CA2692229A1 - Thermoeletric 2-phase gravity condenser & methods of improving existing heat pipe systems - Google Patents

Abstract

In Heat pipes, thermoelectric module boils a refrigerant in a closed container called: boiler. The evporating refrigerant passes through a natural convection condenser. The condenser subtracts heat from the refrigerant and changes its vapour phase into liquid. This liquid refrigerant returns to the boiler, Then, the process repeats its self.

Description

INVENTION OF THERMOELETRIC 2-PHASE GRAVITY CONDENSER &
METHODS OF IMPROVING EXISTING HEAT PIPE SYSTEMS.
INVENTORS:

Mr. Arun Kulkarni, President, Koolatron Inc.
Dr. Godwill M. Igwe, D.Eng. (Milan, Italy), P.Eng. (Ontario, Canada).
(Research Professional Engineer).
SPECIFIATION

1) As the liquid heats up in the boiler and changes its phase to vapour, it absorbs heat from the module till latent heat of the refrigerant is totally absorbed .
This latent heat absorption results in phase change from liquid into vapour.

2a) The present state of the art boils the refrigerant which evaporates and is cooled in the condenser. Boiling of the refrigerant would create pressure in the boiler and pipes. The system prior to its being charged with refrigerant would under go pressure testing to ascertain it could withstand the desired level of pressure (about 100 to 350 psig or more). After successful pressure test, the system would undergo moisture evacuation.
During usage, the pressure generated in the boiler could be so high that it could be greater than the securely tested pressure. This could induce high mechanical stresses in all the elements of the system. It could happen that the combined stress in one location could be higher than the material yield stress. Then permanent mechanical deformation would occur. And it might even be higher than the ultimate boiler material strength. Thus, material failure would occur and there could even be an explosion. These are the deficiencies of the present state of heat pipes.

The higher the temperature of the refrigerant the higher the refrigerant pressure.
Hence, in hot, very hot climates, if the unit is left outside in he sun, the refrigerant pressure induced by the sun radiation and hot ambient temperature could be so high that mechanical failure or explosion could occur as mentioned earlier.
This is another deficiency in the present state of the art of heat pipes.

And more so, the thermoelectric module while heating the refrigerant in the boiler by furnishing the refrigerant latent heat of evaporation, the vapour must be cooled in the condense so as to return to the boiler as liquid. If due to circumstances such as high ambient temperature, malfuncion of the cooling fan if forced heat transfer is used, or blockage of air flow, etc., the vapour might not be condensed into liquid. It would return to boiler hot. And more thermoelectric beat would be pumped into it, thus rasing higher the temperture, consequently raising the pressure higher.
If this would continue, the pressure could be so high that the aforementioned consequences of mechanical failure or explosion might occur. This is another deficiency of the present state of the art.

2b) In the present heat pipes, as the refrigerant evaporates and flows into the condenser to be cooled, at a certain point the saturation temperaure of some of the molucles of the refrigerant would be reached. From then, satuaration would start.
Refrigerant liquid formation would gradually continue as the liquid drips out of the cooled vapour. All the vapour does not change into liquid instantaneously, but takes time. Thus depending on the system heat transfer characteristics, it might happen that the time taken to saturateall the vapour into liquid might be longer than the time the boiler takes to evaporate the liquid. This would induce starvation of liquid in the boiler. It would lead to poor efficiency, very high boiler temperature and so very high boiler pressure. The same high pressure consequences mentioned earlier might occur.
This is another deficiency of the present heat pipe state of the art.

2c) The present state of the art uses a convectional heat transfer condenser.
It has refrigerant tubes bent in counter direction (a Z form) in one, two, .... 4 or more parallel planes. All are connected to a header.
This convectional heat transfer system needs a very large area to expose it to ambient air flow. Thus its usage must allow a very large exposed area with unobstructed air flow. This large area requirement is another present heat pipe state of the art deficiency.

