Heat Exchanger System

The present invention is in the field of heat exchangers, and more particularly in the field of evaporative, porous metal heat exchangers.

Conventional evaporative heat exchangers are generally of tubular construction and include discrete expansion valves. The coolant is typically contained in a separate stand-by reservoir and requires an active means to pump the coolant to the expansion valves. With this construction, such heat exchangers are characterized generally by relatively poor thermal "footprints", i.e. they do not provide a substantially uniform temperature outer surface. Furthermore, such heat exchangers require separate stand-by coolant reservoirs and an active means for pumping coolant to the expansion valves.

More recently, the heat exchangers have been developed where a coolant fluid is passed at relatively low flow velocity through a relatively large contact surface area, small fluid passages. Generally, the heat exchange matrix is in the form of a fine metal screen, or sintered metal powder, for example. These structures provide a desired porosity matrix which is characterized by a relatively high thermal conductivity. The resultant structures provide a relatively low flow impedance. However, these heat exchangers too are characterized by relatively poor thermal footprints.

It is an object of the present invention to provide an improved evaporative, porous metal heat exchanger.

Briefly, the present invention is directed to a system for transferring heat from a first surface through a second surface where the second surface is characterized by a substantially uniform envelope. The system includes an inner porous homogenous layer adjacent to the first surface. That inner layer is characterized by relatively high porosity and thermal conductivity. Moreover, that layer is adapted to house coolant.

An outer porous homogenous is positioned between the inner layer and an outer member. The preferred surface of the outer layer defines the second surface, which is characterized by a uniform thermal envelope. A vapor chamber is positioned between the second surface and the outer member. The outer layer is characterized by a relatively low porosity and relatively high thermal conductivity. The outer layer is thermally coupled to the inner layer and the pores of the outer layer are coupled to the pores of the inner layer, thereby permitting coolant flow therebetween.

In operation, a selectively operable controller adjusts the vapor pressure in the vapor chamber so that the coolant housed within the porous inner layer has a liquid-vapor phase transition within the outer layer.

Thus, the heat exchanger of the present invention is a sandwich construction, with a high porosity inner layer providing a coolant reservoir. A conductive heat transfer path is provided to an evaporator. The outer low porosity layer functions as the evaporator, providing a high pressure loss coolant flow path within its structure. ln this sandwich, the liquid and vapor state of the coolant are controlled to co-exist over a range of pressures by the control of the back pressure in the vapor chamber. The back pressure, as controlled by the controller, governs the evaporation rate of the coolant. Lowering this pressure increases the evaporation rate of coolant in the low porosity layer and increases the heat flow from the surface-to-be-cooled. Thus, the temperature of the surface-to-be-cooled is controlled by modulating the vapor pressure chamber.

In one form of the invention, the coolant flows in a closed loop, wherein vaporized coolant is collected from the coolant chamber exit and routed to an external system where it is condensed, pressurized, and pumped back to the internal reservoir within the inner layer. In another operational mode, the coolant vapor extracted from the vapor chamber is exhausted to the environment, thereby depleting the coolant.

The foregoing and other objects of this invention, the various features thereof, as well as the invention itself, may be more fully understood from the following description, when read together with the accompanying drawings in which:

FIG. 1 shows in section a portion of a heat exchanger system according to the present invention;

FIG. 2 shows in detailed block diagram form, an embodiment of the present invention;

FIG. 3 shows in section a segment of a spherical heat exchanger; and

FIG. 4 shows a block diagram of the control system of the embodiments of FIG. 2.

FIG. 1 shows in section a portion of a system 10 for transferring heat from a first (or inner) surface 12 through a second (or outer) surface 13, where the outer surface 13 is characterized by a substantially uniform thermal envelope. The system 10 includes a multilayer porous structure thermally coupled to the surface 12. In the embodiment of FIG. 1, the multilayer structure incorporates two layers 16 and 20. The peripheral surface of the outer layer 20 functions as the second surface 13 in this embodiment. An outer member 14 is positioned in order to establish a vapor chamber 24 between it and the second surface 13.

