WO2000075577A2

WO2000075577A2 - Cardioplegia heater/cooler system

Info

Publication number: WO2000075577A2
Application number: PCT/US2000/015053
Authority: WO
Inventors: Paul D. Brinda; Richard P. Goldhaber; Jeffrey C. Toy
Original assignee: Lifestream International, Inc.
Priority date: 1999-06-04
Filing date: 2000-06-01
Publication date: 2000-12-14
Also published as: AU5311200A; WO2000075577A3

Abstract

A device and method for providing heat transfer fluid to a medical heat exchanger (42) including warm (16) and cold (14) transfer fluid reservoirs for holding transfer fluid (38) at atmospheric pressure connected to a rotary vane positive displacement fluid supply pump (18) and heat exchanger. The heat transfer fluid is provided to the heat exchanger below atmospheric pressure and a pressure lower than the controlled fluid, such as cardioplegia fluid. The pump is downstream of the heat exchanger, draws the heat transfer fluid from the heat exchanger and recirculates the heat transfer fluid back to the reservoirs. The device may include a heat transfer fluid recirculation control system (200 that switches between warm and cold heat transfer fluid and returns the heat transfer fluid to the originating reservoir by delaying the switch of the return valve (24). The switching may be delayed by an adjustable temperature or time delay.

Description

CARDIOPLEGIA HEATER/COOLER SYSTEM

BACKGROUND AND SUMMARY OF THE INVENTION

This invention relates generally to the field of medical heat exchanger fluid delivery systems. In particular, it relates to a heater/cooler device and method for providing heat transfer fluid to a cardioplegia heat exchanger.

A new and useful heater/cooler device and method is needed that overcomes the problems associated with conventional methods of delivering heat transfer fluid to medical heat exchangers by delivering the heat transfer fluid at a pressure lower than the blood or cardioplegia fluid side of a medical heat exchanger. In addition, a new and useful heater/cooler device and method is needed that overcomes the problems associated with conventional methods of delivering heat transfer fluid to medical heat exchangers by providing a heat transfer fluid recirculation control system.

It is an object of the apparatus and method of the heater/cooler device in accordance with the present invention to solve the problems outlined above that have heretofore inhibited the successful heating and cooling of cardioplegia fluid and other bodily fluids.

The foregoing object is realized according to the invention by providing a heater/cooler apparatus and method that provides for a low pressure heat transfer fluid delivery system. More specifically, the apparatus and method in accordance with the present invention provides a device for delivering heat transfer fluid to a medical heat exchanger at a pressure below the pressure of the cardioplegia or blood side of the associated heat exchange device. The unique heater/cooler device featuring a low pressure heat transfer fluid delivery system for delivering heat transfer fluid to a heat exchanger in accordance with the present invention broadly includes a) a reservoir for holding heat transfer fluid at atmospheric pressure and for fluid connection to the heat exchanger and b) a pump for fluid connection to the heat exchanger; the pump for transferring the heat transfer fluid from the reservoir to the heat exchanger wherein the heat transfer fluid in the heat exchanger is below atmospheric pressure. The device in accordance with the present invention may locate the pump downstream of the heat exchanger to achieve the below atmospheric pressure transfer. The device in accordance with the present invention may also provide the heat transfer fluid at a lower pressure than a controlled fluid in the heat exchanger.

The pump of the device in accordance with the present invention may be provided to draw the heat exchange fluid from the reservoir through the heat exchanger and recirculate the heat exchange fluid to the reservoir.

The pump of the device in accordance with the present invention may be a rotary vane positive displacement pump.

The device in accordance with the present invention may also include a) a second fluid reservoir, b) a supply valve fluidly connected to the reservoirs and to the heat exchanger, the supply valve selectively supplying heat transfer fluid from each reservoir; c) a return valve fluidly connected to the reservoirs and to the heat exchanger, the return valve selectively returning heat transfer fluid from the heat exchanger to the reservoirs; and d) a delay mechanism for controlling the return valve so that heat transfer fluid from the first reservoir returns to the first reservoir while the supply valve supplies heat transfer fluid from the second reservoir.

