) slab affects a relatively large proportion of the total area of the evaporator (compared to multiple slabs connected in series).
As a result of these effects it has been found that in some applications the temperature of the refrigerated compartment of the display cabinet (e.g. for certain products) cannot be maintained in the required range for refrigerated products according to ISO 23953-2 when using a single-pass, single-slab microchannel heat exchanger as an evaporator.
Even if the required temperature can be maintained with a microchannel heat exchanger, the resultant decrease in efficiency of the evaporator requires the circulation of more refrigerant in order to maintain a certain temperature in a refrigerated compartment of a refrigeration unit.
A refrigeration unit 200 for a display cabinet 201 is shown in Figure 2. This refrigeration circuit is similar in some aspects to that shown in Figure 1. The refrigeration circuit 200 comprises a variable speed compressor 202, a condenser 203, an internal heat exchanger 204, and an evaporator 205. However, in contrast to the conventional system shown in Figure 1, the refrigeration circuit 200 also comprises a further heat exchanger 211.
Similar to the conventional refrigeration circuit 100 shown in Figure 1, in the refrigeration circuit 200 the evaporator 205 has an inlet 206 and an outlet 207 for refrigerant. However, the evaporator 205 is a single-pass single-slab microchannel heat exchanger with a length L₂. Micro-channel heat exchangers are particularly useful when the space available in the display cabinet is 201 limited as they are much more compact than conventional evaporators. For example, in the case where the width L₀ of the display cabinet 101 is limited the heat exchanger length L₂ can be chosen to substantially fill this limited space and still provide the necessary heat exchange in a single pass and single slab.
The refrigeration unit 200 also comprises a thermostatic expansion valve 208 and a thermostatic bulb 209 for measuring the temperature of the refrigerant as it leaves the further heat exchanger 211 (as opposed to the evaporator outlet 107 in the refrigeration circuit 100 shown in Figure 1).
Compressed refrigerant flows from the variable speed compressor 202, through the condenser 203 and to the internal heat exchanger 204. In the internal heat exchanger 204, the refrigerant leaving the further heat exchanger 211 subcools the refrigerant in the main line 212 (after the refrigerant in the main line leaves the condenser 203) so that a greater overall cooling capacity can be achieved. The subcooled refrigerant then passes from the heat exchanger 204 into the display cabinet 201 and to the further heat exchanger 211.
Following this, a similar subcooling effect is achieved using the further heat exchanger 211, as the subcooled refrigerant coming from the main line 212 is subcooled yet again by the refrigerant leaving the evaporator 205.
The subcooled refrigerant then passes from the further heat exchanger 211 to the thermostatic expansion valve 208. The thermostatic expansion valve 208 regulates the flow and expansion of refrigerant into the inlet 206 of the evaporator 205. The flow and expansion of refrigerant is based on a temperature measurement made by the thermostatic bulb 209 that is positioned after the outlet 213 of the further heat exchanger 211 where refrigerant flows from the further heat exchanger 211 toward the internal heat exchanger 212. The amount of refrigerant released into the evaporator 205 by the thermostatic expansion valve 208 is controlled based on this temperature by ensuring that the refrigerant that leaves the evaporator 205 is in the saturated liquid-vapour state (i.e. a mixture of saturated liquid and vapour) and that the refrigerant that leaves the further heat exchanger is vapour/superheated vapour and/or is of a predetermined temperature.
Once again, this results in a transition length 210 where the refrigerant transitions from a saturated liquid to a mixture of saturated liquid and vapour and subsequently to a superheated vapour. However, in this arrangement the transition length 210 occurs after the evaporator outlet 207, in the further heat exchanger 211 (rather than the evaporator) as it absorbs heat from the (warmer) refrigerant that has come from the main line 212.
By positioning the bulb 209 after the further heat exchanger 211 to ensure the refrigerant leaving the further heat exchanger comprises superheated vapour and/or is of a predetermined temperature and taking into account a known amount of heat exchange that occurs in the further heat exchanger 211, it can be ensured that the transition length 210 occurs within the further heat exchanger 211 and not the evaporator 205.
In this case, the further heat exchanger 211 is another internal heat exchanger and is a tube-and-tube heat exchanger with a length L₃. With a known length, the resultant heat exchange for a range of operating temperatures and compressor speeds can be calculated and so refrigeration units can be designed with a further heat exchanger 211 of sufficient length to ensure that the cooling requirement of the refrigeration unit is met whilst preventing superheating of refrigerant in the evaporator 205 and ensuring the evaporator 205 is flooded along its entire length. This can be determined for the entire range of speeds at which the variable speed compressor 202 can run and/or for the full range of operating temperatures of the refrigeration unit (i.e. temperature ranges of the refrigerated compartment).
By ensuring no superheated vapour is present in the microchannel evaporator, the effective surface area for heat exchange in the evaporator is maintained and a reduction in efficiency is avoided.

