REFRIGERATION CYCLE APPARATUS

Abstract

A refrigeration cycle apparatus is provided with a refrigerant circuit through which a zeotropic refrigerant mixture circulates and a heat exchanger that allows the zeotropic refrigerant mixture and a fluid to exchange heat with each other, in which, to the heat exchanger, pressure loss is applied such that refrigerant-outlet temperature that is the temperature of refrigerant at an outlet decreases by a set amount.

Description

TECHNICAL FIELD

The present disclosure relates to a refrigeration cycle apparatus configured to condition air in an air-conditioning target space.

BACKGROUND

In recent years, refrigerants such as an HFO refrigerant, which is a low global warming potential refrigerant and is also referred to as a low GWP refrigerant, have been used for environmental protection. In a case in which a low-pressure refrigerant, such as an HFO refrigerant, is singly used, the capability of a heat exchanger, however, significantly decreases. For this reason, when a low-pressure refrigerant is to be used, the refrigerant is mixed with a high-pressure refrigerant, such as R32, into a zeotropic refrigerant mixture, which has been used. Patent Literature 1, for example, is referable.

PATENT LITERATURE

• • Patent Literature 1: International Publication No. 2018/025305

Incidentally, in a case in which a zeotropic refrigerant mixture is to be in use, problems lie in that the amount of a high-pressure refrigerant to be mixed is limited to maintain its GWP at a low level and the difference between high pressure and low pressure changes phases and a heat exchanger is thus inevitably decreased in capability and is increased in size.

SUMMARY

The present disclosure is to address the problems in the related art described above and is to provide a refrigeration cycle apparatus that has a heat exchanger of which decreased capability and increased size are limited even in a case in which a zeotropic refrigerant mixture is in use.

A refrigeration cycle apparatus according to an embodiment of the present disclosure provided with a refrigerant circuit through which a zeotropic refrigerant mixture circulates is provided with a heat exchanger that allows the zeotropic refrigerant mixture and a fluid to exchange heat with each other, in which, to the heat exchanger, pressure loss is applied such that refrigerant-outlet temperature that is the temperature of refrigerant at an outlet decreases by a set amount.

According to an embodiment of the present disclosure, pressure loss is applied to a heat exchanger such that refrigerant-outlet temperature decreases by a set amount and the heat exchanger is thus configured to have an increased capability that is higher than the capability of a heat exchanger to which no pressure loss is applied. Decrease in capability and increase in size of the heat exchanger are therefore limited even in a case in which a zeotropic refrigerant mixture is in use.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a circuit diagram that illustrates an exemplary configuration of an air-conditioning apparatus according to Embodiment 1.

FIG. 2 is a Mollier chart that illustrates temperature characteristics of a zeotropic refrigerant mixture and single-phase refrigerant.

FIG. 3 is a graph with which the temperature of a zeotropic refrigerant mixture in a heat exchanger is described.

FIG. 4 is a graph with which a relationship between pressure loss at the heat exchanger and the difference in temperature between refrigerant-outlet temperature and refrigerant-inlet temperature is described.

DETAILED DESCRIPTION

An embodiment of the present disclosure is described below with reference to drawings. The present disclosure is not limited to the embodiment described below and may also be variously changed without departing from the spirit of the present disclosure. In addition, the present disclosure includes any combination of combinable configurations among configurations described in the embodiment described below. In addition, elements that have the same reference signs in the drawings are the same or equivalent elements and the reference signs are common in the full text of the specification.

Embodiment 1

A refrigeration cycle apparatus according to Embodiment 1 is described below.

[Configuration of Air-Conditioning System]

FIG. 1 is a circuit diagram that illustrates an exemplary configuration of an air-conditioning apparatus according to Embodiment 1. As illustrated in FIG. 1, an air-conditioning apparatus 100, which serves as the refrigeration cycle apparatus, is provided with a heat-source unit 10, which has a refrigerant circuit, and a use-side unit 20, which has a use-side circuit.