As mentioned above, the heater/cooler device may recirculate the heat transfer fluid back to the reservoir. In particular, the device preferably temporarily continues to return heat transfer fluid to a first reservoir after switching the supply source from the first reservoir to a second reservoir. The heat transfer fluid is then recirculated to a second reservoir after such a delay, to reduce a temperature variation of the heat transfer fluid in the second reservoir. Thus, by way of example, the device of the present invention may include a first reservoir that holds warm heat transfer fluid and a second reservoir that holds cold heat transfer fluid, and the recirculation is controlled so that when the delivery mode is switched from warm delivery to cold delivery, the spent warm heat transfer fluid is temporarily returned to the first reservoir while the supply valve supplies cold heat transfer fluid to the heat exchanger. The delay mechanism may be adjustable, and may be temperature dependent and/or time dependent. Thus, the delay mechanism may be a time delay switch that may be adjusted based on the volume of recirculating heat transfer fluid. The volume of heat transfer fluid may be estimated based on the length of the connecting lines.

The unique heater/cooler device featuring the heat transfer fluid recirculation control system for delivering heat transfer fluid to a heat exchanger in accordance with the present invention broadly includes a) a first and second reservoir each for holding heat transfer fluid; b) a pump fluidly connected to the reservoirs and for fluid connection to the heat exchanger; the pump for transferring the heat transfer fluid from the reservoirs to the heat exchanger; c) a supply valve fluidly connected to the reservoirs and to the heat exchanger, the supply valve selectively supplying heat transfer fluid from each reservoir; d) a return valve fluidly connected to the reservoirs and to the heat exchanger, the return valve selectively returning heat transfer fluid from the heat exchanger to the reservoirs; e) a delay mechanism for controlling the return valve so that heat transfer fluid from the first reservoir returns to the first reservoir while the supply valve supplies heat transfer fluid from the second reservoir.

The present invention may also include a method of providing heat transfer fluid to a medical heat exchanger comprising: a) providing a reservoir for holding heat transfer fluid at atmospheric pressure, the reservoir fluidly connected to the heat exchanger; b) providing a pump fluidly connected to the heat exchanger; and c) flowing the heat transfer fluid from the reservoir through the heat exchanger below atmospheric pressure. The method in accordance with the present invention may locate the pump downstream of the heat exchanger to achieve the below atmospheric pressure transfer. The method in accordance with the present invention may also provide the heat transfer fluid at a lower pressure than a controlled fluid in the heat exchanger.

The pump of the method in accordance with the present invention may be provided to draw the heat exchange fluid from the reservoir through the heat exchanger and recirculate the heat exchange fluid to the reservoir.

The method of the present invention may also include a) providing a second reservoir for holding heat transfer fluid at atmospheric pressure, the second reservoir fluidly connected to the heat exchanger; b) selectively supplying heat transfer fluid from the reservoirs to the heat exchanger; c) selectively returning heat transfer fluid to the reservoirs; and d) returning the heat transfer fluid from the first reservoir to the first reservoir while supplying heat transfer fluid from the second reservoir to the heat exchanger.

A feature of the device is that the blood side pressure of the heat exchanger is greater than the heat transfer side of the heat exchanger. Another feature is that the heat transfer fluid pump is downstream of the heat exchanger. Another feature is that the pressure of the heat transfer fluid between the heat transfer fluid reservoir and the fluid pump is below atmospheric pressure. One advantage of this pressure profile is that heat transfer fluid will not accidentally leak into the blood or cardioplegia side of the heat exchanger. Instead, any leaks in the system will be from the blood side to the heat transfer fluid side, and any leaks of blood into the heat transfer fluid may be easily detected as a discolorization of the heat transfer fluid.

Another advantage of the present invention is that the variation in temperature of the heat transfer fluid in the reservoirs is reduced.

Another advantage is that the recovery time to return the temperature of the heat transfer fluid to the set point temperature is reduced. Thus, yet another advantage is that energy consumption for heating is reduced.

A further advantage is that energy for cooling is reduced. If ice is used, ice consumption is reduced. If a refrigeration system is used, less energy is used.

Another advantage is that the reduction in temperature variance of the heat transfer fluid permits a quicker changeover between different temperature heat transfer fluids. The reduction in temperature variance of the heat transfer fluid may also allow the use of a lower energy consumption heater. These and other objects and advantages of the present invention will become apparent during the course of the following detailed description and appended claims. The invention may best be understood with reference to the accompanying drawings, wherein an illustrative embodiment is shown.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGURE 1 is a perspective view of the device of the present invention.

