Two-pipe system for refrigerant isolation

A heat pump system for heating and cooling including a vapor compressor driven by a heat engine in which the refrigerant is isolated from a space being conditioned, by a two-pipe loop that, in a heating mode transfers both refrigeration derived heat and heat rejected by the heat engine in an efficient serial manner. The system circuitry includes a counterflow heat transfer fluid to air heat exchanger and a heat transfer fluid to potable water heater exchanger.

BACKGROUND OF THE INVENTION 
The invention relates to improvements in heat pump systems and, in 
particular, to a fuel-fired heat pump system coupled to a load including 
an air handler and a heat storage medium by an isolation circuit. 
PRIOR ART 
U.S. Pat. Nos. 4,976,464, 5,192,022, 5,226,594 and 5,253,805 assigned to 
the assignee of the present invention, disclose methods and apparatus that 
utilize a heat engine to drive the refrigeration compressor of a heat pump 
circuit for space conditioning. Such circuits are typically interconnected 
to a heat storage medium in the form of a tank-type water heater that can 
be heated both from the heat pump unit and from a fuel burner associated 
with the tank. 
These space conditioning systems can improve operating efficiency by 
reducing cycling losses and by utilizing heat rejected from the engine. 
Additionally, the systems can provide a back-up heat source in the event 
of an engine malfunction. The aforementioned U.S. Pat. No. 5,253,805 
teaches a technique to isolate the refrigerant from the space being 
conditioned. 
SUMMARY OF THE INVENTION 
The invention provides a fuel-fired heat pump system for conditioning the 
air of an enclosed space that has its heat engine and refrigeration 
circuit outdoors and is thermally coupled to the indoor load in a 
simplified manner. The indoor load is represented as a heat exchanger for 
heating or cooling the air in the enclosed space and a heat exchanger for 
heating water stored in a potable hot water tank. In the preferred 
embodiment the heat exchange between the heat pump and, in heating 
service, the heat engine with the load is accomplished with a heat 
transfer fluid circulated through a closed loop circuit that 
advantageously isolates the refrigerant from the interior of the space. 
The circuit requires only two pipes between the heat exchangers for the 
heat pump and heat engine and the load. 
The disclosed simplified two-pipe system is capable of efficient operation 
by virtue of a unique arrangement of the associated heat exchangers. In 
particular, for heating service, at the outdoor section, the circuit 
directs heat transfer fluid first through the heat pump heat exchanger and 
then through the heat engine heat exchanger. Indoors, the circuit conducts 
the heat transfer fluid in counterflow relation to forced air flow in an 
associated duct. Where the hot water storage tank requires heat, the 
circuit delivers heat transfer fluid first to the water tank heat 
exchanger and then to the air heat exchanger. 
The disclosed simplified two-pipe system can reduce original equipment cost 
as well as installation cost while still achieving high operating 
efficiency and reliability. Advantageously, the hot water storage tank is 
a fuel-fired heater type for potable hot water. A novel aspect of the 
invention involves a heat exchanger construction integrated with such a 
tank in a cost effective manner that avoids the risk of contamination of 
the potable water.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 illustrates a system 10 for air conditioning, i.e. space heating and 
cooling a residential, work or recreational area such as a house, 
apartment, office or like occupied space. The system 10 includes a heat 
pump compressor 11 driven by a fuel-fired prime mover or engine 12 and a 
storage-type hot water heater 13. The system 10 further includes heat 
exchanger coils 16a, b in a duct 18 through which air from the space being 
conditioned is circulated. The closed space being conditioned by the 
system 10 is schematically illustrated by the broken line 19. A building 
represented by the zone to the left of a broken line 15 in FIG. 1 
containing the conditioned space 19 may also contain other enclosed areas 
either occupied or normally non-occupied areas such as a boiler room or 
other equipment room. 