Heat-Source Unit 10

The heat-source unit 10 is provided with a compressor 11, a heat-medium heat exchanger 12, an expansion device 13, an evaporator 14, a fan 15, an inlet-temperature sensor 16, and an outlet-temperature sensor 17. The compressor 11, the heat-medium heat exchanger 12, the expansion device 13, and the evaporator 14 are connected by refrigerant pipes and the refrigerant circuit is thus formed through which refrigerant circulates. In addition, in Embodiment 1, a zeotropic refrigerant mixture is used as the refrigerant that flows through the refrigerant circuit. A zeotropic refrigerant mixture is, for example, a mixture of a low-pressure refrigerant, such as an HFO refrigerant, and a high-pressure refrigerant, such as R32.

The compressor 11 is configured to suck low-temperature and low-pressure refrigerant and compress the sucked refrigerant and discharge high-temperature and high-pressure refrigerant. The compressor 11 is a compressor such as an inverter compressor of which capacity that is the amount of refrigerant transmitted per unit time is controlled by, for example, changing its operation frequency.

The heat-medium heat exchanger 12 allows refrigerant that flows through a portion of the refrigerant circuit connected to a refrigerant flow passage and a heat medium that flows through a portion of a heat-medium circuit described later and connected to a heat-medium flow passage to exchange heat with each other. The heat-medium heat exchanger 12 serves as a condenser that transfers the heat of refrigerant to heat medium and thus condenses the refrigerant. The expansion device 13 is, for example, an expansion valve, and configured to reduce the pressure of refrigerant and thus expand the refrigerant. The expansion device 13 is, for example, a valve such as an electronic expansion valve of which opening degree is controllable.

The evaporator 14 allows refrigerant and air supplied by the fan 15 to exchange heat with each other. Specifically, the evaporator 14 evaporates refrigerant and, by use of resultant heat of evaporation generated at this time, cools air. The fan 15 is driven by an unillustrated motor and thus configured to supply air to the evaporator 14. The rotation frequency of the fan 15 is controlled by an unillustrated controller and the amount of air transmitted to the evaporator 14 is thus adjusted.

The inlet-temperature sensor 16 is provided to a refrigerant inlet of the evaporator 14 and configured to detect refrigerant-inlet temperature, which is the temperature of refrigerant that flows into the evaporator 14. The outlet-temperature sensor 17 is provided to a refrigerant outlet of the evaporator 14 and configured to detect refrigerant-outlet temperature, which is the temperature of refrigerant that flows out from the evaporator 14.

Use-Side Unit 20

The use-side unit 20 is provided with a primary-side pump 21, a tank 22, a secondary-side pump 23, and a load 24. The use-side circuit included in the use-side unit 20 is a circuit through which a heat medium circulates and is composed of a primary-side heat-medium circuit and a secondary-side heat-medium circuit. The primary-side pump 21, the heat-medium heat exchanger 12 of the heat-source unit 10, and the tank 22 are connected by pipes and the primary-side heat-medium circuit is thus formed through which a heat medium circulates. In addition, the secondary-side pump 23, the tank 22, and the load 24 are connected by pipes and the secondary-side heat-medium circuit is thus formed through which a heat medium circulates. As a heat medium that circulates through the primary-side heat-medium circuit and the secondary-side heat-medium circuit, water or brine is used.

The primary-side pump 21 is driven by an unillustrated motor and thus configured to transmit a heat medium that has flowed out from the tank 22 and supply the heat medium to the heat-medium flow passage in the heat-medium heat exchanger 12. The primary-side pump 21 is thus configured to circulate a heat medium through the primary-side heat-medium circuit. The tank 22 receives and stores a heat medium that is heated by exchanging heat with refrigerant in the heat-medium heat exchanger 12 and that flows into the tank 22.