Claims

A refrigeration circuit comprising:
a compressor;

a condenser;

an expansion valve and sensor, wherein the expansion valve sensor is for controlling the degree of opening of the expansion valve in accordance with the temperature and/or pressure of refrigerant fluid at the sensor;

an evaporator, wherein the evaporator is a microchannel or microtube heat exchanger; and

a further heat exchanger, wherein the further heat exchanger is positioned in the circuit between a refrigerant outlet of the evaporator and the expansion valve sensor and is configured for internal heat exchange between refrigerant in a return flow line leading from the evaporator and refrigerant in a condensed flow line leading to the expansion valve.
The refrigeration circuit as claimed in claim 1, wherein the further heat exchanger is configured to ensure that during use the evaporator is flooded along its entire length.
The refrigeration circuit as claimed in claim 1 or 2, wherein during use refrigerant from the evaporator is superheated in the further heat exchanger as a result of the internal heat exchange with refrigerant in the condensed flow line leading to the expansion valve.
The refrigeration circuit as claimed in any preceding claim, wherein the evaporator is a single-pass microchannel heat exchanger.
The refrigeration circuit as claimed in any preceding claim, wherein the evaporator is a single-slab microchannel heat exchanger.
The refrigeration circuit as claimed in any preceding claim, wherein the compressor is a variable speed compressor and the further heat exchanger is configured such that, in use, superheating is prevented in the evaporator for the full range of speeds that the compressor can perform.
The refrigeration circuit as claimed in any previous claim, wherein the further heat exchanger is a tube-and-tube heat exchanger.
The refrigeration circuit as claimed in any previous claim, wherein the further heat exchanger has refrigerant paths of sufficient length to prevent superheating in the evaporator.
The refrigeration circuit as claimed in any previous claim, wherein the further heat exchanger has refrigerant paths of sufficient length to ensure the evaporator is flooded along its entire length.
The refrigeration circuit as claimed in any previous claim, wherein the further heat exchanger is a first internal heat exchanger and the refrigeration circuit comprises an additional internal heat exchanger positioned in the circuit between the expansion valve sensor and the compressor, the additional internal heat exchanger being for heat exchange between refrigerant in a return flow line from the first internal heat exchanger and refrigerant in a condensed flow line leading toward the first internal heat exchanger.
The refrigeration circuit as claimed in any previous claim, wherein the expansion valve and sensor control the rate of flow of refrigerant through the further heat exchanger and the evaporator, and wherein no additional control is used to prevent superheating in the evaporator during use.
A refrigeration unit comprising the refrigeration circuit as claimed in any of claims 1 to 11 and a refrigerated compartment that is cooled via heat absorption at the evaporator, preferably wherein the refrigeration unit is a display cabinet.
The refrigeration unit of claim 12, wherein the further heat exchanger is configured such that, in use, superheating is prevented in the evaporator for the full range of operating temperatures of the refrigerated compartment.
A method of manufacturing a refrigeration unit as claimed in claim 12 or 13, comprising:
determining the length and/or size of the further heat exchanger required in order to meet the cooling capacity of the refrigeration unit whilst preventing superheating in the evaporator and ensuring the evaporator is flooded along its entire length; and

providing the further heat exchanger for internal heat exchange between refrigerant in a return flow line leading from the evaporator and refrigerant in a condensed flow line leading to the expansion valve.
A method of operating the refrigeration circuit of any of claims 1 to 11, the method comprising controlling the degree of opening of the expansion valve via the expansion valve sensor in order to reduce or prevent superheated refrigerant in the evaporator.