The prime mover 12 is an internal combustion engine or other heat engine 
such as a Sterling, steam or gas driven unit and is preferably fueled by 
natural gas or other combustible fuel supplied by a line 20. The 
illustrated heat pump compressor 11 is preferably a refrigerant vapor 
compressor producing a reverse Rankine vapor compression cycle. It will be 
understood that various types of compressors such as reciprocating, screw, 
vane, or centrifugal can be used. Further, a reverse Brayton heat pump 
cycle can also be used. Typically, the engine 12 and compressor 11 are 
situated outdoors of the building 15 and are contained in a common cabinet 
25. Ordinarily, the mechanical power output capacity of the engine 12 is 
matched to the rated power requirement of the heat pump compressor 11. 
Operation of the system 10 is described herein first with reference to 
heating service and later with reference to cooling service. In heating 
service, a refrigerant fluid, when the heat pump compressor 11 is 
operating and a four-way cross over valve 14 is appropriately positioned 
by a controller 62, circulates through a heat exchanger 28 located 
outdoors in or adjacent the cabinet 25 and through another coil or heat 
exchanger 21 also located outdoors in or adjacent the cabinet through 
interconnecting lines 22-24. Heat is absorbed by the refrigerant fluid at 
the outdoor heat exchanger 21 and is exchanged from this fluid to a heat 
transfer fluid, typically a liquid, at the heat exchanger 28 as more fully 
discussed below. A refrigerant liquid expansion valve 26 in the line 23 
causes the refrigerant to enter the outdoor heat exchanger coil 21 
partially vaporized at low pressure and low temperature. The outdoor coil 
21 is in heat exchange relation to outdoor or environmental air which may 
be circulated across the coil by a powered fan 27. Alternatively, the 
outdoor coil 21 may be in heat exchange relation with a subsurface media 
such as ground water or with a solar pond. Heat absorbed by the 
refrigerant as it passes through the coil 21 causes it to be vaporized. 
The compressor 11 elevates the pressure of the vaporized refrigerant and, 
therefore, the condensing temperature of the refrigerant fluid before it 
enters the heat exchanger 28. The refrigerant condenses in the heat 
exchanger 28 giving up heat. 
The heat exchanger 28 has a coil 29 through which the refrigerant 
circulates and a coil 30 through which a heat transfer fluid circulates. 
The coils 29, 30 are in thermal communication with each other. The heat 
transfer fluid in the coil 30 is preferably a liquid such as a water and 
ethylene glycol solution or another liquid capable of absorbing and 
transferring heat and not freezing in normally expected winter air 
temperatures at the site of the building. The coils 29, 30 allow heat to 
be transferred from the fluid contained in one coil to the fluid contained 
in the other coil while maintaining the fluids physically isolated from 
one another. 
A heat exchanger 43, conveniently a liquid-to-liquid type, is arranged, 
selectively through a valve "A", to transfer heat rejected by the engine 
12 to the heat transfer fluid circulated in the heat exchanger coil 30 
associated with the refrigeration circuit and related supply and return 
lines 44, 45 respectively. A pump 47 mechanically driven by the engine 12 
circulates engine coolant through lines 48, 49 connecting it with a coil 
50 of the heat exchanger 43 and through lines 51, 52 connecting it with a 
liquid-to-air heat exchanger 53 served by the fan 27. The engine coolant 
circulating in these lines typically is arranged to absorb heat being 
rejected by the engine in its jacket and, if desired, in its exhaust. 
Rejected heat from the engine 12 is available at a higher temperature than 
the temperature reached by the heat pump refrigerant. This relationship is 
accounted for by arranging a coil 56 of the engine coolant heat exchanger 
43 downstream, with reference to flow of the heat transfer fluid in the 
lines 44, 45 of the coil 29, of the refrigerant heat exchanger 28, when 
the coil 56 is active. 