The inner layer 16 is characterized by a relatively high porosity and thermal conductivity. The second or outer layer 20 is characterized by relatively low porosity and relatively high thermal conductivity. The two layers 16 and 20 are thermally coupled to each other, and the pores of the outer layer are coupled to the pores of the inner layer. In the preferred form, the outer layer 20 is made from sintered copper. By way of example, this material may be 100 FX Series, manufactured by Astro Met Associates, Inc., Cincinnati, Ohio.

The inner layer 16 is preferably made from an element selected from the group consisting of spherical particle metal powders, foam metals, felt metals, and ceramic materials. By way of example, this material may be an open cell, reticulated copper foam, such as Type 210-10 manufactured by Astro Met Associates, Inc. In other embodiments, additional intermediate layers With successively decreasing porosity (or graded porosity) may be positioned between the layers 16 and 20. In yet other embodiments, a relatively high porosity layer may be positioned between the outer layer 20 and the second surface 13.

As shown in FIG. 1, a pump 30 and associated pressure sensor 32 are coupled to the inner porous layer 16. Elements 30 and 32 are adapted to provide a liquid phase coolant in the layer 16. A pump and valve assembly 36 is coupled to the interior of the vapor chamber 24. A controller 40 is coupled to the vapor chamber by way of a temperature sensor 42 and a pressure sensor 44. The controller 40 is adapted to control the vapor pressure in the vapor chamber 24 so that the coolant between elements 12 and 14 has a liquid-vapor phase transition within the outer layer 20. In the present embodiment, the pump 30 and pressure sensor 32 is adapted to provide a continuous flow of coolant into the layer 16. In alternate embodiments, the system may be initially charged with a coolant in layer 16, without means for replenishing that supply. In the latter form, the pump, and valve assembly 36 controls the vapor pressure chamber to maintain the liquid-vapor phase transition within the outer layer 20, and thus providing effective heat exchange operation, until the supply of coolant in layers 16 and 20 has been exhausted.

In operation, the relatively high porosity inner layer provides a low resistance liquid flow path within the structure. This layer 16 effectively establishes a coolant reservoir adjacent to the surface-to-be-cooled 12. The outer layer 20 is characterized by relatively low porosity, and provides a continuous surface with a restrictive liquid/vapor flow path for coolant within its structure. With this configuration, the liquid and vapor state of the coolant can coexist at different pressures in the functioning system. By controlling the vapor pressure on the far side of the outer layer 20, i.e. in the vapor chamber 24, the temperature of the surface 12 is controlled by utilizing the unique equilibrium state of the coolant's vapor pressure-temperature relationship. Heat is transferred from the heat source (surface 12) through the porous metal structure (layers 16 and 20) by a combination of conduction, mass transport and vapor/condensation cells within the structure. The rate of evaporation depends on the heat flow and the rate at which vapor can leave the vapor chamber outside the porous metal shell.

In some forms of the invention, the coolant extracted from the vapor chamber 24 may be condensed and reinjected into the high porosity inner layer 16 to extend the operational utility of the heat exchanger. As a consequence of this operation, the thermal footprint associated with the second (or outer) surface 13 is substantially uniform. This is in part established by the transverse heat flow within the structure formed by homogenous layers 16 and 20.

In another form of the invention, the system may be configured for adaptively initiating operation. For example, the pores of the outer layer may initially be filled with a relatively low melting point material (rendering layer 20 initially non-porous). When the surface 12 gets sufficiently hot, the pore-filling material melts and opens up the pores of layer 20 and the above-described heat transfer begins.

FIG. 2 shows, detailed exemplary embodiment of a heat exchanger system in accordance with the present invention. The system of FIG. 2 is adapted for cooling a spherical body within a heat exchanger configured according to the present invention. In FIG. 2, elements corresponding to elements in the embodiment of FIG. 1 are denoted by identical reference designations.

