US20120060534A1

US20120060534A1 - Heat pump water heater

Info

Publication number: US20120060534A1
Application number: US12/976,378
Authority: US
Inventors: Maoyong Zhang; Jianliang Wang
Original assignee: AO Smith China Water Heater Co Ltd; AO Smith Corp
Current assignee: AO Smith China Water Heater Co Ltd; AO Smith Corp
Priority date: 2009-12-26
Filing date: 2010-12-22
Publication date: 2012-03-15
Also published as: CN101839547A; CN101839547B; CA2726394A1

Abstract

A heat pump water heater that includes a compressor, a condenser, an expansion device, and an evaporator. The condenser is in a heat exchange relationship with a water storage tank and includes a heat exchange coil with a first section and a second section. The first section has a first flow area. The second section has a second flow area that is less than the first flow area and the second section is downstream from the first section. The compressor, the condenser, the expansion device, and the evaporator are connected in series.

Description

BACKGROUND OF THE INVENTION

The present invention relates to heat pump water heaters, and more particularly, to condensing heat exchangers for heat pump water heaters.

SUMMARY OF THE INVENTION

The present invention provides, in one aspect, a heat pump water heater including a compressor, a condenser, an expansion device, and an evaporator. The condenser is in a heat exchange relationship with a water storage tank and includes a heat exchange coil with a first section and a second section. The first section has a first flow area. The second section has a second flow area that is less than the first flow area and the second section is downstream from the first section. The compressor, the condenser, the expansion device, and the evaporator are connected in series.
Other features and aspects of the invention will become apparent by consideration of the following detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a condensing heat exchanger.

FIG. 2 is a side view of the condensing heat exchanger of FIG. 1.

FIG. 3 is a front view of the condensing heat exchanger of FIG. 1.

FIG. 4 is a top view of the condensing heat exchanger of FIG. 1.

FIG. 5 is a schematic view of an internal coil water storage tank including the condensing heat exchanger of FIG. 1.

FIG. 6 is a schematic view of an external coil water storage tank including the condensing heat exchanger of FIG. 1.

FIG. 7 is a schematic view of a heat pump water heater including the condensing heat exchanger of FIG. 1.

FIG. 8 is a sectional view of the first section of the heat exchange coil.

FIG. 9 is a sectional view of the second section of the heat exchange coil.

Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting.