The secondary-side pump 23 is driven by an unillustrated motor and thus configured to transmit a heat medium that has flowed out from the tank 22 and supply the heat medium to the load 24. The secondary-side pump 23 is thus configured to circulate a heat medium through the secondary-side heat-medium circuit. The load 24 is a device that uses heat supplied by a heat medium. As the load 24, for example, an air-conditioning apparatus, floor heating, or a water heater is used.

[Actions of Air-Conditioning Apparatus 100]

Actions of the air-conditioning apparatus 100, which is configured as described above, are described next. First, in the heat-source unit 10, refrigerant that flows through the refrigerant circuit is compressed and discharged by the compressor 11. The refrigerant discharged from the compressor 11 flows into the heat-medium heat exchanger 12. The refrigerant that has flowed into the heat-medium heat exchanger 12 heats a heat medium that flows through the primary-side heat-medium circuit by exchanging heat with the heat medium and transferring heat while condensing and then flows out from the heat-medium heat exchanger 12.

The refrigerant that has flowed out from the heat-medium heat exchanger 12 is reduced in pressure and expanded by the expansion device 13 and then flows out from the expansion device 13. The refrigerant that has flowed out from the expansion device 13 flows into the evaporator 14. The refrigerant that has flowed into the evaporator 14 exchanges heat with air, receives heat, evaporates, and then flows out from the evaporator 14. The refrigerant that has flowed out from the evaporator 14 is sucked into the compressor 11. The refrigerant then repeats the circulation describe above.

In the use-side unit 20, on the other hand, the primary-side pump 21 drives and a heat medium thus flows out from the tank 22. The heat medium that has flowed out from the tank 22 flows into the heat-medium heat exchanger 12. The heat medium that has flowed into the heat-medium heat exchanger 12 is heated by exchanging heat with refrigerant and then flows out from the heat-medium heat exchanger 12. The heat medium that has flowed out from the heat-medium heat exchanger 12 flows into the tank 22 and is then stored in the tank 22

In addition, the secondary-side pump 23 drives and a heat medium thus flows out from the tank 22. The heat medium that has flowed out from the tank 22 flows into the load 24. Heat of the heat medium that has flowed into the load 24 is used by the load 24 and the heat medium then flows out from the load 24. The heat medium that has flowed out from the load 24 flows into the tank 22. The heat medium then repeats the circulation describe above in the primary-side heat-medium circuit and the secondary-side heat-medium circuit.

[Capability of Heat Exchanger]

The capability of a heat exchanger is described below. The capability of a heat exchanger is usually expressible by use of mathematical formula (1). In mathematical formula (1), Q indicates a heat-exchange capability [kW] and A indicates a heat-transfer area [m²]. In addition, K indicates an overall heat transfer coefficient [W/(m²·K)] and ΔT indicates a logarithmic mean temperature difference.

$\begin{array}{cc}Q=A\times K\times \Delta T& \left(1\right)\end{array}$

In addition, the logarithmic mean temperature difference ΔT is expressible by use of mathematical formula (2). In mathematical formula (2), ΔTa is the difference between outdoor air temperature and the temperature of refrigerant at an inlet of the heat exchanger and is calculated by use of mathematical formula (3). ΔTb is the difference between outdoor air temperature and the temperature of refrigerant at an outlet of the heat exchanger and is calculated by use of mathematical formula (4).

$\begin{array}{cc}\Delta T=\left(\Delta \mathrm{Ta}-\Delta \mathrm{Tb}\right)/\left(\ln \left(\Delta \mathrm{Ta}/\Delta \mathrm{Tb}\right)\right)& \left(2\right)\end{array}$ $\begin{array}{cc}\Delta \mathrm{Ta}=\mathrm{outdoor}\mathrm{air}\mathrm{temperature}-\mathrm{refrigerant}-\mathrm{inlet}\mathrm{temperature}& \left(3\right)\end{array}$ $\begin{array}{cc}\Delta \mathrm{Tb}=\mathrm{outdoor}\mathrm{air}\mathrm{temperature}-\mathrm{refrigerant}-\mathrm{outlet}\mathrm{temperature}& \left(4\right)\end{array}$

