US2171407A - Thermostatic control for multipass evaporators - Google Patents

Description

Aug. 29, 1939. I L. SH-RODE 7 07 THERIQSTATIC CONTROL FOR MULTIPASS EYAPORATORS Filed May 14, 1936 3 Sheets-sheaf 1 COMP/M1530 J. L. SHRODE THERIOSTATIQ CONTROL FOR MULTIPASS EVAPORATORS 3 Sheets-Sheet 2 Filedlay 14, 1936 Au 29, 1939. J, L, SHRODE 2,171,407

THERIOSTATIC CONTROL FOR MULTIPASS EVAPORATORS Filed May 14, 1936 3 Sheets-Sheet 3 LIL. 519127415 Patented Aug. 29, 1939 UNITED STATES THERMOSTATIG CONTROL FOR. MULTIPAss EvAPonAToRs John L. Sin-ode, St. Louis, Mo., assignor to Alco Valve Company, Incorporated, St. Louis, Mo., a corporation of Missouri Application May 14,

' 4 Claims.

This invention relates to the control of multipass evaporators and it has for its general object-the provision of means, in Vaporizers of the direct expansion type, for close and flexible control of the .supply of refrigerant to individual tubes or groups of tubes, for maintaining an optimum value of thermal efficiency in all elements of the evaporator throughout the entire load range, and with regard to the variable load incident to the location of different parts of the evaporator.

Most specifically, the invention relates to a control system for multi-pass evaporators, em-

. ploying independent expansion valves for each tube or for groups of tubes, and controlled by thermostatic means-, likewise individual to the tubes or groups of tubes and regardless of whether the tube loops 'of a controlled unit lie in one or a plurality of planes, said thermostatic means being located at the suction end of the evaporator and quickly responding to a temperature of superheat at suction pressure just sufliciently high to prevent refrigerant in liquid form from being drawn over to the compressor.

Other objects of the invention will appear as the following description of a preferred and practical embodiment thereof proceeds.

It is of course a matter of elemental knowledge that the capacity of a refrigerant to absorb heat is of consequential magnitude only while the liquld is changing state, and that after the liquid has become a gas it has very little absorptive capacity. The saturation point of the gas is, therefore, the limit of its thermal efficiency. To obtain the maximum efficiency from a given length of evaporator coil, it is thereforev necessary 'to admit the liquid refrigerant to the coil at such a rate relative to the refrigerating demands that it will be changing its state clear to the posterior end of the coil, that is to say, the end remote from the expansion valve.

A number of practical factors generally supervene to prevent realization of this ideal condition, for instance, the control means may be sluggish in responding to load variations. If the load increases, the saturation point may shift from the posterior or suction end of the coil, toward the anterior end of the coil. The length of coil between this saturation point and the suction end of the evaporator may be regarded as substantially functionless in the performance of useful refrigeration since it is occupied by gas above the saturation temperature and with very little heat absorbing capacity. If the load should suddenly diminish, the saturation point would move rearward, altogether out of the coil and toward the compressor. Since it is very important that the liquid refrigerant be prevented from passing over to the compressor, the control means is ordinarily so set as to as- 1936, Serial No. 79,765

sure that the saturation point will remain in the coil under the lightest load, and under such setting of the control mechanism, it follows that under heavy load the saturation point advances quite a material distance toward the anterior portion of the coil. The refrigerant gas to the rear of the saturation point in the coil, due to its small mass and density quickly" rises in temperature, toward the temperature 'of the cooled medium outside the coil and this rise, over the temperature at the point of saturation is known as superheat.

As stated, a certain amount of superheat is essential to prevent liquid being drawn over into the compressor, which, stated in other words, means that a certain length of the coil must be devoted to the safeguarding of the compressor and thus withdrawn from functioning in a refrigerating capacity. A further amount of superheat must be developed to create enough pressure in the thermostat to operate the diaphragm of the expansion valve against the tension of its closing spring, which means that a still further length of the coil shall be devoted to a nonrefrigerating purpose.

Asid from these inherent negative factors, the efficiency of a single coil may be maintained at an optimum value by placing the thermostatic control at the. posterior end of the coil where it will respond to the superheat temperature at suction pressure, and employing the same refrigerant in the thermostatic capsule as is used in the refrigerating system. Such expedients of control are known.

In a multi-pass evaporator however, a different situation is confronted. It is seldom that load conditions are idential for all the passes and consequently, a single expansion valve for the entire evaporator with a single thermostatic control is incapable of determining maximum efficiency in all of the passes. The present invention seeks to provide a control in a multi-pass evaporator which will establish uniform and optimum efficiency in all of the passes regardless of the load range and regardless of variations in load for the several passes.

Referring now to the drawings in which similar characters of reference have been employed to designate identical parts of the several views:

Figure l is a front elevation of a multi-pass r show a section on a lower plane of a modified form of the invention;

Figure 4 is a. section taken along the line 4-4 of Figure 3;

Figure 5 is afragmentary view showing an alternative means for partitioning off a conduit, in the exercise of the invention;

Figure 6 is a rear view on a reduced scale of the evaporator shown in Figure 1, parts being shown in section;

Figure '7 is a front elevation of an evaporator in which all of the tube loops under a single control are in one plane; and

Figure 8 is a side elevation of the evaporator shown in Figure 7.

Referring now in detail to the several figures, and first to that form of the invention shown in Figures 1, 2, 5 and 6, the numeral I represents in general an evaporator employed in air conditioning and consisting of an upper header 2 and a lower header 3 and the tube loops 4. The arrow shown in Figure 1 shows that the air is blowing against the evaporator perpendicularly to the planes of thepasses. This is only by way of example for the air or other medium to be cooled might flow in any relation to the evaporator coils without transcending the spirit and scope of the invention.