During heating service, heat transfer fluid circulated by a pump 41, 
operated by the controller 62, through supply and return lines 44, 45 
first picks up heat from the refrigerant coupled coil 30 and then from the 
engine coolant or rejected heat coil 56. From the latter coil 56, the heat 
transfer fluid is conducted through the line 45 to a valve housing 54 from 
which it is directed to the air duct coils 16a, b or, a heat exchanger 55 
associated with the hot water tank 13 or, serially to both the hot water 
tank exchanger 55 and the duct heat exchangers 16a, b. It will be 
understood that the controller 62 has control lines to the various 
directional control valves A, B and C in the cabinet 25 as well as 
individual valve components D through H in the housing 54. 
The refrigerant coupled coil 30, engine heat coupled coil 56 and the supply 
and return lines 44 and 45 form an isolation circuit that transfers heat 
from the heat pump circuitry and from the engine 12 indoors to the tank 13 
and/or the air duct coils 16a, b in the building while maintaining 
refrigerant out of the building. 
Relatively high temperature heat storage is preferably provided by the unit 
13 in the form of a storage-type hot water heater. Particularly suited for 
this application are appliances which comply to American National 
Standards Institute Standard Z-21.10. The heater 13 is of generally 
conventional construction with the exception of the addition of the heat 
exchanger 55 provided as a coil wrapped on the exterior of a tank proper 
31 as described in greater detail below. 
The tank 31 has a capacity in the range of 30 to 50 gallons, for example, 
and a burner 32 with a capacity in the range of 36,000 to 100,000 btu/hr., 
for example, centrally located at its bottom. The burner 32 mixes natural 
gas from a supply line 35 and air and supports combustion of the same. 
Combustion products from the burner 32 pass through a vertical stack 33 
through the center of the tank 31 to heat water stored therein in a known 
manner. 
A conventional thermostatic control valve 34 responds to the temperature of 
water in the tank 31 and operates the burner 32 whenever the temperature 
falls below a predetermined limit, for example, 120.degree. F. An outlet 
36 on the heater tank 31 supplies potable hot water through a line 37 to 
sink taps and the like at the space 19. A source of cold potable water, 
such as a public utility line, supplies an inlet 39 of the tank 31 through 
a line 38 to make up for water use at the taps. 
The heat exchanger coil 55 is preferably comprised of metal tubing wrapped 
about the exterior of the heater tank 31. Ordinarily, the tank is a 
cylindrical structure formed of steel and the tube forming the coil 55 can 
be wrapped in direct contact with the tank and held in place by solder or 
other suitable thermally conductive material such as an adhesive. The tube 
can be made of copper, steel or other suitable metal. In one example, with 
a 50 gallon tank adequate heat transfer was achieved using a 1/2" diameter 
copper tube coiled in a double helix about the lower 2/3 portion of the 
tank. The coil was soldered along essentially its full length to the 
exterior of the tank with adjacent turns on a 1-1/2" spacing. The external 
construction of the heat exchanger coil 55 on the outside surface of the 
tank 31 eliminates the risk that the heat exchange fluid carried therein 
can enter the tank in the event of a leak and contaminate the potable 
water contained in the tank. The heat exchanger 55 is coupled to the valve 
housing 54 by lines 63, 64. 
A blower 58 circulates air from the space 19 being conditioned through the 
duct 18 in the direction indicated by the arrows 59 in order to heat this 
air at the exchangers 16a, b. The blower 58 is operated under the command 
of the controller 62. A thermostat 61 monitors the temperature of air 
within the space 19 and provides a signal to the controller 62. The 
controller 62, in response to a signal from the thermostat 61 that there 
is a demand for heat, causes the engine 12 to start-up and drive the heat 
pump compressor 11 thereby moving heat from the outdoor coil 21 to the 
other heat pump coil 29. At the same time, rejected engine heat is 
delivered to the coil 50 of the associated heat exchanger 43. 