In a relationship indicated by mathematical formula (2), in a case in which the heat exchanger has the predetermined values of the heat-transfer area A and the overall heat transfer coefficient K, as the logarithmic mean temperature difference ΔT decreases, the capability Q also decreases. On the other hand, to maintain the capability of the heat exchanger at a fixed capability irrespective of the value of the logarithmic mean temperature difference ΔT, as the logarithmic mean temperature difference ΔT decreases, the value from a mathematical formula of A×K, which is the product of the heat-transfer area A and the overall heat transfer coefficient K, has to be increased. The heat exchanger thus has to be increased in size.

In addition, in Embodiment 1, as refrigerant that flows through the refrigerant circuit of heat-source unit 10, a zeotropic refrigerant mixture, which is a mixture of a low-pressure refrigerant and a high-pressure refrigerant, is used. For these reasons, a case is considered in which a zeotropic refrigerant mixture flows through such a heat exchanger.

FIG. 2 is a Mollier chart that illustrates temperature characteristics of a zeotropic refrigerant mixture and single-phase refrigerant. In FIG. 2, solid lines are each a temperature line, which represents the temperature of a zeotropic refrigerant mixture, and a dotted line is a temperature line, which represents the temperature of single-phase refrigerant. As illustrated in FIG. 2, the temperature lines of zeotropic refrigerant mixtures each have a temperature gradient, which is caused by phase change in the difference between high pressure and low pressure, and that is different from a temperature gradient of the temperature line of single-phase refrigerant.

FIG. 3 is a graph with which the temperature of a zeotropic refrigerant mixture in a heat exchanger is described. In FIG. 3, a horizontal axis represents positions in a direction in which refrigerant flows in the heat exchanger and a vertical axis represents the temperature of a zeotropic refrigerant mixture. “INLET” represents a position at which a refrigerant inlet of the heat exchanger is located and, for example, in a case of the evaporator 14 in Embodiment 1, represents a position at which the inlet-temperature sensor 16 is located. Also, “OUTLET” represents a position at which a refrigerant outlet of the heat exchanger is located and, for example, in a case of the evaporator 14 in Embodiment 1, represents a position at which the outlet-temperature sensor 17 is located.

As illustrated in FIG. 3, the temperature of a zeotropic refrigerant mixture that flows through the heat exchanger changes as illustrated in a straight line A represented by a solid line. Specifically, in this example, when a zeotropic refrigerant mixture flows into the heat exchanger, the refrigerant-inlet temperature is 0 [degrees C.], and, when a zeotropic refrigerant mixture flows out from the heat exchanger, the refrigerant-outlet temperature is 7 [degrees C.].

Incidentally, when pressure loss is applied to the heat exchanger, the temperature of a zeotropic refrigerant mixture changes as illustrated in a curve line B represented by a dotted line illustrated in FIG. 3. When the refrigerant-outlet temperature decreases by, for example, ΔTm as illustrated in the curve line B, the value of ΔTb expressed by mathematical formula (4) increases and is larger than the value in a case in which pressure loss is not applied. The logarithmic mean temperature difference ΔT expressed by mathematical formula (2) accordingly increases. The capability Q of the heat exchanger expressed by mathematical formula (1) consequently increases and decrease in capability and increase in size of the heat exchanger are thus limited.

In a case in which, for example, an outdoor air temperature is 8 [degrees C.] and the temperature of a zeotropic refrigerant mixture changes as illustrated in the straight line A illustrated in FIG. 3, ΔTa and ΔTb are calculated as described below by use of mathematical formula (3) and mathematical formula (4).

$\Delta \mathrm{Ta}=\mathrm{outdoor}\mathrm{air}\mathrm{temperature}-\mathrm{refrigerant}-\mathrm{inlet}\mathrm{temperature}=8-0=8\left[\mathrm{degrees}C.\right]$ $\Delta \mathrm{Tb}=\mathrm{outdoor}\mathrm{air}\mathrm{temperature}-\mathrm{refrigerant}-\mathrm{outlet}\mathrm{temperature}=8-7=1\left[\mathrm{degrees}C.\right]$

The logarithmic mean temperature difference ΔT in this case is therefore calculated as described below by use of mathematical formula (2).