3a) As enunciated earlier, high pressure in the system beyond the tested pressure level is very dangerous. It could be catastrophic. Our invention solves this problem. We added Pressure Relief (PR) valve and also Pressure Control sytem.
Temperature Control (TC) system could also be included.
The Pressure Relief valve would induce a little loss of refrigerant to the ambient.
This is not very desirable because of refrigerant ozone layer depletion effect.
Pressure Control valve is far mush better than Pressure Relief valve because it shuts off the power to the module. And it does not let refrigerant into the atmosphere.
The above solution is beyond the present state of the art of heat pipes.

3b) As mentioned earlier, the saturation rate in the condenser could be lower than the evaporation rate in the boiler with the consequences mentioned in the preceding paragraphs.

n To solve this problem, we added a reservoir of refrigerant container. This could be viewed as an Accumulator-Receiver unit of the system. With this, the boiler would be assured of constant liquid flow into it, irrespective of the cooling rate of the saturation phase.
This is another solution beyond the present state of the heat pipe system.

3c) As was mentioned in paragraph (2c) the large area requirement of the present heat pipe system is a handicap and a deficiency when it would be needed in a small restrictive area.
Our system is a Gravity Condenser which has parallel tubes with rectangular or circular fins. Multi tubes are in parallel in one plane. There could also be multi-planes with their parallel pipes. These pipes in the planes are joined in series. It has a fan for forced convectional heat transfer. Usage of fins would increase the heat transfer surface area, higher than in the present heat pipe system. The fan blowing air across it would generate higher surface heat transfer coefficient higher than convectional heat transfer.
This would be superior to the present heat pipe system for compactness, increased heat transfer characteristics, and very efficient system.

4a) Figure 1 shows the sketch of the general assembly of the invention.
GC1= gravity condenser;
AR=Accumulator-Receiver;
PR- Pressure Relieve Vaalve;
PC=Pressure Control Unit.
In order to ensure that he refrigerant pressure does not exceed a set limit to avoid disaster, PR and /or PC are inserted in series in the system. These could be of any type, shape or form so long as it controls the max pressure set.

4b) To solve the problem of refrigerant saturation rate possibly being less than the evaporation rate in the boiler, the AR (Accumulator-Receiver) is added in series in the system. Its dimension and thickness would depend on available area to accommodate it and on the max pressure expected in the system.
4c) To solve the problem of compactness, figures (1) and (2) show the sketch of the Gravity Condenser system views. The pipe could be of any diameter depending on the expected and analyzed volume of refrigerant flowing through it. Normaly this would be metal of high thermal conductivity.
The fins could be rectangular or circular. Normally, these would be aluminium or any other material with high thermal conductivity.
The pipes in one plain are connected to the pipes in another plain such that the liquid would flow down from the condenser by gravity into the boiler.

4d) Another form of the boiler system would be to have the base area of contact to the thermoelectric module to be larger over 10 % to 400% or more. This will distribute the heat from thermoelectric to a large area, thus reducning the inter-face surface temperature. The lower the hot interface temp , the more efficient would be the thermoelectric system. See and compare B1-B2 in fig I to B3-B4 in fig.
3.
4e) Figure 2 shows a larger Boiler volume than in fig. 1 . The larger the boiler volume using the same refrigerant quantity, the lower would be the pressure in the boiler. The lower the pressure, so also would be lower the saturation temp.
Hence the module interface temp would be lower. This as was mentioned earlier would induce high thermoelectric efficiencTnd lower cold plate temp..

(5) The above described invention could not only be used in thermoelectric system, but in all other situations of fluid heat transfer, with or without phase change, is required.

Claims

THE EMBODIMENT OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

(1) Any heat pipe which comprises of any type of Pressure Relief valve, Pressure Contol unit, Temperature Control Unit or /and any other system that would limit pressure rise beyond a set limit.

(2) Any heat pipe with or without includes any type of refrigerant reservoir feeding into the boiler.

(3) Any Gravity Condenser as described above with parallel pipes in one plain, and connected to another parallel plain in counter flow direction. Many plains and tubes could be used depending on the thermal requirements of the system.
The plains may or may not be in parallel. Also the pipes may or may not be in parallel. The fins could be of any geometry: rectangular, circular, hexagonal, etc.
(4) Any boiler whose surface area of contact with the hot side of the thermoelectric module is 10% to 400% or more of the thermoelectric hot surface area.

(5) Any boiler whose volume is greater than 2.5 cubic inches.