FIG. 3 shows (in section) one segment of the hemispherical shell of one form of heat exchanger structure in accordance with this invention. The segment includes two separate different porosity layers of copper bonded together. The relatively thick inner layer 16 is a metal foam made of 11% dense copper, while the outer layer 20 is a metal foam made of 86% dense copper. On the innermost surface, of layer 16, i.e. adjacent to surface 12, there is radial and circumferential grooving to provide ducts for distributing coolant fluid within the inner portion of that layer. The outer member 14, or vapor shell, is a machined aluminum structure having lands cast into the spherical inner surface for the purpose of providing bonding seats for the copper foam segments. The outer layer 20 is epoxy-bonded to these seats. The innermost portion of the copper foam is layer 16A crushed upon assembly in order to improve and control the thermal conductivity across the gap from the surface 12 to the heat exchanger. Alternatively, a crushable copper, such as Foam Metal CU 45, manufactured by Hogen Industries Inc. of Willoughby, Ohio, may bridge the gap between layer 16 and the surface-to-be-controlled 12. The arrows generally show the flow of coolant in the illustrated quadrant from the input port 26 to the output port 28.

In the present embodiment, the liquid supply pressure P.sub.F is controlled by a liquid supply pump 30. The temperature of the surface 12 is controlled by varying the amount of heat removed from the system by vaporization of a Freon R-114 cooling fluid, by control of the mass flow of vapor leaving the vapor chamber 24. The valve 36 restricts the vapor flow out of the vapor chamber 24 by means of an electronically controlled variable orifice. The liquid pressure P.sub.F and vapor chamber pressure P.sub.U are sensed by pressure transducers 32 and 44 located on the heat exchanger liquid inlet and at the vapor chamber outlet, respectively. The temperature of surface 12 is sensed by a thermistor 42.

FIG. 4 shows a block diagram of the control system for this embodiment. Three control loops are implemented: a liquid pressure loop which regulates the Freon 114 liquid supply pressure, a temperature control loop, which controls the surface 12 temperature by commanding a change in vapor chamber pressure and a minor control loop, which controls a Freon 114 vapor control valve.

The liquid pressure loop compares a liquid pressure set point, P.sub.FS, to the actual liquid pressure, P.sub.F, and generates a pump control signal to increase or decrease the speed of the liquid supply pump 30. This pump control signal is input to a pump drive electronics module which controls the pump motor. Pump speed is thus used to maintain liquid supply pressure at the set point.

The temperature control loop functions by controlling the pressure in vapor chamber 12. Liquid supply pressure is selected to cause that flow required to satisfy heat load requirements.

Vaporization of the coolant within or at the surface of the porous layer 20 occurs in accordance with conditions of temperature, pressure, and heat transfer to the fluid (Freon 114). Vapor chamber pressure determines the Freon 114 vaporization temperature at the surface of (or within) the porous layer 20, and thereby determines the driving temperature differential (across the porous metal) for heat transfer out of the surface 12. Therefore, control of vapor chamber pressure allows control of heat flow out, thus allowing control of surface 12 temperature. Accordingly, a temperature error is used to call for a pressure change, modifying heat flow out of the system, therefore controlling temperature.

The vapor pressure control loop, a minor loop in the temperature control system, nulls the quantity

where

.DELTA.PC=Change in pressure command from temperature loop

P.sub.U =Vapor pressure

P.sub.b =Pressure bias value

Pressure bias is nominally set to operating point pressure (P.sub.U).

The vapor chamber pressure, P.sub.U, is varied by the vapor control valve 36, which is an electronically controlled variable flow orifice valve. The pressure control law produces a valve command which sets the valve flow area, in combination with an independently set valve bias voltage adjustment, V.sub.b. The overall range of control is governed by the flow area limits of the valve and the downstream pressure.

In the preferred form, each pressure of transducer modules 32 and 44 includes a Type DHF pressure cell (0-50 psig), manufactured by BLH Electronics, and an associated amplifier circuit. The units are calibrated such that the overall output is scaled at 1 volt/psi, with a bias of 0 volts at 20 psia for vapor pressure transducer, and 0 volts at 30 psig for liquid modules.

