METHOD FOR CONTROLLING FROST ON A HEAT TRANSFER DEVICE

Abstract

A method for controlling frost on a heat transfer device includes the steps of providing an air flow device adapted to provide a stream of air at a high flow rate, and using the air flow device to subject a heat transfer device to the stream of air during frosting times. The stream of air interacts with the heat transfer device to remove water droplets disposed on the heat transfer device, thereby preventing frost from accumulating on the heat transfer device.

Description

This application claims the benefit of Provisional Application No. 61/481,978 filed on May 3, 2011.

BACKGROUND OF THE INVENTION

The present invention relates to an improvement in the thermal efficiency of a heat transfer device under frosting conditions. More particularly, the invention relates to a method for controlling the accumulation of frost on the outside coil of a heat transfer device such as a heat pump.

Frost accumulation occurs during operation at low outdoor temperatures and high air relative humidity. It is a primary cause of winter peak demand when electric backup heat must replace capacity of compromised heat pumps. Ice crystals form on refrigerant-to-air heat exchangers generally when the evaporator surface temperature is below ˜28° F. Surface crystals insulate the coil and cause a reduction in refrigerant temperature, beginning the cascading formation of thick frost which in relatively short order completely blocks heat transfer.

BRIEF SUMMARY OF THE INVENTION

These and other shortcomings of the prior art are addressed by the present invention, which provides Accordingly, there is a need for a method of controlling the accumulation of frost and improving the overall thermal efficiency of a heat pump or other heat transfer device.

According to one aspect of the present invention, a method for controlling frost on a heat transfer device includes the steps of providing an air flow device adapted to provide a stream of air at a high flow rate, and using the air flow device to subject a heat transfer device to the stream of air during frosting times. The stream of air interacts with the heat transfer device to remove water droplets disposed on the heat transfer device, thereby preventing frost from accumulating on the heat transfer device.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter that is regarded as the invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which:

FIGS. 1A-1C show a coil defrosting using the method according to an embodiment of the invention;

FIG. 2 shows pressure increase due to frost formation;

FIGS. 3A-3B show the effect of higher velocities of air on frost and water removal;

FIG. 4 shows heat transfer rates;

FIG. 5 shows pressure increase due to frost formation at constant air velocities;

FIGS. 6A-6B show the effect of higher air velocities on frost formation;

FIG. 7 shows heat transfer rates at constant air velocities;

FIG. 8 shows pressure increase due to frost formation at 40 Pa; and

FIG. 9 shows pressure increase due to frost formation at 60 Pa.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings, an exemplary method for controlling frost on a heat transfer device according to an embodiment of the invention is disclosed. The method removes liquid water from a heat transfer coil of a heat transfer device, such as a heat pump, during or shortly after the coil is defrosted by imposing a high velocity air flow through the coil.

Increasing the air flow rate for a short time removes liquid water from the coil. This removal of water allows the heat transfer device to operate for a longer period of time before the next defrosting is required. This increase in the operating time between thermal defrosts of the coils improves the overall thermal efficiency of the heat transfer device. FIG. 1 shows a coil during the described defrosting procedure using a high air flow rate after frost formation at 0 minutes, 5 minutes, and 10 minutes.

A set of experiments were conducted to remove frost from the heat transfer device using a combination of thermal defrost and increased air velocities. An additional thermal bath was added to the experimental setup. The ‘cold’ alcohol was supplied to simulate the heating mode in a heat pump, while the ‘hot’ alcohol was applied to simulate a defrost cycle of a heat pump. A valve system was employed to switch between the thermal baths.

Table 1 summarizes the test conditions for the frost reduction experiments. The base line test (1.0-D3-F0 (76)) was conducted with a constant air velocity for the entire test duration. The defrost cycles started by providing alcohol at 3° C. after each hour for 3 min. Additionally, increasing air velocities by approximately a factor of 6.5 (medium air velocity increase) were applied during the defrost cycle in tests 6.5-D3-F3 (77) and 6.5-D3-F5(79) for 3 min and 5 min, respectively. The test 6.5-D3-F180 (80) was conducted with the increased air velocity for the entire duration of the test. The duration of the test was 3 hours (three defrost cycles). The environmental conditions were kept the same as those in the previous experiments.

TABLE 1
Matrix of test conditions using defrost and high air velocities
Initial Air			Air Velocity	Air Inlet	Defrost	Duration of Air	Max. Pressure
Velocity	Test Name	Test #	Increase Factor	Temperature	Time	Velocity increase	Drop
0.47 m/s	1.0-D3-F0	76	1.0	2° C.	3 min	NA	NA
(18 cfm)	6.5-D3-F3	77	6.5**			3	min
	6.5-D3-F5	79				5	min
	6.5-D3-F180	80				180	min
	1.0-D2-F0	96	1.0		2 min	NA	40 Pa
	6.5-D2-F2	94	6.5**			2	min
	1.0-D2-F0	97	1.0			NA	60 Pa
	6.5-D2-F2	98	6.5**			2	min
	1.0-D2-F0	114	1.0			NA
	6.0-D2-F2	110	6.0**			2	min
	12.0-D2-F2	112	12.0***
	1.0-D2-F0	120	1.0	0° C.		NA
	6.0-D2-F2	121	6.0**			2	min
	12.0-D2-F2	126	12.0***
	1.0-D2-F0	116	1.0	−2° C.		NA
	6.0-D2-F2	119	6.0**			2	min
	12.0-D2-F2	117	12.0***
	**medium air velocity increase,
	***high air velocity increase.