FIGURE 2 is a perspective view of the device of the present invention.

FIGURE 3 is a schematic drawing of the device of the present invention showing the fluid path of the present invention.

FIGURES 4A-B and 5A-B are schematic drawings of the electrical controls of the present invention.

FIGURE 6 is a front view of the control panel of the device of the present invention.

FIGURE 7 is a perspective view of the backside of the device.

FIGURE 8 is an exploded view of the inside portion of the device.

DETAILED DESCRIPTION OF THE INVENTION

GENERAL ASSEMBLY

Referring to FIGURES 1 through 8, the heater/cooler device 10 in accordance with the present invention broadly includes a housing 12, a cold fluid reservoir 14, a warm fluid reservoir 16, a fluid supply pump 18, a control panel 20, a supply valve 22, a return valve 24, connecting lines 26, heaters 28, temperature switch 30 (located inside heaters), level switches 31 , 32, 33, reservoir temperature sensor 34, circulating temperature sensor 50, and drain valves 36.

The housing 12 is used to house the various components of the device 10. The housing 12 is preferably constructed out of plastic or other suitable polymer resin. The housing may also be constructed out of metal or composite materials. The reservoirs 14, 16 may be integral with the housing 12 or separately inserted. The reservoirs 14, 16 are preferably integrally molded with the housing 12, but may also be constructed out of suitable metal such as stainless steel and may also be inserted into the housing 12.

The cold fluid reservoir 14 is designed to hold a heat transfer fluid 38 at atmospheric pressure. The heat transfer fluid 38 in the cold fluid reservoir 14 is preferably ice water. However, the heat transfer fluid 38 in the cold fluid reservoir 14 may also be cooled by a mechanical refrigeration system. The cold fluid reservoir 14 is preferably large enough to hold enough ice to cool a minimum of 2 liters of cardioplegia fluid from 37 degrees C to less than 9 degrees C. The cold fluid reservoir 14 is preferably large enough to accommodate a standard cardioplegia delivery coil type heat exchanger with a 4 inch coil diameter and a 6 inch coil stack height. The cold fluid reservoir 14 also preferably incorporates an overflow system to collect overflow from the cold fluid reservoir 14 if the device is overfilled such as when ice is added to the cold fluid reservoir 14.

The warm fluid reservoir 16 is designed to hold a heat transfer fluid 38 such as water at atmospheric pressure. The warm fluid system 40 preferably includes the warm fluid reservoir 16, three level switches 31 , 32, 33 (a first low level switch 31 , a second low level switch 32 and a high level switch 33), two heaters 28, and a reservoir temperature sensor 34. The level switches 31 , 32, 33 are positioned in the warm fluid reservoir 16 at various fluid levels so that high and low heat transfer fluid 38 levels can be detected. Preferably a first low level switch 31 is located near the bottom of the warm fluid reservoir 16 to indicate a low heat transfer fluid 38 situation. Preferably, the first low level switch protects the fluid supply pump 18 from operating in a low fluid situation. The second low level switch 32 is preferably located above the reservoir temperature sensor 34 to prevent heating if the heat transfer fluid 38 is below the reservoir temperature sensor 34. A high level switch 33 is located near the top or at full point to indicate full or overfull conditions. The two heaters 28 are preferably 450 Watt electrical immersion heaters. The heaters 28 heat the heat transfer liquid 38 from room temperature to about 42 degrees C. The reservoir temperature sensor 34, located above the heaters 28, monitors the temperature of the heat transfer fluid 38 in the warm fluid reservoir 16. The temperature of the heat transfer fluid 38 is indicated on the control panel 20 on the warm fluid reservoir temperature indicator 46.

The fluid flow of the device 10 generally is preferably from the fluid reservoirs 14, 16 through a supply valve 22, to the heat exchanger 42, to the fluid supply pump 18, to the return valve 24, and back to the fluid reservoirs 14, 16.