The pump 30 is a Tuthill magnetic gear pump, Model 9175. The pump control responds to an external voltage input. The drive module contains a switch which can select either manual pump control, by means of a potentiometer on the drive module box, or automatic control in response to an external input. The vapor control valve 36 is a Bertea vapor control valve used to control vapor flow and pressure. The valve 36 contains an electronically controlled variable orifice, with the flow area decreasing as the voltage to the valve is increased.

The controller 40 includes five analog control circuits: temperature decoder 62, temperature control 64, monitor 66, pressure control 68, analog pump control 70, and a TI 990/101M microprocessor system 80 and an associated analog input/output network 82 and power supplies (not shown). The five analog circuits perform the following functions:

Converts an input temperature signal to an analog voltage, required for control.

Analog temperature control.

Creates the pressure error signal and temperature error signals required.

Contains the analog vapor pressure control circuit, the valve drive circuit, and a precision voltage generation circuit.

Analog liquid pressure control.

The CPU control switch 53 controls the output of the microprocessor DAC2. The computer controller 80 executes both the temperature (outer loop) control, which has a .DELTA.P.sub.C output command, and the vapor chamber pressure (minor loop) control, which has a valve drive command output. If both digital loops are run simultaneously, then the .DELTA.P.sub.C command from the temperature control loop is used internally as the input to the vapor pressure control loop, and the output from the computer (DAC2) is a valve command. In this case, the CPU control switch 53 is set on valve drive CMD. This case is commanded in the software by setting FLAG =0. If digital temperature control only is desired, along with the analog vapor pressure control minor loop, FLAG is set to a nonzero value in the software. The output to DAC2 is then a .DELTA.P.sub.C voltage command to be input to the analog vapor chamber pressure control board. In this case, the CPU control switch 53 is set in the PRESS CMD position.

The analog vapor pressure CMD switch 52 determines the source of the .DELTA.PC command input to the analog vapor pressure control board: CPU, in which the command is derived from the microprocessor temperature control loop, or ANALOG, in which the .DELTA.PC command is derived from the analog temperature control loop on the temperature control board.

The LIQUID PUMP CMD switch 56 determines the source of command input to the liquid pump; the analog liquid pressure control output in the ANALOG position, or the output from the digital liquid pressure loop in the CPU position.

The VALVE CMD switch 51 determines the source of the valve drive command. This drive command is summed with the V.sub.b value, set on the VALVE BIAS potentiometer, to yield the final overall value drive voltage. In the ANALOG position, the command is derived from the analog vapor pressure control loop. In the CPU position, the command is derived from the digital pressure control system.

The microprocessor-based digital control system 80 implements control functions with relatively long time constants and to provide the capability for additional control logic and modes of control. The system 80 includes a microprocessor having a Texas Instruments TM 990/302 software development board. This board provides a ROM based, operating system with a text editor, 2 pass assembly, program loader, program debugger, and ROM programmer, all contained in 8K of ROM memory. The system 80 incorporates a TI Silent 700 terminal as the I/O device, and two standard cassette records as program storage devices. A RAM expansion memory board, TM 990/206, is included for software development work, and a ROM memory board, TM 990/201, was included for operational use. An Analog Devices RTI-1243 analog I/O board 82 provides A/D and D/A capability.

In the operational configuration, the control program is stored in two TMS 2716 ROMS, contained in location 2000.sub.16 -2FFF.sub.16 on the 201 memory board. The 302 software development operating system is also present and used to initialize the control program. After power-up the operating system is obtained by pressing the RESET button, and performing a carriage return on the terminal, the 302 software development monitor is then entered. (The 302 software development system is fully described in the TI users manual). The control program is started by entering the .DP (Debug Program) mode. An initial workspace pointer (WP) of 1000.sub.16 is selected. Two Program entry points are available; 2000.sub.16, which reinitializes all control constants and program running variables from ROM, or 2100.sub.16, which does not initialize anything. The program must be first run at location 2000.sub.16 to initialize the constants. The program may be interrupted and control returned to the monitor by pressing the RESET button, and performing a terminal carriage return. At this point, the .DP (Debug Program) mode may be entered again. Memory constants can now be changed by using the .IM (Inspect Memory) command at the desired program location. Control law gains may be changed in this manner. The program is started by initializing the Program Counter to 2100.sub.16, and issuing an .EX command.

