Dual-Circuit Series Counterflow Chiller with Intermediate Waterbox
Background of the Invention
[0001] This invention relates generally to water cooled chillers and, more
specifically, to the interconnection of two vapor compression refrigeration systems in a series-counterflow arrangement.
[0002] Water cooled chillers in a series-counterflow arrangement consist of
two independent vapor compression refrigeration systems with chilled water and condenser water circuits that are common to both circuits and are arranged in series. This arrangement allows for an increased coefficient of performance (COP) over a single refrigeration circuit design because the separate circuits with series counterflow have a lower average pressure differential between the evaporator and condenser, thus requiring less energy to compress refrigerant from the evaporator to the condenser.
[0003] In such a system, water in each of the evaporators and the condensers
flows through a plurality of tubes that span bom refrigeration circuits, with the refrigeration circuits being separated by a tubesheet which is located at the middle of the tubes, and with each tube being hermetically sealed to the tubesheet, typically by expansion of the tube to the tubesheet.
[0004] One problem that arises is that of servicing the tubes such as may be
required if a tube fails in operation. Such removal of a tube requires cutting the tube at all locations where it has been expanded and then pulling the tube out. It is not possible to completely remove a tube since there is no access to cut the tube at the center tubesheet location, which is inside the refrigerant boundary. If a tube is cut internally, or if a tube fails in operation, a leak path is created between the circuits that does not allow for operation of either circuit, thus adversely impacting both reliability and serviceability.
[0005] Another problem with a dual circuit system is that of control. A
critical parameter for control of a water cooled chiller is the use of the leaving temperature differential, which is the difference in the temperature of the water leaving a heat exchanger and the refrigerant temperature within the heat exchanger. Since the water tubes span both refrigerant circuits in a dual system, it is not
possible to obtain the leaving water temperatures of the upstream circuit's condenser or evaporator.
[0006] In addition to serviceability and control as discussed hereinabove,
prior art heat exchanger tubes that span dual circuits pose problems of reliability, shipping and performance. That is, because the common tubes extend across both circuits, it is impossible to optimize the heat transfer tubes in each circuit independently, and shipping of machines that are longer due to the longer tubes can be difficult
Summary of the Invention
[0007] Briefly, in accordance with one aspect of the invention, each circuit
has unique tubesheets that separate the refrigeration circuit from the cooling
medium. Between each circuit is an intermediate waterbox that passes water from
the upstream circuit to the downstream circuit The waterbox is removable for
service and enables the transporting of the units in pieces with shorter length
requirements.
[0008] In accordance with another aspect of the invention, since each circuit
has its separate and unique tubes, a tube failure in either circuit no longer creates a
refrigerant leak path to the adjacent circuit, such mat operation of the nonfailed
circuit can be maintained, thereby increasing reliability.
[0009] By another aspect of the invention, since the intermediate waterbox is
accessible from the outside, temperature measurement instrumentation can be
installed to obtain the leaving temperature differential of the first circuit, thereby
providing better control of the system.
[0010] In accordance with another aspect of the invention, the intermediate
waterbox causes mixing of the water that leaves the upstream circuit before entering
tbe downstream circuit, thereby increasing heat transfer effectiveness and COP.
[0011] By yet another aspect of the invention, use of the waterbox allows for
multiple parameters that can be varied in order to optimize the efficiency of each of
the circuits. In addition to varying the length of each circuit, the tube material, the
tube heat transfer enhancement, and the number of tubes are configurable, and can
be unique to each circuit
[0012] In the drawings as hereinafter described, a preferred embodiment is
depicted; however, various other modifications and alternate constructions can be made thereto without departing from the spirit and scope of the invention.
Brief Description of the Drawings
[0013] FIG. 1 is a schematic illustration of the temperatures in a single
circuit chiller in accordance with the prior art.
[0014] FIG. 2 is a schematic illustration of the temperatures in a dual-circuit
chiller in accordance with the prior art
[0015] FIG. 3 is a schematic illustration of the condensers and evaporators of
a dual-circuit chiller in accordance with the prior art.
[0016] FIG. 4 is a schematic illustration of dual-circuit chiller system in
accordance with the present invention.
[0017] FIG. 5 is a schematic illustration of the condenser and evaporators in
a dual-circuit system of the present invention.
[0018] FIG. 6 is a schematic illustration of the waterbox portion of the dual-
circuit system in accordance with the present invention.
[0019] FIG. 7 is a perspective view of the waterbox portions of a dual-circuit
system in accordance with the present invention.
[0020] FIG. 8 is an end view of the waterbox portion of a dual-circuit system
in accordance with the present invention.
Description of the Preferred Embodiment
[0021] Figure 1 shows a condenser 11 and a cooler or evaporator 12 of a
single circuit chiller that is typical of the prior art. As shown, the condenser water
and evaporator water flows in a counterflow relationship, and the resulting
temperatures entering and leaving the condenser and evaporator are as shown.
