FIELD

The present invention relates to systems of thermal machines having a condensable working fluid and methods of operating such systems.

BACKGROUND

Thermal machines such as heat pumps are known devices. Heat pumps are generally used to heat indoor spaces or supply hot water to a user. Use of heat pumps is desirable as they provide for more sustainable heat than heating devices which use fossil fuels. Heat pumps transfer thermal energy from a low- temperature heat source to a high-temperature heat sink.

A refrigerant is used in the heat pump system. The refrigerant is a specially selected fluid which absorbs or rejects heat as it circulates through the heat pump system. A compressor in the form of a displacement device is used to pressurise the refrigerant and move the refrigerant through the system.

An expansion valve is used as a controlling device which controls the balance between the operating pressures/temperatures and refrigerant/working fluid flow in combination with the compressor. The expansion valve provides a simple means for reducing the pressure, and hence the temperature, between the condenser section and the evaporator section of the heat pump, thereby completing the thermodynamic cycle, as will be easily understood by a person skilled in the art.

Many industrial processes require heat at high temperatures in the form of, for example, steam or hot water, which is extremely energy-intensive to produce, especially when primary energy sources are used. Examples of industries utilising heat at high temperatures include paper, food and beverages, chemicals, automotive, metal, plastic, engineering, textiles and wood. For example, in the food and beverage industry, heat at high temperatures is used in processes such as drying, evaporation, pasteurisation, sterilisation, boiling, distillation, blanching, scalding, concentrating, tempering and smoking, to name merely a handful of examples.

Industrial waste heat is usually not utilised due to the low temperature of such waste heat, which is lower than the temperature required in many industrial processes. This waste heat can be upgraded using a high-temperature thermal machine such as a high-temperature heat pump, and thus reused, which has clear economic and environmental benefits.

Current high-temperature heat pumps face challenges with regard to the working fluid and with regard to maintaining the desired properties of the lubricant in the compressor. In screw, scroll and vane compressors, a substantial amount of lubricant is typically added to the working fluid to achieve good lubrication as well as good sealing between the flank of the screw/scroll/vane and the housing. When the temperature becomes high, for example >100°C, the lubricant will degrade more quickly and performance is greatly reduced, resulting in reduced run time, increased service costs and reduced service life. At even higher temperatures the lubricant may become completely degraded, and mechanical breakdown may even occur. Therefore, it is very important to maintain a good lubricant quality and integrity over the entire lifetime of the equipment.

Reciprocating compressors are best equipped to operate at temperatures above 100°C due to their largely similar structure to internal combustion engines designed for operating temperatures typically up to 900°C in the working chamber and with lubricant temperatures up to about 100°C.

Current industrial heat pumps can provide temperature lifts to around 160°C. At these temperature levels, the evaporator temperature is usually above 110°C, and there is a relatively high pressure and temperature in the compressor housing which affects the viscosity of the oil.

Using a reciprocating compressor results in small amounts of working fluid blowby (working fluid leaking past the pistons) entering the internal volume filled with lubricant. The working fluid condenses and mixes with the lubricant, reducing the viscosity of the lubricant. A way to reduce the interference of working fluid in the lubricant is to heat the lubricant to a higher temperature to evaporate the working fluid therefrom. However, heating the lubricant also results in reduced viscosity and shorter service life.

At least one aim of the invention is to obviate or at least mitigate one or more drawbacks of prior art.

SUMMARY

According to a first aspect of the invention, there is provided a working fluid extraction system for a displacement machine for removing a condensable working fluid from a working fluid/lubricant mix in an interior volume of the displacement machine separate from a working chamber of the displacement machine, the system comprising a condenser operatively connected to the interior volume, wherein the condenser is settable to a lower temperature than the temperature of the interior volume such that working fluid is drawn from the interior volume to the condenser in use, or is settable to a lower pressure than the pressure of the interior volume such that working fluid is drawn from the interior volume to the condenser in use, or is settable a lower temperature and lower pressure than the temperature and pressure of the interior volume such that working fluid is drawn from the interior volume to the condenser in use, or is settable to a lower temperature than the saturation temperature of the working fluid in the interior volume such that working fluid is drawn from the interior volume to the condenser is use.

The condenser may be configured to condense the working fluid drawn from the interior volume.