The supply valve 22 enables switching between heat transfer fluid 38 from the cold fluid reservoir 14 and the warm fluid reservoir 16. The supply source switch 44 on the control panel 20 controls the supply valve 22. The supply valve 22 is preferably a single three way valve, however supply valve 22 may be a combination of two-way valves. The return valve 24 operates in conjunction with the supply valve 22 preferably after a delay. The return valve 24 returns the heat transfer fluid 38 back to the proper fluid reservoir 14, 16. Return valve 24 is preferably a three-way valve, however, return valve 24 may be a combination of two-way valves. The return valve 24 is preferably operated in conjunction with the supply valve 22 after a delay to allow the heated or cooled heat transfer fluid 38 to return to the appropriate fluid reservoir 14, 16. The delay is caused by a delay mechanism 74, which is preferably an adjustable time delay switch 82. The time delay switch 82 may be adjusted based upon the length of the connecting lines 26, particularly the length of tubing from the device 10 to and from the heat exchanger 42.

A drain valve 36 is preferably provided for each fluid reservoir 14, 16 to drain the heat transfer fluid 38 to a drain 80 after use. The drain valves 36 are connected to the connecting lines 26 between the fluid reservoirs 14, 16 and the supply valve 22.

The fluid supply pump 18 is located downstream of the heat exchanger 42. The fluid supply pump 18 draws the heat transfer fluid 38 from the upstream connecting line 26 and returns the heat transfer fluid to the fluid reservoirs 14, 16 through the return valve 24. The fluid supply pump is preferably a Varna™ Mag-Drive Vane pump Model V3- 306 available from Micropump®.

Connecting lines 26 fluidly connect the fluid reservoirs 14, 16 to the valves 22, 24, 36, the heat exchanger 42, and the fluid supply pump 18. The connecting lines 26 are preferably V≥ inch or % inch reinforced PVC tubing.

Control panel 20, shown in detail in Fig 6, is attached to the top front of the housing 12. The control panel 20 provides a central location for the controls and indicators. An electrical schematic of the electrical control system is shown in FIGURES 4A-B and 5A-B. The control panel 20 includes a warm fluid reservoir temperature indicator 46, a circulating fluid temperature indicator 48, a supply source switch 44, a standby/priming/deliver switch 52, a low water alarm indicator 54, and an over temperature alarm indicator 56. The warm fluid reservoir indicator 46 indicates the temperature of the heat transfer fluid 38 in the warm fluid reservoir 16 as sensed by the reservoir temperature sensor 34. The warm fluid reservoir indicator 46 also indicates and adjusts the warm fluid reservoir set temperature. Three adjustment buttons 58, 60, 62 located next to the warm fluid reservoir indicator 46 adjust the set point of the warm fluid reservoir temperature. The set temperature is indicated on the warm fluid reservoir indicator when the set point adjustment button 58 is held down. The set temperature may be adjusted with the up and down adjustment buttons 60, 62 while the set point adjustment button 58 is held down.

The circulating fluid temperature indicator 48 indicates the temperature of the heat transfer fluid 38 as sensed by circulating temperature sensor 50 located before the heat transfer fluid 38 exits the heater/cooler device 10 at the heat transfer fluid outlet 76 and before the heat exchanger 42.

Supply source switch 44 chooses the source of the heat transfer fluid 38. Heat transfer fluid 38 may be supplied from either the warm fluid reservoir 16 or the cold fluid reservoir 14. The supply source switch 44 controls the supply valve 22 and the return valve 24. The supply source switch 44 is selectively switchable between a cold fluid position 64 and a warm fluid position 66. The standby/priming/deliver switch 52 switches between a standby position 68, a priming position 70 and a delivery position 72. The standby/priming/deliver switch 52 controls the operation of the fluid supply pump 18. In the standby position 68, the pump is not activated, however, the heaters 28 are activated. In the priming position 70, the pump provides heat transfer fluid 38 to the heat exchanger 42 at a positive pressure. Flow is from the reservoirs 14, 16 directly to the fluid supply pump 18 to eliminate air from the system. Preferably the fluid supply pump 18 operates in reverse to supply the heat transfer fluid at positive pressure. However, the flow of heat transfer fluid 38 through the heat exchanger 42 may also be reversed through additional piping and valves instead of reversing the flow of the fluid supply pump 18.

The low water alarm indicator 54 is located on the control panel 20 and preferably is an indicator light. In addition, an audible alarm preferably sounds when the indicator light is activated. The low water alarm indicator 54 is activated when the heat transfer fluid 38 in the warm fluid reservoir 16 is below either of the low level switches 31 , 32. In addition, the heaters 28 and fluid supply pump 18 are preferably deactivated when an alarm is received.