$\Delta T=\left(\Delta \mathrm{Ta}-\Delta \mathrm{Tb}\right)/\left(\ln \left(\Delta \mathrm{Ta}/\Delta \mathrm{Tb}\right)\right)=3.37\left[\mathrm{degrees}C.\right]$

On the other hand, in a case in which an outdoor air temperature is 8 [degrees C.], the refrigerant-outlet temperature is 5 [degrees C.], and the temperature of a zeotropic refrigerant mixture changes as illustrated in the curve line B illustrated in FIG. 3, ΔTa and ΔTb are calculated as described below by use of mathematical formula (3) and mathematical formula (4).

$\Delta \mathrm{Ta}=\mathrm{outdoor}\mathrm{air}\mathrm{temperature}-\mathrm{refrigerant}-\mathrm{inlet}\mathrm{temperature}=8-0=8\left[\mathrm{degrees}C.\right]$ $\Delta \mathrm{Tb}=\mathrm{outdoor}\mathrm{air}\mathrm{temperature}-\mathrm{refrigerant}-\mathrm{outlet}\mathrm{temperature}=8-5=3\left[\mathrm{degrees}C.\right]$

The logarithmic mean temperature difference ΔT in this case is therefore calculated as described below by use of mathematical formula (2).

$\Delta T=\left(\Delta \mathrm{Ta}-\Delta \mathrm{Tb}\right)/\left(\ln \left(\Delta \mathrm{Ta}/\Delta \mathrm{Tb}\right)\right)=5.1\left[\mathrm{degrees}C.\right]$

In other words, in a case in which the temperature of a zeotropic refrigerant mixture changes as illustrated in the curve line B, the logarithmic mean temperature difference ΔT increases and is larger than the difference in a case in which the temperature of a zeotropic refrigerant mixture changes as illustrated in the straight line A.

To address the problem, in Embodiment 1, pressure loss is intentionally applied to the evaporator 14, which is the heat exchanger, to limit decrease in capability and increase in size of the evaporator 14. Specifically, a pipe diameter, the number of paths, a flow speed at which refrigerant flows inside a pipe, or other factor of the evaporator 14 is changed such that pressure loss is applied to the evaporator 14 by a set amount. In other words, at least one of a pipe diameter, the number of paths, and a flow speed at which refrigerant flows is determined and pressure loss is thus applied to the evaporator 14.

In a state in which, for example, the number of paths and a flow rate at which refrigerant flows inside a pipe of the evaporator 14 are left unchanged, as a pipe diameter is decreased, or pressure loss applied to the evaporator 14 accordingly increases. In addition, in a state in which a pipe diameter and a flow rate at which refrigerant flows inside a pipe of the evaporator 14 are left unchanged, as the number of paths is decreased, a flow rate at which refrigerant flows in each pipe accordingly increases. Pressure loss applied to the evaporator 14 thus increases. In addition, in a state in which a pipe diameter and the number of paths of the evaporator 14 are left unchanged, as a flow rate at which refrigerant flows inside a pipe increases, pressure loss applied to the evaporator 14 accordingly increases. In this case, by increasing the rotation frequency of an inverter of the compressor 11 such that the operation frequency of the compressor 11 is higher than the current operation frequency, the flow rate of the refrigerant is achieved to be increased.

FIG. 4 is a graph with which a relationship between pressure loss at the heat exchanger and the difference in temperature between refrigerant-outlet temperature and refrigerant-inlet temperature is described. In FIG. 4, a horizontal axis represents pressure loss at the heat exchanger and a vertical axis represents an outlet-inlet temperature difference, which is the difference in temperature between refrigerant-outlet temperature and refrigerant-inlet temperature at the heat exchanger.