The working fluid extraction system may further comprise a first fluid pump configured to pump working fluid from the interior volume to the condenser, to assist in the draw of working fluid from the working fluid/lubricant mix in the interior volume to the condenser.

The working fluid extraction system may further comprise a receptacle fluidly connected to the condenser to receive condensed working fluid in use.

The working fluid extraction system may further comprise a second fluid pump configured to pump condensed working fluid from the condenser to a working fluid circuit of a thermal machine comprising the displacement machine, such that the working fluid drawn from the working fluid/lubricant mix in the interior volume can be reintroduced into a working chamber of the displacement machine.

The working fluid extraction system may further comprise a non-return valve arranged in fluid communication with the second fluid pump and the working fluid circuit, and configured to allow fluid to be pumped from the second fluid pump into the working fluid circuit, but not from the working fluid circuit to the second fluid pump.

The non-return valve may be configured to deliver fluid to a pressure reducing valve in the thermal machine.

The working fluid extraction system may further comprise a vent connected to the interior volume, and configured to allow excess working fluid to be expelled from the interior volume. The vent may be a non-return valve. The vent may be fluidly connected to a working fluid circuit of the thermal machine, such that expelled working fluid can be reintroduced into a working chamber of the displacement machine.

The displacement machine may be a compressor.

The working fluid extraction system may further comprise a fluid separator system for separating the condensable working fluid from the lubricant in the interior volume, the fluid separator system comprising: a fluid separator chamber; a fluid separator inlet operatively connected to the interior volume and configured to allow fluid communication between the interior volume and the fluid separator chamber; a fluid separator shielding member arranged at the fluid separator inlet and configured to restrict a quantity of working fluid/lubricant mix entering the fluid separator inlet; wherein the fluid separator chamber is configured in use to provide a deceleration of the working fluid/lubricant mix entering the fluid separator chamber from the fluid separator inlet, such that, in use, the working fluid/lubricant mix entering the fluid separator chamber is decelerated, thereby separating at least some working fluid from the lubricant.

The fluid separator system may further comprise a fluid separator channel fluidly connected to the fluid separator chamber, such that in use separated working fluid can be evacuated from the fluid separator chamber through the fluid separator channel.

The condenser may be operatively connected to the interior volume via the fluid separator system, such that in use, working fluid can be separated from the working fluid/lubricant mix by the fluid separator chamber, and delivered to the condenser by the fluid separator channel.

According to a second aspect of the invention, there is provided a thermal machine comprising: a displacement machine comprising a working chamber and an interior volume; an evaporator; a pressure reducing valve; a recuperator; a receiver; a filter; a first condenser; a condensable working fluid; a shut-off valve located between the recuperator and the displacement machine; and a working fluid extraction system for removing working fluid from a working fluid/lubricant mix in the interior volume of the displacement machine, comprising a second condenser operatively connected to the interior volume, wherein the second condenser is settable to a lower temperature than the temperature of the interior volume such that working fluid is drawn from the interior volume to the second condenser in use, or is settable to a lower pressure than the pressure of the interior volume such that working fluid is drawn from the interior volume to the second condenser in use, or is settable to a lower temperature and lower pressure than the temperature and pressure of the interior volume such that working fluid is drawn from the interior volume to the second condenser in use, or is settable to a lower temperature than the saturation temperature of the working fluid in the interior volume such that working fluid is drawn from the interior volume to the second condenser is use.

The displacement machine may be a compressor.

The thermal machine may further comprise a fluid separator system for separating the condensable working fluid from the lubricant in the interior volume, the fluid separator system comprising: a fluid separator chamber; a fluid separator inlet operatively connected to the interior volume and configured to allow fluid communication between the interior volume and the fluid separator chamber; a fluid separator shielding member arranged at the fluid separator inlet and configured to restrict a quantity of working fluid/lubricant mix entering the fluid separator inlet; wherein the fluid separator chamber is configured in use to provide a deceleration of fluid entering the fluid separator chamber from the fluid separator inlet, such that, in use, the working fluid/lubricant mix entering the fluid separator chamber is decelerated, thereby separating at least some working fluid from the lubricant.

The fluid separator system further comprises a fluid separator channel fluidly connected to the fluid separator chamber, such that in use separated working fluid can be evacuated from the fluid separator chamber through the fluid separator channel.