The over temperature alarm indicator 56 is preferably an indicator light located on the control panel 20 and is activated in a heat transfer fluid 38 over temperature condition. An audible alarm may also sound when the indicator light is activated. Preferably, the over temperature indicator 56 is activated when either the reservoir temperature sensor 34 or the circulating temperature sensor 50 sense a heat transfer fluid 38 temperature equal to or over 43 degrees C. Activation of the over temperature alarm indicator 56 preferably deactivates the fluid supply pump 18 and the heaters 28. IN OPERATION

The device 10 can be described with reference to several different systems including a cooling system, a heating system, a pumping system, a temperature control system, and operation.

The cooling system effectiveness is preferably provided by ice.

The ice is preferably crushed or flaked. The ice is placed in the cold fluid reservoir 14, which has enough capacity to cool a delivery of 2 two liters of cardioplegia solution from 37 degrees C to less than 9 degrees C. The cold fluid reservoir 14 is large enough to accommodate a standard cardioplegia delivery coil heat exchanger of 4 inches in diameter and 6 inches stack height.

The heating system uses an electric heater 28 to warm the heat transfer fluid 38 such as water. The warm fluid reservoir 16 is filled from the top and also accommodates a standard coil heat exchanger. The heating system is capable of heating at least one liter of cardioplegia solution from 25 degrees C to 37 degrees C. The heating and temperature monitoring equipment in the warm fluid reservoir 16 is protected from contact with a standard coil heat exchanger.

The pumping system provides heat transfer fluid 38 from the device 10 to a flow through type heat exchanger 42 via Hansen connections. The pumping system creates a below atmospheric pressure in the fluid path through the heat exchanger 42. This below atmospheric pressure system is safer than a high pressure system because a leak in the system of the present invention will result in blood entering the heat transfer fluid 38 instead of heat transfer fluid 38 entering the blood. In addition any leak of blood into the heat transfer fluid 38 can be detected by a pink color in the heat transfer fluid 38 in the reservoirs 14, 16. The temperature control system maintains the temperature of the heat transfer fluid 38 in the warm fluid reservoir 16. A desired temperature may be entered via the control panel 20 and the temperature is then maintained automatically. In addition, the reservoir temperature sensor 34 provides the control panel 20 with a temperature signal. If an over temperature condition exists, such as a temperature of over 43 degrees C, an alarm will activate and the heaters 28 will be disabled. The temperature of the heat transfer fluid 38 in the cold fluid reservoir 14 is maintained at approximately 4 to 6 degrees C by maintaining sufficient ice in the cold fluid reservoir 14.

The device 10 is operated via the controls on the control panel 20. The operator hooks up the heat exchanger 42 via the exterior tubing to the heat transfer fluid inlet 76 and the heat transfer outlet 78 of the device 10. The operator fills the reservoirs 14,16 with heat transfer fluid 38 and preheats the heat transfer fluid 38 in the warm fluid reservoir 16 by turning the standby/prime/deliver switch 52 to the standby position 68. The operator may also adjust the preset temperature of the warm fluid reservoir 16 by holding the setpoint adjustment button 58 and either pushing the up adjustment button 60 or the down adjustment button 62 to change the indicated setpoint.

The operator primes the system by turning the standby/prime/deliver switch 52 to the prime position 70. In this position, the device flows heat transfer fluid 38 through the heat exchanger 42 in reverse or in the direction opposite of the normal delivery flow direction.

The operator may deliver heat transfer fluid 38 to the heat exchanger 42 by turning the standby/prime/deliver switch 52 to the deliver position 72 and turning the supply source switch 44 to either the cold fluid position 64 or the warm fluid position 66. The operator may switch between cold and warm heat transfer fluid as needed. When the supply source switch 44 is switched from one position to the other the return flow from the heat exchanger 42 and connecting lines 26 is returned to the originating reservoir. The return valve 24 switches after an adjustable delay. The delay may be temperature or time activated. However, an adjustable time delay is preferred. The time delay may be operator adjusted based on the length of the exterior tubing. The time delay is preferably adjustable from 0 to 30 seconds.

The following Table details the general features and equipment specifications of a presently preferred, exemplary embodiment.

Table 1 - General Features and Equipment Specifications (Example 1)

Although the description of the preferred embodiment has been presented, it is contemplated that various changes, including those mentioned above, could be made without deviating from the spirit of the present invention. It is therefore desired that the present embodiment be considered in all respects as illustrative, not restrictive, and that reference be made to the appended claims rather than to the foregoing description to indicate the scope of the invention.