A line X illustrated in FIG. 4 represents a relationship between pressure loss and an outlet-inlet temperature difference in a case in which decrease temperature ΔTm of the temperature of refrigerant at an outlet of the heat exchanger is caused to be first decrease temperature. In this example, the first decrease temperature is 3 [degrees C.]. In addition, a line Y represents a relationship between pressure loss and an outlet-inlet temperature difference in a case in which decrease temperature ΔTm is caused to be second decrease temperature. In this example, the second decrease temperature is 5 [degrees C.], which is higher than the first decrease temperature. The first decrease temperature and the second decrease temperature are not limited to this example and may also be changed to be suited as required.

In the example illustrated in FIG. 4, an area located between the line X and the line Y and marked with diagonal lines is an area in which the heat exchanger effectively produces its performance and the logarithmic mean temperature difference ΔT is achieved to be increased. In other words, the heat exchanger is formed such that the pressure loss is larger than or equal to the amount of pressure loss in a case in which a decrease amount of refrigerant-outlet temperature is caused to be the first decrease temperature and the pressure loss is less than or equal to the amount of pressure loss in a case in which a decrease amount of refrigerant-outlet temperature is caused to be the second decrease temperature, which is high. The heat exchanger is provided such that the pressure loss is in this area marked with diagonal lines and the logarithmic mean temperature difference ΔT is thus increased and decrease in capability and increase in size of the heat exchanger are also thus limited. This area marked with diagonal lines is changed to be suited to the specifications and required performance of the heat exchanger.

As described above, in the air-conditioning apparatus 100 according to Embodiment 1, pressure loss is applied to the evaporator 14 such that refrigerant-outlet temperature decreases by a set amount. The logarithmic mean temperature difference ΔT is thus caused to be increased and the capability Q of the evaporator 14 is achieved to be increased. Even when a zeotropic refrigerant mixture is in use, decrease in capability and increase in size of the heat exchanger are therefore limited.

While Embodiment 1 is described above, the present disclosure is not limited to Embodiment 1 described above and may also be variously changed and applied without departing from the gist of the present disclosure. For example, while the air-conditioning apparatus 100 is described as an example of the refrigeration cycle apparatus in Embodiment 1, the refrigeration cycle apparatus is not limited to this example and may also be any refrigeration cycle apparatus as long as a refrigerant circuit through which refrigerant circulates is provided to the refrigeration cycle apparatus.

Claims

1. A refrigeration cycle apparatus comprising: a refrigerant circuit through which a zeotropic refrigerant mixture circulates; anda heat exchanger that allows the zeotropic refrigerant mixture and a fluid to exchange heat with each other,to the heat exchanger, pressure loss being applied such that refrigerant-outlet temperature that is temperature of refrigerant at an outlet decreases by a set amount,the heat exchanger being formed such that the pressure loss is larger than or equal to an amount of pressure loss in a case in which a decrease amount of the refrigerant-outlet temperature is caused to be first decrease temperature and the pressure loss is less than or equal to an amount of pressure loss in a case in which a decrease amount of the refrigerant-outlet temperature is caused to be second decrease temperature that is higher than the first decrease temperature.

2. The refrigeration cycle apparatus of claim 1, wherein at least one of a pipe diameter, the number of paths, and a flow speed at which the zeotropic refrigerant mixture flows is determined such that the pressure loss is applied to the heat exchanger.

3. (canceled)

4. The refrigeration cycle apparatus of claim 1, wherein the heat exchanger comprises an evaporator that allows the zeotropic refrigerant mixture and air to exchange heat with each other.

5. The refrigeration cycle apparatus of claim 4, wherein the first decrease temperature is temperature higher than 0 degrees, andthe second decrease temperature is temperature lower than a difference between refrigerant-inlet temperature that is temperature of refrigerant at an inlet and the refrigerant-outlet temperature at the heat exchanger to which the pressure loss is not applied.