The condenser may be operatively connected to the interior volume via the fluid separator system, such that in use, working fluid can be separated from the working fluid/lubricant mix by the fluid separator chamber, and delivered to the condenser by the fluid separator channel.

According to a third aspect of the invention, there is provided a method of extracting working fluid from a working fluid/lubricant mix in an interior volume of a displacement machine, the method comprising the steps of: providing a working fluid extraction system according to the first aspect of the invention; and one of the steps of: setting the temperature of the condenser to less than the temperature of the interior volume; or setting the pressure of the condenser to less than pressure of the interior volume; or setting the temperature of the condenser to less than the temperature of the interior volume and setting the pressure of the condenser to less than the pressure of the interior volume, such that working fluid in the interior volume is drawn to the condenser. According to a fourth aspect of the invention, there is provided a method of extracting working fluid from a working fluid/lubricant mix in an interior volume of a displacement machine, the method comprising the steps of: providing a working fluid extraction system according to the first aspect of the invention; separating the working fluid from the working fluid I lubricant mix in the fluid separator system; and one of the steps of: setting the temperature of the condenser to less than the temperature of the interior volume; or setting the pressure of the condenser to less than pressure of the interior volume; or setting the temperature of the condenser to less than the temperature of the interior volume and setting the pressure of the condenser to less than the pressure of the interior volume, such that separated working fluid is drawn to the condenser.

According to a fifth aspect of the invention, there is provided a fluid separator system for separating a condensable working fluid from a lubricant in a working fluid/lubricant mix, the fluid separator system comprising: a fluid separator chamber; a fluid separator inlet; a fluid separator shielding member arranged at the fluid separator inlet and configured to restrict a quantity of working fluid/lubricant mix entering the fluid separator inlet; wherein the fluid separator chamber is configured in use to provide a deceleration of fluid entering the fluid separator chamber from the fluid separator inlet, such that, in use, the working fluid/lubricant mix entering the fluid separator chamber is decelerated, thereby separating at least some of the working fluid from the lubricant. The fluid separator system may further comprise a fluid separator channel fluidly connected to the fluid separator chamber, such that in use separated working fluid can be evacuated from the fluid separator chamber through the fluid separator channel.

According to a sixth aspect of the invention, there is provided a method of separating a working fluid from a working fluid/lubricant mix the method comprising the steps of: providing a fluid separator system according to the fifth aspect of the invention; passing a working fluid/lubricant mix past the shielding member, thereby delivering a restricted quantity of working fluid/lubricant mix to the fluid separator inlet; passing the working fluid/lubricant mix through the fluid separator inlet to the fluid separator chamber; decelerating the working fluid/lubricant mix in the working fluid chamber, thereby separating at least some of the working fluid from the working fluid I lubricant mix.

The method may further comprise a step of evacuating separated working fluid from the fluid separator chamber through the fluid separator channel.

This extraction system may be advantageous in that it may allow improved performance of lubricants in thermal machines by removing working fluid from the working fluid/lubricant mix in the interior volume without requiring the temperature to be increased in the interior volume.

BRIEF DESCRIPTION OF THE DRAWINGS There will now be described, by way of example only, embodiments of the invention with reference to the following drawings, in which:

Fig. 1 shows a standard heat pump system using a piston compressor;

Fig. 2 shows a detailed view of a piston/cylinder arrangement of the piston compressor used in the heat pump of Fig. 1 ;

Fig. 3 shows an example of a working fluid extraction system in use with a displacement machine in a thermal machine in accordance with an aspect of the invention; and

Fig. 4 shows a piston compressor comprising a fluid separator system.