Claims

1. A device for delivering heat transfer fluid to a medical heat exchanger comprising: a reservoir for holding heat transfer fluid at atmospheric pressure and for fluid connection to the heat exchanger; a pump for fluid connection to the heat exchanger; said pump for transferring the heat transfer fluid from said reservoir to the heat exchanger wherein the heat transfer fluid in the heat exchanger is below atmospheric pressure.

2. The device of claim 1 wherein the heat transfer fluid is recirculated back to said reservoir.

3. The device of claim 1 wherein said pump is located downstream of the heat exchanger.

4. The device of claim 1 wherein the heat transfer fluid has a lower pressure than a controlled fluid in the heat exchanger.

5. The device of claim 1 wherein the heat transfer fluid is from -5 to - 500 mmHg absolute pressure in the heat exchanger.

6. The device of claim 1 wherein the heat exchanger is used for cardioplegia fluid.

7. The device of claim 1 wherein said pump draws the heat exchange fluid from said reservoir through the heat exchanger and recirculates the heat exchange fluid to the reservoir.

8. The device of claim 1 wherein said pump is a rotary vane positive displacement pump.

9. The device of claim 1 further comprising: a second fluid reservoir, a supply valve fluidly connected to said reservoirs and to the heat exchanger, said supply valve selectively supplying heat transfer fluid from each said reservoir; a return valve fluidly connected to said reservoirs and to the heat exchanger, said return valve selectively returning heat transfer fluid from the heat exchanger to said reservoirs; a delay mechanism for controlling said return valve so that heat transfer fluid from said first reservoir returns to said first reservoir while said supply valve supplies heat transfer fluid from said second reservoir.

10. A device for providing heat transfer fluid to a medical heat exchanger comprising: a first and second reservoir each for holding heat transfer fluid; a pump fluidly connected to said reservoirs and for fluid connection to the heat exchanger; said pump for transferring the heat transfer fluid from said reservoirs to the heat exchanger; a supply valve fluidly connected to said reservoirs and to the heat exchanger, said supply valve selectively supplying heat transfer fluid from each said reservoir; a return valve fluidly connected to said reservoirs and to the heat exchanger, said return valve selectively returning heat transfer fluid from the heat exchanger to said reservoirs; a delay mechanism for controlling said return valve so that heat transfer fluid from said first reservoir returns to said first reservoir while said supply valve supplies heat transfer fluid from said second reservoir.

11. The device of claim 10 wherein said first reservoir holds warm heat transfer fluid and said second reservoir holds cold heat transfer fluid.

12. The device of claim 11 wherein said warm heat transfer fluid is temporarily returned to said first reservoir while said supply valve supplies cold heat transfer fluid to the heat exchanger.

13. The device of claim 10 wherein said delay mechanism is adjustable.

14. The device of claim 10 wherein said delay mechanism is temperature dependent.

15. The device of claim 10 wherein said delay mechanism is time dependent.

16. The device of claim 10 wherein said delay mechanism is a time delay switch.

17. A method of providing heat transfer fluid to a medical heat exchanger comprising: providing a reservoir for holding heat transfer fluid at atmospheric pressure, said reservoir fluidly connected to the heat exchanger; providing a pump fluidly connected to the heat exchanger; and flowing the heat transfer fluid from said reservoir through the heat exchanger below atmospheric pressure.

18. The method of claim 17 wherein said heat transfer fluid is recirculated back to said reservoir.

19. The method of claim 17 wherein said pump is located downstream of the heat exchanger.

20. The method of claim 17 wherein the heat transfer fluid in the heat exchanger has a lower pressure than a controlled fluid in the heat exchanger.

21. The method of claim 17 wherein said pump draws the heat exchange fluid from said reservoir through the heat exchanger and returns the heat transfer fluid to said reservoir.

22. The method of claim 17 further comprising: providing a second reservoir for holding heat transfer fluid at atmospheric pressure, said second reservoir fluidly connected to the heat exchanger; selectively supplying heat transfer fluid from said reservoirs to the heat exchanger; selectively returning heat transfer fluid to said reservoirs; and returning the heat transfer fluid from the first reservoir to said first reservoir while supplying heat transfer fluid from said second reservoir to the heat exchanger.