FIG. 2 shows the pressure drop increases due to frost formation. For an easier visual comparison of the curves, the pressure spikes caused by the increase of air flow rates were removed. The pressure increase curves are nearly identical for the first hour. However, a delay in the rise of the pressure drop is noticeable for 2^nd and 3^rd cycles for which high air velocities are applied. This delay in the blockage of the coil can be attributed to a more thorough liquid water removal by the high velocity air. The liquid water remaining on the coil after the defrost cycle quickly refreezes and shortens the time for the coil to become reclogged with frost.

The refrozen water can be differentiated from frost crystals of the frosted coil, as shown in FIG. 3. In experiments with high velocity air, any liquid water remaining on the coil is in form of very small droplets. Therefore, the application of high air velocity at the end of the defrost cycle (6.5-D3-F5 (79)) delays the next pressure drop increase as the coil begins to accumulate frost (FIG. 2). It also should be noted that the initial pressure drop reading after the defrost cycle is steadily increasing with every cycle. This is attributed to the increase to residual water that remains on the coil after the defrost cycle.

FIG. 4 shows the heat transfer rates from the same series of the defrost cycles. As was seen in the pressure drop data, a larger heat transfer rate is maintained for a longer portion of the test if high velocity air is applied during the defrost cycle.

FIG. 5 compares two tests each conducted with different constant air velocities applied during the entire test (including the defrost cycle of the test). The pressure drop is higher for the high velocity air test. However, the pressure drop during this high velocity air test does not begin to level off, which indicates that the coil is less blocked with frost than the test conducted at lower air velocity. This difference is attributed to the more complete removal of water by the higher velocity air. Direct visual observation shows a large difference between these tests, as shown in FIG. 6. These figures show the condition of the heat exchanger before the 1^st defrost cycle (one hour after the beginning of the experiment). The heat exchanger coil is completely blocked with frost tested at a moderate air velocity, while less frost has accumulated on the coil tested at a higher air velocity.

The obvious advantage of using high air velocity is depicted in FIG. 7. At the low air velocity, the heat transfer rate drops rapidly, while it slightly decreases at the high air velocity. This increase in the heat transfer rate at higher air velocities is not surprising, but what may not be widely known is that higher air velocity retards frost formation (a separate effect from the removal of liquid water after defrosting). Thus, the fan speed of heat pumps may be optimized for non-frosting conditions, such as through the use of a variable speed fan in the heat transfer device. In this way, the fan can operate at a higher air velocity during frosting conditions and reduce defrost frequency.

To investigate the effect of high air velocity on defrost cycles, additional tests were carried out. The maximum pressure drops were set to 40 Pa or 60 Pa as the conditions for the defrost cycle start. The defrost time was 2 minutes for all experiments. The test 1.0-D2-F0 (96) was conducted with a constant low air velocity, and the test 6.5-D2-F2 (94) was conducted with the application of a higher air velocity for the duration of the defrost cycle. FIG. 8 compares the pressure drops as functions of time for the two tests with a 40 Pa maximum pressure set-point. The pressure drop curves are nearly identical at the beginning of the tests. However, some delay in the pressure drop increase becomes noticeable after the 6^th defrost cycle. The effect of the high velocity air seems to be not very significant in this case, maybe because the 40 Pa set-point represents only a minimum amount of frost accumulation.

The effect on the defrost cycle time using a pressure drop set-point of 60 Pa is much more significant, as shown in FIG. 9, which compares tests using moderate and higher air velocity with a 60 Pa defrost set-point. After the first cycle, the coil operated with a moderate air flow rate needs to be defrosted every twenty minutes. The defrost cycle is not removing the liquid water from the heat exchanger. The water refreezes causing a higher pressure drop at the beginning of each heating cycle, and consequently a faster pressure drop rise with each operating cycle. The high air velocity removes more water from the heat exchanger during the defrost cycle, returning the initial pressure drop reading to nearly the initial level, and reducing the defrost frequency. Instead of the six defrost cycles over this three hour period required for the moderate air velocity test, the high air velocity test only required four defrost cycles—a 33% reduction in the energy required for defrosting a heat transfer device.

The foregoing has described a method for controlling frost on a heat transfer device. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation.

Claims

1. A method for controlling frost on a heat transfer device, comprising the steps of:

(a) providing an air flow device adapted to provide a stream of air at a high flow rate; and

(b) using the air flow device to subject a heat transfer device to the stream of air during frosting times, such that when the stream of air interacts with the heat transfer device, water droplets disposed on the heat transfer device are removed from the heat transfer device by the stream of air, thereby preventing frost from accumulating on the heat transfer device.

2. The method according to claim 1, wherein the air flow device is programmed to provide the high flow rate of air during periods of time when frosting may occur and to provide a reduced flow rate of air during periods of time when frosting is unlikely to occur.

3. The method according to claim 1, wherein the air flow device is a variable speed fan programmed to adjust its fan speed in accordance with atmospheric conditions to prevent frost from accumulating on the heat transfer device.

4. The method according to claim 1, further including the step of installing the air flow device in the heat transfer device.

5. The method according to claim 4, wherein the air flow device is a variable speed fan adapted to replace the heat transfer device's fan and programmed to provide a high flow rate of air during periods of time when frosting may occur.