DETAILED DESCRIPTION OF THE DRAWINGS

Fig 1 shows a prior art thermal machine comprising a displacement machine which utilises a condensable working fluid. More specifically, Fig 1 shows a prior art heat pump system 100 comprising a compressor 110 in the form of a piston compressor, an evaporator 120, a pressure reducing valve 130, a recuperator 140, a receiver 150, a filter 160, a condenser 170, working fluid 180 and lubricant 181 . The compressor 110 comprises a housing 111 , a heating element 112 and a vent 113 which will be explained in more detail below. As will be easily understood by those skilled in the art, a plurality of pistons 114, 114’, 114”, 114”’ are arranged with connection to a crank shaft 114”” which is driven by a motor 116. The compressor 110 has a working chamber 117 where the working fluid 180 is compressed by the pistons 114, 114’, 114”, 114’” and an internal volume 118 comprising the lubricant 181. Fig 2 shows further details of one of the pistons 114 of Fig. 1 . In use working fluid 180 is compressed by the piston 114 in the working chamber 115. Some working fluid 180 may leak past the piston 114 and move from the working chamber 115 into the internal volume 118 which is substantially filled with lubricant 181 to prolong the service life of the compressor 110. When this happens, working fluid 180 mixes with the lubricant 181 and negatively modifies the properties of the lubricant 181 , thereby resulting in decreased service life of the compressor 110 and/or more regular maintenance required. To remove the leaked working fluid 180 from the lubricant 181 in the internal volume 118, the heating element 112 heats the lubricant 181 I working fluid 180 mix in the internal volume 118 to ensure that the working fluid 180 evaporates from the lubricant 181 and to raise the pressure inside the internal volume 118 to force the evaporated working fluid 180 out of the internal volume 118 through the vent 113. The vent 113 is typically in the form of a non-return valve. A major drawback of this system can include the aforementioned negative effects of increasing the temperature of the lubricant 181 - namely decreased viscosity and decreased performance.

Fig. 3 shows an example of an improved thermal machine in the form of a heat pump system 200 using a working fluid extraction system. Similarly to the system 100 described with reference to Fig. 1 , the system 200 comprises a displacement machine in the form of a compressor 210, in the form of a piston compressor, comprising a housing 211 . A plurality of pistons 214, 214’, 214”, 214”’ are arranged with connection to a crank shaft 214”” which is driven by a motor 216. The compressor 210 has a several working chambers which can be grouped together as a working chamber 217 to aid explanation. It will be understood that the compressor 210 may have one or any number of working chambers depending on the number of pistons 214, 214’, 214”, 214’” in the system 200, as will be apparent to a person skilled in the art. In the working chamber 217, working fluid 280 is compressed by the pistons 214, 214’, 214”, 214’” when the system 200 is in use. The compressor 210 also comprises an internal volume 218 comprising a lubricant 281. The internal volume 218 is a space inside the compressor where working fluid 280 is not substantially compressed by the pistons 214, 214’, 214”, 214’”.

The system 200 further comprises an evaporator 220, a pressure reducing valve 230, a recuperator 240, a receiver 250, a filter 260, a first condenser 270, working fluid 280 and lubricant 281. The internal volume 218 is substantially filled with lubricant 281 to prolong the service life of the compressor 210. As previously described, some working fluid 280 may mix with the lubricant 281 in the internal volume 218 to provide a working fluid 2801 lubricant mix 281 in the internal volume 218. The heat pump system 200 further comprises a shut-off valve 241 between the recuperator 240 and the compressor 210. Advantageously, the heat pump system 200 further comprises a second condenser 271 operatively connected to the internal volume 218 such that working fluid 280 in the working fluid 280 / lubricant 281 mix can be drawn from the internal volume 218 to the second condenser 271 where it is condensed. In some instances, the working fluid 280 may flow without assistance from the internal volume 218 to the second condenser 271 . The greater the temperature difference between the hotter internal volume 218 and the cooler second condenser 271 , the more efficiently the working fluid 280 will flow to the second condenser 271 without assistance. In this regard, it is preferable, in use, to maintain the internal volume 218 at a higher temperature than the second condenser 271 . In some examples, the system 200 may optionally further comprise a first fluid pump 290 located between the second condenser 271 and the internal volume 218. The first fluid pump 290 is arranged to pump working fluid 280 from within the internal volume 218 to the second condenser 271. Such a first fluid pump 290 is particularly useful if the required temperature differential between the internal volume 218 and the second condenser 271 cannot be achieved. However, even when the required temperature differential between the internal volume 218 and the second condenser 271 can be achieved, the system 200 may still comprise a first fluid pump 290 to improve the efficiency of the removal of working fluid 280 from within the internal volume 218.