[0022] In order to obtain increased COPs, a dual-circuit is connected in
series counterflow arrangement as shown in Fig. 2. Here, two independent vapor compression refrigeration circuits, 13 and 14, are connected by an intermediate tubesheet 15 as shown. The first circuit 13 has a condenser 16 and an evaporator 17,
and the second circuit 14 has its own condenser 18 and evaporator 19. However, the condenser water circuits of the condenser 16 and 18 are common to both circuits and are arranged in series. Also, the chilled water circuits of the evaporators 17 and 19 are common to both circuits and are arranged in series. This can be best seen by reference to Fig. 3.
[0023] It will be seen in Fig. 3 that the condenser tubes 21 are long and span
the length of each of the condensers 16 and 18 of the circuits 13 and 14. While the
intermediate tubesheet 15 isolates and separates the refrigerant in the respective
circuits 13 and 14, the water flow through the condenser tubes 21 is continuous from
the entrance of the condenser 16 to the outlet of the condenser 18.
[0024] Similarly, the evaporator tubes 22 are unitary members that extend
across both circuits 13 and 14, with the intermediate tubesheets providing isolation only for the refrigerant in the systems 13 and 14, but allow for the evaporator water to flow continuously from the inlet end of the evaporator 19 to the outlet end of the evaporator 17.
[0025] As discussed hereinabove, such dual-circuit systems with heat
exchanger tubes that span both circuits present problems with respect to service, reliability, shipping, performance and control.
[0026] Referring now to Fig. 4, a system is shown to overcome the above-
discussed problems. A first circuit, 23, includes a condenser 24, an expansion
device 26, an evaporator 27 and a compressor 28, which operate in serial flow
relationship in a well-known manner. A second circuit, 29, includes a condenser 31,
an expansion device 32, an evaporator 33 and a compressor 34 which also are
connected in serial flow relationship and operate in a well known manner. The two
circuits 23 and 29 are interconnected in a manner similar to that shown in Fig. 3 but
with a different structure at the interface between the two circuits and different
structure with respect to the tubes within both the condensers and the evaporators.
[0027] As shown in Figs. 4 and 5, at an intermediate position between the
two evaporators 27 and 33 is an evaporator waterbox 36, and at an intermediate position between the two condensers 24 and 31 is a condenser waterbox 37. Further, unlike the systems described hereinabove wherein the tubes are unitary tubes extending across both circuits, the condenser tubes 38 of circuit 1 are separate and
independent from the condenser tubes 39 of circuit 2, and the evaporator tubes 41 in circuit 1 are separate and distinct from the evaporator tubes 42 of circuit 2. That is, the condenser tubes 38 are fluidly connected to one side of the waterbox 36 and the condenser tubes 39 are fluidly connected to the other side thereof. Similarly, the evaporator tubes 41 are fluidly connected to one side of the waterbox 37 and the evaporator tubes 42 are fluidly connected to the other side thereof. The waterboxes 36 and 37 therefore act as intermediate receptacles for the water as it passes between the first circuit 23 and second circuit 29.
[0028] The advantages of the above-described design are numerous. First of
all, rather than having long unitary tubes, the tubes, and therefore the refrigeration
circuits, are generally only about half as long and can be more easily handled and
shipped to a site, with the tubes, and therefore the refrigeration circuits, being
independent and separatable from the waterboxes. Second, since the tubes are
independent, they can be configurable to optimize performance in each circuit. That
is, in addition to the variation in length of the tubes in each circuit, the number of
tubes within the second circuit can be different from those in the first circuit as
shown in Fig. 5, and other variations can be made, such as different tube material, or
different heat transfer enhancements. This allows the designer to optimize the
desired capacity, efficiency, pressure drop, or cost for each circuit.
[0029] Other advantages of the present system can be seen by reference to
Fig. 6. Because the water from the upstream tubes is discharged along one side of
the waterbox 36 (or waterbox 37 in the case of the evaporator), it tends to cause a
turbulence within the waterbox such that the individual flow streams are mixed so as
to become a reservoir of water with a relative uniform temperature before it enters
the tubes of the downstream circuit. This mixing is beneficial to the heat transfer
effectiveness, thereby increasing COP of the total system.
[0030] By using the waterbox 36 as described, the intermediate waterbox 36
is now accessible from the outside and temperature measurement instrumentation 43
can easily be used to obtain the leaving temperature differential of the upstream heat
exchangers, thus providing improved control of the system.
[0031] Another advantage of the use of waterboxes as described is mat of
facilitating service and repair. That is, since the waterbox is attached to the tube
circuits in a manner that allows removal of the waterbox, as will be described
hereinafter, the removal of the waterbox allows service of the tubes at each circuit's
tubesheet, thereby substantially improving serviceability. Further, since a tube
failure in either circuit does not create a refrigerant leak path to the adjacent circuit,
the reliability of the system is substantially enhanced.