The valve housing 54 is conveniently situated indoors and can be integrated 
with the pump 41 if desired. The controller 62 selectively operates 
individual valves D, E, F, G and H in the housing 54 and valves A, B, C in 
the outdoor cabinet 25. 
The chart of FIG. 2 pictorializes the position of the valves A-H for 
various operating modes of the space conditioning system 10. In all of the 
heating modes, the pump 41 and relevant valves are operational to 
circulate heat transfer fluid through the loop formed by the lines 44 and 
45 and the heat exchanger coils 30 and 56 with such fluid serially passing 
first through the refrigerant coupled coil 30 and then through the engine 
heat coil 56. In Mode 1, heat from the heat pump system is used 
exclusively for heating the air of the space 19. The line 45 returns the 
heat transfer fluid to the valve housing 54 from which it is directed 
through a line 65 to the air duct heat exchange coils 16a, b. Fluid flow 
in the coils 16a, b is in counter-flow relation to forced air flow in the 
duct 18 for maximum heat transfer efficiency. 
In Mode 2, the system 10 operates to provide space heating of air at low or 
moderate levels and to store the balance of heat being developed by the 
heat pump and its engine in the hot water storage tank 13. In this 
operational mode, the valves E-H direct hot heat transfer fluid returning 
from the heat exchangers 30 and 56 serially first through the heat 
exchanger coil 55 associated with the hot water storage tank 13 and then 
through the air duct heat exchangers 16a, b. 
In Mode 3, the engine 12 and heat pump compressor 11 are not operated and 
heat is provided solely from that existing in the tank 13. This is 
accomplished by operating the pump 41 to circulate heat transfer fluid 
through the hot water coil 55 and then serially through the heat 
exchangers 16a, b. It will be understood that in Mode 3 heat in the tank 
13 can be used to heat the space 19 at appropriate times between periods 
of operation of the engine 12 and the heat pump compressor 11. In a simple 
effective control strategy. The controller 62 for successive periods of 
heat demand can alternate modes of heat supply between operation of the 
heat pump (Modes 1 or 2) and exchange of heat from water in the tank 13 
without heat pump operation in this Mode 3. 
In Mode 4, the heat pump system 10 operates to store heat, i.e. to heat 
potable water only and does not provide space heating. As can be seen in 
the chart of FIG. 2, valves isolate the circulating heat transfer fluid 
from the air duct coils 16a, b and limit its circulation for heat delivery 
to the hot water tank heat exchanger coil 55. Modes 5 and 6 in the chart 
of FIG. 2 represent alternative constructions of the valves F and H, 
respectively. In these modes, heat is supplied for space air heating 
simultaneously from the heat pump system as well as the burner of the hot 
water tank 13. 
Operation of the system 10 for cooling the space 19 is represented by Mode 
6 in the table of FIG. 2. In this mode, the controller 62 shifts the valve 
14 to reverse the flow of refrigerant fluid through the lines 22-24 and 
initiates operation of the engine 12 and compressor 11. When the 
compressor 11 operates in this mode, heat is absorbed by refrigerant in 
its coil 29 from the heat transfer fluid in the coil 30 and this heat is 
discharged to the atmosphere from the refrigerant coil 21. The valves B 
and C direct engine coolant carrying heat rejected by the engine through 
line 51 to the heat exchanger 53 to discharge such heat to the 
environmental air. The engine coolant returns through line 52. The valve A 
assumes a position that shunts heat transfer fluid leaving the coil 30 in 
a chilled condition away from the engine heat coil 56 (which is inactive) 
and conducts it directly to the line 45. The valves in the housing 54 
direct this chilled heat transfer fluid to the coils 16a, b for cooling 
air flowing to the duct 18. 
It should be evident that this disclosure is by way of example and that 
various changes may be made by adding, modifying or eliminating details 
without departing from the fair scope of the teaching contained in this 
disclosure. The invention is therefore not limited to particular details 
of this disclosure except to the extent that the following claims are 
necessarily so limited.