DETAILED DESCRIPTION

Heat pump waters heaters have good energy saving effects and have experienced rapid development in recent years. In an internal coil heat pump water heater, a water heat exchanger is positioned in a hot water storage tank and the high temperature refrigerant passes through the heat exchanger to heat the water stored in the storage tank. Alternatively, in an external coil heat pump water heater, the heat exchanger surrounds the outer surface of the hot water storage tank. This helps solve corrosion problems associated with positioning the heat exchanger in the storage tank. The heat transfer capability of these types of heat pump water heaters is relatively low. The reason is mainly that the refrigerant vapor condenses as it passes through the heat exchanger coil, thereby causing complex two-phase heat exchange between the refrigerant in the heat exchanger and the water in the storage tank. The condensed liquid refrigerant forms a liquid film along the inner wall of the coil. The liquid film gets thicker as the refrigerant passes through the coil. The liquid film hinders heat exchange between the refrigerant vapor and the water in the storage tank and leads to a reduced heat exchange capability of the heat exchanger. Additionally, these types of heat exchangers are relatively expensive and heavy, consume a lot of materials and occupy a large space, and tend to cause reliability problem due to corrosion and stress concentration. Overcoming these shortcomings by using a smaller heat exchanger saves cost, but also results in a reduction of heating capability and coefficient of performance (COP) of the heat pump. The COP of a heat pump is the heat supplied to a heat reservoir divided by the work consumed by the heat pump.
FIG. 7 illustrates a heat pump water heater 100 that includes a compressor 105, a condensing heat exchanger or condenser 110, an expansion device 115, and an evaporator 120 connected in series. The heat pump water heater 100 functions according to a vapor-compression heat pump cycle.
As shown in FIG. 5, the heat exchanger 110 is positioned in a water storage tank 125 and includes a generally spiral-shaped heat exchange coil 130 formed by two tubular sections 135 and 140. As shown in FIGS. 1-3, the first section 135 is connected to the second section 140 in series so that refrigerant flows through the first section 135 and then through the second section 140. In other words, the second section 140 is downstream from the first section 135. As shown in FIG. 8, the first section 135 has an inner diameter 145, an outer diameter 150, and a first section length. As shown in FIG. 9, the second section 140 has an inner diameter 155, an outer diameter 160, and a second section length. The inner diameter 145 is larger than the inner diameter 155 so that a flow area 157 of the first section 135 is larger than a flow area 159 of the second section 140. The flow area ratio of the flow area 157 to the flow area 159 can vary. Preferably, the flow area ratio is greater than or equal to two and less than or equal to four. The outer diameter 150 is greater than the outer diameter 160. In some embodiments, a first section wall thickness 162 between the inner diameter 145 and the outer diameter 155 is the same as a second section wall thickness 164 between the inner diameter 155 and the outer diameter 160. The length ratio of the first section length to the second section length can vary. Preferably, the length ratio is greater than or equal to 0.8 and less than or equal to one.
As shown in FIGS. 1 and 3, the heat exchanger 110 also includes a refrigerant inlet 165 connected to the beginning of the first section 135 by a connecting tube 170 and a refrigerant outlet 175 connected to the end of the second section 140. The connecting tube 170 is located outside of the coil 130. As shown in FIG. 5, the storage tank 125 includes a hot water outlet 180, a cold water inlet 185, and a drain 190. The storage tank 125 is surrounded by a jacket or shell 195. Insulation 197 is provided between the jacket 195 and the storage tank 125.
In use, refrigerant vapor enters the first section 135 through the refrigerant inlet 165, then passes through the second section 140, and exits the heat exchanger 110 through the refrigerant outlet 175. Cold water from an external water source enters the storage tank 125 through the cold water inlet 185 and is stored in the storage tank 125. The water in the storage tank 125 is in a direct heat exchange relationship with the coil 130 in order to heat the water in the storage tank 125. Hot water is supplied from the storage tank 125 through the hot water outlet 180.
The heat pump water heater 100 with the condensing heat exchanger 110 having a reducing inner diameter coil 130 provides relatively high heat exchange efficiency and lower manufacturing costs when compared to a heat pump water heater with a condensing heat exchanger having a constant inner diameter coil.
For the constant inner diameter coil, at the beginning portion of the coil, the condensation heat transfer of the refrigerant is quick and the heat transfer coefficient is high. However, as condensation continues along the length of the coil, the liquid film of the condensed refrigerant accumulates and attaches to the inner wall of the coil, especially at the middle and end portions of the coil. As the liquid film accumulates on the tube wall, the thermal resistance to the heat transfer from the refrigerant vapor to the water in the storage tank increases and leads to a reduction of the heat transfer performance of the entire coil.
The heat pump water heater 100 improves the heat transfer performance of the entire coil 130 across the beginning, middle, and end portions of the coil 130. The reduced inner diameter 155 increases the flow rate of the refrigerant through the coil 130, which reduces the thickness of the liquid film that accumulates on the inner wall of the coil 130 and thereby improves the heat transfer performance in the second section 140 as compared to the constant inner diameter coil.
The heat transfer capacity of the coil 130 is improved over a constant inner diameter coil so the performance of the heat pump is also improved when using the same heat exchange area and the same working conditions for the heat pump. The improved heat transfer capacity of the coil 130 also allows for performance similar to a constant inner diameter coil, but with a reduced heat exchange area and weight of the coil 130 as compared to the constant inner diameter coil, which results in savings on material and manufacturing costs, as well as reducing structural size, which increases reliability.
The reducing inner diameter coil 130 provides improved heat transfer at all portions of the coil 130 by reducing the thickness of the liquid film that accumulates on the inner wall of the coil 130. At the same time, the flow restriction or resistance in the coil 130 is not increased beyond the capacities of the compressor 105, the expansion device 115, and the evaporator 120.
A comparison test of the heat pump water heater 100 (#2) and a heat pump water heater with a constant inner diameter coil (#1) is shown in the table below (the test conditions are nominal conditions for a heat pump water heater with an original water temperature of 15° C. (59° F.)).
It can be seen that an advantage of the heat pump water heater 100 is that because the reducing inner diameter coil 130 improves the heat exchange performance in the middle and end heat exchange stages, the overall heat exchange is strengthened and good heat exchange effect is achieved. Additionally, manufacturing difficulty is not increased and material consumption and cost is effectively reduced. Therefore, the heat pump water heater 100 (#2) is a higher efficiency heat pump water heater than the heat pump water heater with a constant inner diameter coil (#1).