The control program performs two major functions:

(1) Data logging on terminal

(2) Control of liquid pressure

(3) Control of either vapor chamber pressure and temperature simultaneously, or control of temperature only (in which case vapor chamber pressure control is done by the analog control board).

A program listing and RAM variable list for microprocessor 80 is contained in Appendices 1 and 2.

The program is interrupt driven with a main routine and two interrupt levels. The highest priority interrupt is the clock interrupt--typically run at 32 Hz. The eight A/D values are converted in this routine and summed such that eight converted, summed values of each A/D channel are passed out to the control program-which runs typically at 4 Hz. The A/D sums are performed in the vector ADCT. Summed A/D variables are as follows:

Since the A/D is 12 bits, with two's complement scaling for polarity, the summed eight values are always contained within the 16 bit word. Therefore, the overall scale factor in bits/volt for each channel is ##EQU1##

Every eight clock interrupts (for QCNT =8) the A/D sums are copied to the variable list and the control program run. The control program is run on Interrupt 6. This program is called by setting output bit P6 of the TM 990/101M board high, then low. This bit is wired to the input of INT6 on the same board. Thus, a hardware INT6 is generated and the interruptable control programs runs.

Time counters for each of the three control loops are kept. These counters are decremented and the routines run when the counter equals 0. Typically, the liquid pressure and temperature loops are run at 1 Hz, while vapor pressure loop runs at 4 Hz.

Control calculations are performed in the PID subroutine. This subroutine is called with the current error (calculated in control setup routines VCTL, PCTL, TCTL) in R0, and R9 containing the address pointer of a control data table. All references in P/D to control variables are then referenced as offsets to the data table base address. The data tables are RAM vectors of length 66 bytes: TDAT, PDAT, FDAT. The individual variable location within these tables are shown in the RAM variable list of Appendix 2.

The PID control subroutine contains three modes of operation--depending on the value of MODE. The modes are defined as follows:

The value of MODE is initialized at 0, and increments once per control call, until MODE =MOD2, when it is fixed at that value. In the STARTUP mode proportional, integral, derivative control is performed with control error limit checks. If the control error is greater than CHI, or less than CLO, the output OUI is set to OTHI or OTLO, respectively, and the integrator, EI, is cleared. In the RUN Mode, entered after the startup period, an out-of-limit error causes a RESET mode. In a RESET, the value of MODE is initialized to -MOD1, and the output set to MOD0. This fixed output continues until MODE increments to 0, when STARTUP is entered. RESET mode may be bypassed by setting MOD1 equal to a negative value (whose absolute value is less than MOD2).

The PID terms are all computed, scale, and limited before summing into the output OUT. Scaling of the proportional term is ##EQU2##

The output is limited to .+-.5 volts output. The integral term EI, is computed as follows: ##EQU3##

At a 1 Hz control rate, the integrator integrates at ##EQU4##

The error rate, RATE, is computed by first computing the delta error between the current (unfiltered) error, located at E 0 in the error table, and some error at a previous time, located in the table. This delta error time is set by the variable ERRL. The number of control periods over which the delta error is computed is equal to ERRL/8. This delta error is now filtered by a 1st order digital filter to produce RATE. The rate term is equal to ##EQU5##

For ERRL =8 (4 sec period to compute rate over) the rate terms scaled as follows: ##EQU6##

Upon entry at 2000.sub.16 the RAM variables are initialized. The interrupts are setup and the clock interrupt started. The main routine now enters a data logging loop--initiated by the counter LTIM, which is decremented in the clock interrupt routine. At the time when LTIM =0, the eight A/D variables are output in scaled form. (psi or Degrees F). Also the following variables are output for each control law (1 line):

ERR

RATE

EI

MODE

OUT

The main program can be entered at 2100.sub.16 if RAM initialization is not desired.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. ##SPC1## ##SPC2## ##SPC3##