In some examples (not shown in the figures), working fluid 280 which passes (blow-by) the pistons 214, 214’, 214”, 214”’ may be expelled from the system 200 once it has been extracted from the internal volume 218, to for example a suitable storage receptacle (not shown). In such an example, the working fluid 280 in the system may be replenished to replace lost working fluid 280 discharged to the storage receptacle by introducing new working fluid 280 into the system from a working fluid reservoir (not shown). The storage receptacle and working fluid reservoir may be the same vessel, or may be different vessels which may be fluidly connected to allow transfer from the storage receptacle to the working fluid reservoir. Alternatively, the storage receptacle and working fluid reservoir may be different vessels which may not be fluidly connected to allow transfer from the storage receptacle to the working fluid receptable. In such a case, the working fluid in the storage receptacle may need to be discharged and/or additional working fluid 280 may need to be added to the working fluid reservoir during servicing of the heat pump system 200.

Referring again to the system 200 shown in Fig 3, i.e. where the working fluid 280 is not discharged to a storage receptacle but is instead recycled within the system 200, a second fluid pump 291 is connected to the second condenser 271 to transport the working fluid 280 extracted from the internal volume 218 back to the normal working fluid circuit of the heat pump system 200. In this way, the working fluid 280 which has leaked past any of the pistons 214, 214’, 214”, 214”’, commonly referred to as ‘blow-by’, is efficiently extracted from the working fluid 2801 lubricant 281 mix and reintroduced back into the working fluid circuit where it is desirable that it is maintained such that it does not change the carefully selected properties of the lubricant 281 . Still referring to Fig 3, in the presently described example, the working fluid 280 recovered from the internal volume 218 is reintroduced through a nonreturn valve 292 into the working fluid circuit and mixed with the working fluid 280 in the working fluid circuit before the combined working fluid 280 enters the pressure reducing valve 230. In some alternative examples, the working fluid 280 recovered from the housing 211 may be reintroduced through a non-return valve 292 into the working fluid circuit and mixed with the working fluid 280 in the working fluid circuit after the pressure reducing valve 230, i.e. downstream of the pressure reducing valve 230.

Advantageously, using the presently described system 200 allows the condensation temperature at the second condenser 271 to be adjusted (for example by a control system (not shown)) independently of the evaporation temperature at the evaporator 220, and allows the condensation temperature of the second condenser 271 to be maintained sufficiently low relative to the evaporation temperature of the evaporator 220 such that a negative pressure occurs in the internal volume 218, which sucks the working fluid 280 from within the internal volume 218 towards the second condenser 271 .

A differential pressure between the evaporator side and internal volume 218 sufficient to ensure that the working fluid 280 leaking past the piston evaporates immediately when it enters the internal volume 218, and stays as a vapour due to the lower pressure in the internal volume 218, is highly advantageous.

The presently described system 200 can thus allow a lower pressure and temperature to be maintained in the internal volume 218 which can ensure evaporation of working fluid 280 from the working fluid 2801 lubricant 281 mix, thereby maintaining the desired properties of the lubricant 281 including the desired viscosity.

The heat pump system 200 of Fig 3 may in some examples further comprise a vent (not shown in Fig 3), similar to the vent 113 of the system 100 described in Fig 1 , as a safety feature to ensure that working fluid 280 breaching the pistons can always be vented from the internal volume 218.

Fig 4 shows a simplified drawing of a piston compressor 310 in a so called “V”-configuration, comprising a compressor block 311 , a first cylinder head 312 and a second cylinder head 313. The compressor 310 further comprises first and second pistons 314, 314’ angularly separated by about 90° as shown. The pistons 314, 314’ are arranged with connection to a crank shaft 314”” which is driven by a motor (not shown). The crank shaft 314”” comprises counterweights 315 to reduce vibration. In other examples, the compressor 310 may comprise many more pistons 314, 314’. Preferably, there are an even number of pistons 314, 314’, such as sixteen pistons for example, however an odd number of pistons 314, 314’ is feasible. The compressor 310 has a several working chambers which can be grouped together as a working chamber 317 to aid explanation. It will be understood that the compressor 310 may have one or any number of working chambers 317 depending on the number of pistons 314, 314’, as will be apparent to a person skilled in the art. The compressor 310 further comprises a first inlet valve 316A and a first outlet valve 316B associated with the first piston 314 and a second inlet valve 316’A and a second outlet valve 316’B associated with the second piston 314’. Additionally, the compressor comprises standard first valve control components 319 and standard second valve control components 319’ which are configured to open and close the first and second inlet and outlet valves 316A, 316B, 316’A, 316’B as a person skilled in the art will be familiar. In this regard, the configuration of the first and second valves control components 319, 319’ may take many forms, and are not explained here in the interests of brevity and clarity.