	#1 - Heat Pump	#2 - Heat Pump
	Water Heater	Water Heater
	with a	100 with	Difference
	Constant Inner	Reducing Inner	between
Item	Diameter Coil	Diameter Coil	#2 and #1

Compressor	Nominal Cooling	Nominal Cooling
	Capacity of the	Capacity of the
	Compressor	Compressor
Diameter (mm)	14 × 2	14 × 2 + 10 × 2
Coil Length (m)	19.5	11.5 + 8
Weight (kg)	11.6	10	−14%
Average Heat Capacity	2.3	2.3
of Water Temperature
between 15-55° C.
(59-131° F.)
Average COP of	3.4	3.6	5.8%
Water Temperature
between 15-55° C.
(59-131° F.)

FIG. 6 illustrates a heat pump water heater 200 similar to the heat pump water heater 100. Components similar to those of the heat pump water heater 100 described above are numbered in a similar fashion plus one hundred. Some of the differences between the heat pump water heater 200 and the heat pump water heater 100 are described below.
The heat pump water heater 200 is an external coil heat pump water heater in which the coil 230 of the condensing heat exchanger 210 is wound around the outer surface of the storage tank 225 and positioned within the insulation 297. The coil 230 is in a heat exchange relationship with the water in the storage tank 225 to heat the water in the storage tank 225. Tests prove that the heat exchange capacity of the coil 230 of the heat pump water heater 200 is increased over a similar heat pump water heater with a constant inner diameter coil and that the required heat exchange area is effectively reduced. Material and cost savings are also achieved when compared to a heat pump water heater with a constant diameter inner coil.
Various features of the invention are set forth in the following claims.

Claims

What is claimed is:

1. A heat pump water heater comprising:

a compressor;

a condenser in a heat exchange relationship with a water storage tank, the condenser including a heat exchange coil with a first section and a second section, the first section has a first flow area, the second section has a second flow area that is less than the first flow area, and the second section is downstream from the first section;

an expansion device; and

an evaporator;

wherein the compressor, the condenser, the expansion device, and the evaporator are connected in series.

2. The heat pump water heater of claim 1, wherein a flow area ratio comparing the first flow area to the second flow area is greater than or equal to two and less than or equal to four.

3. The heat pump water heater of claim 1, wherein the first section has a first section length;

wherein the second section has a second section length; and

wherein a length ratio comparing the first section length to the second section length is greater than or equal to 0.8 and less than or equal to one.

4. The heat pump water heater of claim 1, wherein the first section has a first section outer diameter; and

wherein the second section has a second section outer diameter that is less than the first section outer diameter.

5. The heat pump water heater of claim 4, wherein the first section has a first section wall thickness; and

wherein the second section has a second section wall thickness that is equal to the first section wall thickness.

6. The heat pump water heater of claim 1, wherein the heat exchange coil is positioned within the water storage tank.

7. The heat pump water heater of claim 1, wherein the heat exchange coil is wound around the outside of the storage tank

8. The heat pump water heater of claim 2, wherein the first section has a first section length;

wherein the second section has a second section length; and

9. The heat pump water heater of claim 8, wherein the first section has a first section outer diameter; and

10. The heat pump water heater of claim 9, wherein the first section has a first section wall thickness; and

11. The heat pump water heater of claim 9, wherein the heat exchange coil is positioned within the water storage tank.

12. The heat pump water heater of claim 9, wherein the heat exchange coil is wound around the outside of the storage tank

13. The heat pump water heater of claim 10, wherein the heat exchange coil is positioned within the water storage tank.

14. The heat pump water heater of claim 10, wherein the heat exchange coil is wound around the outside of the storage tank

15. The heat pump water heater of claim 8, wherein the heat exchange coil is positioned within the water storage tank.

16. The heat pump water heater of claim 8, wherein the heat exchange coil is wound around the outside of the storage tank

17. The heat pump water heater of claim 2, wherein the first section has a first section outer diameter; and

18. The heat pump water heater of claim 2, wherein the heat exchange coil is positioned within the water storage tank.

19. The heat pump water heater of claim 2, wherein the heat exchange coil is wound around the outside of the storage tank

20. The heat pump water heater of claim 3, wherein the first section has a first section outer diameter; and