[0032] Referring now to Figs. 7 and 8, the structural interface of the
intermediate waterbox and the adjacent circuits are shown. As shown the
intermediate waterbox 44 comprises a relatively short cylinder with a plurality of
holes 46 formed longitudinally from one end 47 to the other, for receiving bolts 48
passing through the respective tubesheets 49 and 51. The waterbox, 44, is thus
sandwiched between the tubesheets 49 and 51 of the respective circuits and can be
easily disassembled by removing the bolts, 48, to get access to the tubes for repair
purposes at the tubesheets between the circuits. It will therefore be recognized mat
each of the circuits is independent, and access can be gained to the intermediate tube
to tubesheet joints without disrupting refrigerant boundary of either circuit.
[0033] Although the waterbox 44 is shown in Figs. 7 and 8 as relatively
short in length (i.e. about 4 inches), its configuration, size and shape can be substantially varied while remaining within the scope of the present invention. Further, although described in terms of use with a water cooled chiller, the present invention could also be applicable to an air cooled chiller wherein the evaporators of series connected circuits are interconnected by way of an intermediate waterbox structure.

We Claims :-
1. A chiller system of the type having first and second refrigeration
circuits with each refrigeration circuit having a compressor, a condenser, an
expansion device and an evaporator and with the respective evaporators in the first
and second circuits having a plurality of tubes to conduct the flow of liquid to be
cooled, and with the respective evaporators of the first and second circuits being
interconnected in series relationship such that the liquid to be chilled passes serially
through the respective evaporators of the first and second circuits, comprising:
a waterbox interconnected between the evaporators of the first and second circuits and having a unitary reservoir for conducting the flow of liquid from the tubes in the evaporator of the first circuit and to the tubes of the evaporator of the second circuit.
2. A chiller system as set forth in claim 1 wherein each of said first and second evaporators includes an intermediate tubesheet, and wherein said intermediate waterbox is interconnected between said tubesheets.
3. A chiller system as set forth in claim 2 wherein said intermediate waterbox is cylindrical in form and is connected to said tubesheets at the respective circular ends of the cylinder.
4. A chiller system as set forth in claim 3 wherein said waterbox has a plurality of holes formed longitudinally between its opposite ends and further wherein bolts are passed through the tubesheets and through said holes.
5. A chiller system as set forth in claim 1 and including temperature measurement instrumentation connected to said waterbox to measure the temperature of the water therein.
6. A chiller system as set forth in claim 1 wherein the respective condensers of the first and second circuits are connected in series and are watercooled and include a waterbox interconnected between the condensers.
7. A chiller system as set forth hi claim 6 wherein said evaporators of said first and second circuits are adapted to conduct the flow of cooling liquid in counterflow relationship to the flow of cooling liquid in the condensers of the first and second circuits.
8. A dual-circuit chiller, comprising:
a first circuit having a compressor, a condenser, an expansion device and an evaporator, with the evaporator having a plurality of tubes for conducting the flow of water to be cooled from an inlet end to an outlet end of the tubes;
a second circuit having a compressor, a condenser, an expansion device and an evaporator with the evaporator having a plurality of tubes for conducting the flow of water to be cooled from an inlet end to an outlet end of the tubes; and
an evaporator waterbox fluidly interconnected between said first circuit tube outlet ends and the second circuit tube inlet ends, such mat water to be cooled flows from said first circuit tube outlet ends, into said evaporator waterbox and then into the second circuit tube inlet ends.
9. A dual-circuit chiller as set forth in claim 8 and including a first intermediate tubesheet surrounding said first circuit tube outlet ends and a second intermediate tubesheet surrounding said second circuit tube inlet ends and further wherein said waterbox is connected to said first and second intermediate tubesheets.
10. A dual-circuit chiller as set forth in claim 9 wherein said waterbox is cylindrical in form.
11. A dual-circuit chiller as set forth in claim 10 wherein said cylinder has holes formed longitudinally between its end surfaces and further wherein bolts pass through said first and second intermediate tubesheets and through said holes to secure the waterbox to said first and second intermediate tubesheets, respectively.
12. A dual-circuit chiller as set forth in claim 8 and including temperature measurement instrumentation attached to said waterbox for measuring the temperature of the water therein.
13. A dual-circuit chiller as set forth in claim 8 wherein said condensers of said first and second circuits are watercooled and are connected in serial flow relationship and further include a condenser waterbox interconnected between the condensers of said first and second circuits.
14. A dual-circuit chiller as set forth in claim 6 wherein the flow of water in said evaporators is in counterflow relationship with the flow of water in said condensers.