In the working chamber 317, working fluid is compressed by the pistons 314, 314’ when the compressor 310 is in use. The compressor 310 also comprises an internal volume 318 comprising a lubricant 381. The internal volume 318 is a space inside the compressor 310 where working fluid is not substantially compressed by the pistons 314, 314’.

As previously explained with reference to the heat pump system 200 in Fig 3, externally of the compressor 310, a working fluid may undergo heat exchange in one or more heat exchangers (not shown). The working fluid is led to the compressor typically through a pipe or a manifold.

Some working fluid may leak past the pistons 314, 314’ and move from the working chamber 317 into the internal volume 318 which is substantially filled with lubricant 381 to prolong the service life of the compressor 310. When this happens, working fluid mixes with the lubricant 381 and negatively modifies the properties of the lubricant 381 , thereby resulting in decreased service life of the compressor 310 and/or more regular maintenance required, as previously explained.

It is typically highly desirable to separate at least some of the working fluid from the working fluid I lubricant 381 mix. Preferably, a substantial quantity of working fluid may be separated from the working fluid I lubricant 381 mix. More preferably, all of the working fluid may be separated from the working fluid I lubricant 381 mix. In some configurations, the working fluid may be separable from the lubricant 381 within the compressor 310, and without requiring further processing, i.e. it can be directly recycled or reintroduced into another part of the compressor 310 or to another system, for example a heat pump system, which is compressor is part of.

Alternatively, the compressor 310 may be attached to a second condenser

271 (Fig 3) or to another system configured to extract and/or process the working fluid from the working fluid I lubricant 381 mix. Regardless of the presence of a downstream system which may or may not be attached to the compressor 310 to extract and/or process the working fluid from the working fluid I lubricant 381 mix such that the working fluid can be recycled or reused, the compressor 310 shown in Fig 3 comprises a fluid separator system 340 to separate at least some of the leaked working fluid from the lubricant 381 within the compressor 310.

Such separation ensures that at least some of the working fluid is sufficiently separated to be effectively processed by downstream processes which, as in the example given in Fig 3, may return the working fluid to another part of a heat pump system 200 via the second condenser 271 . In such cases it is often highly desirable to ensure that as much lubricant 381 as possible is removed from the working fluid I lubricant 381 mix, as the presence of lubricant 381 in the working fluid being extracted and processed may hinder the processing of the working fluid and/or affect the performance of other components of the system to which the compressor 310 is attached or forms part of, for example the heat pump system 200 which the compressor 310 may form a part of. In such a case, the presence of lubricant 381 may affect the performance of the second condenser 271. For example, a thin layer of lubricant 381 on a working surface of the second condenser 271 may drastically reduce the heat transfer coefficient of the working surface, thereby drastically affecting performance. The configuration of the fluid separator system 340 to allow separation of at least some of the working fluid from the working fluid I lubricant 381 mix within the compressor 310 is now explained.

The fluid separator system 340 comprises a fluid separator channel 341 , a fluid separator shielding member 342, a fluid separator chamber 343 and a fluid separator inlet 344. The fluid separator channel 341 is fluidly connected to the internal volume 318 of the compressor 310. In this connection, a working fluid I lubricant 381 mix is directed past the fluid separator shielding member 342 and into a fluid separator chamber 343 through the fluid separator inlet 344. The shielding member 342 functions to limit the amount of lubricant 381 droplets which are carried into the fluid separator chamber 343 together with the working fluid. The fluid separator chamber 343 has sufficient volumetric space to ensure that the working fluid velocity is reduced when the working fluid I lubricant 381 mix enters the fluid separator chamber 343, and droplets of lubricant 381 are thereby separated from the working fluid before the working fluid is evacuated through the fluid separator channel 341 and allowed to proceed to a downstream collector or recycling system, such as the above-described second condenser 271 arrangement described with respect to Fig 3, or to be directly used within the compressor 310.

In some examples, a 20-30kW condenser may be used with a 1 MW compressor. In other examples, a 100-150kW condenser may be used with a 5MW compressor. In other examples, a 200kW-300kW condenser may be used with a 10MW compressor.