For convenience in illustrating the invention, the passes have been divided into groups of four, each group being hereinafter referred to as a tube loop unit, said unit consisting of the number of tubes served by a single expansion valve including those headers or those segregate parts of a header which the opposite ends of the tubes are attached and which functions solely in serving that group of tubes to which they are attached. It is obvious from Figure 1 that the group a bears the brunt of the cooling since the air strikes it in an entirely unrefrigerated condition and that the groups b and c carry lesser loads in the order named for the air has been partially cooled after passing the first group and cooled still further after passing the second. It is obvious that under such conditions a single expansion valve and a single thermostatic control could not regulate the admission of refrigerant to all of said passes in such a manner as to assure that the saturation point of the refrigerant would be uniformly close to the posterior ends of each of the passes. If the thermostat and the expansion valve determined a correct control for the passes of the group a, this would not be the correct control for the passes of the groups I) and c which are subject to smaller loads and refrigerant in liquid form, would be drawn from the groups b and 0 back to the compressor.

The present invention contemplates dividing the upper and lower headers 2 and 3 into a plurality of segregate chambers 5, 6 and I by partitions 8 and 9 and in providing independent expansion valves l0, II and I2 [or each of the upper chambers, and independent thermostats I3, -14 and I5 for each of the lower chambers. Thus the tubes of groups a will be under the control of one expansion valve thermostat couple while groups I) and 0 will respectively be under the control of other independent expansion-valve-thermostat couples, each couple being responsiveto the superheat of the effluent refrigerant-in the particular chamber with which the thermostatic element is associated. The several expansion valves are supplied from a conduit l6 leading from the high side of the compressor, while the thermostatic elements are in the branches l1, I8 and IQ of a common eiiiuent conduit 20 leading to the low side of the compressor. It is, of course,

conceded that the individual tubes of each of the groups are subjected to different load conditions so that the control of a group of tubes or passes by a single expansion valve and thermostatic element is merely an approximation of optimum efiiciency, and if it be desired to carry eflieiency value still higher, the groups could be still further sub-divided even to the extent that an independent valve and thermostatic bulb would be provided for each tube. Of course, the expense of equipment would be a prime consideration in determining how finely the independent control of the several passes shall be sub-divided.

Inasmuch as the invention contemplates the independent control of each pass or group of passes, it is imperative that the remote thermostatic elements for the several valves be so arranged that the valve response will be a result of the change in condition in the section of the evaporator which the valve is controlling. The usual method of control with the thermostatic bulbs strapped to the outside of the suction connection from the various sections, introduces many variables which prevent the accurate and positive response of the valve to the controlled condition. The mass effect of the refrigerant conduit, the conductivity of the metal in the refrigerating conduit, and the radiation from the outside atmosphere surrounding the bulb, all tend to interfere with the correct transfer of heat at low temperature gradients from and to the thermostatic bulb. A further objection to this most common method of bulb application (strapped to the suction line) is the conductivity of the suc tion line which causes the suction gas from one section to change the temperature gradient on the thermostatic bulbs of one or more of the other valves, thus creating an unwarranted increase or decrease in refrigerant flow through the sections whose valves were spuriously affected.

With the present invention, the suction connections from the various coil sections are connected 'to common manifold sections so constructed that liquid refrigerant will not trap, and so that the isolated thermostatic bulb l3 may be inserted in the separate suction conduits H! from each section. It will be understood that in the preferred for of the invention, the bulbs l3 are completely insulated from the walls of the surrounding conduit not only by their spaced relation from the lateral walls of said conduit, but by the interposition of an insulating space and support 2| at their base.

It is obvious that the use of the insert type of thermostatic bulb within the refrigerant conduit provides for a control which responds to the temperature of the refrigerant vapor and minimizes the mass and conductivity of the suction line and the necessary fittings as a disturbing factor. It will be recognized that it is only with this type of thermostatic bulb that a speed of response necessary to'preserve minimum superheat with freedom from hunting or cycling is -malntained, and without the dang-er of back flooding.

tubes would make it diflicult to find room for the installation of the plurality of expansion valves and thermostatic elements.

, Figures 5 and 6 suggest the use of partitions iii and 3| blocking the continuity of the liquid and suction conduits IB and 2D, adapting the system to the use of multiple compressors, or the partitions may be employed for obtaining capacity reduction in conjunction with stop valves 35 and 36 placed on one or both sides of the liquid conduit 6, for example, even when the two sides of the conduit are connected to the same compressor.

In Figure 5 a simple expedient for installing the partition 3| is suggested. The partition 3| comprises adisk having a concave periphery 32 arranged in the conduit l6 perpendicular to the axis thereof and having the conduit peened or upset inwardly forming beads which seat in the channel in the partition [6 and maintain it permanently in position.

Referring now to Figures 7 and 8, a form of invention is shown in which the coils of each controlled unit lie in a single plane. As shown, the unit consists of three groups of coils 36', 3'! and 38 arranged one beneath the other. Where coil sections are thus vertically arranged, a problem arises regarding the uniform distribution of liquid refrigerant to each of the groups. This is accomplished in the present instance by the provision of a common liquid manifold 39 having independent tubular branches 40, 4| and 42 leading to the influx ends of the several coils, and a vertical suction manifold 43 to which the several effiuent ends of the coil sections are connected at different levels. Since the manifold39 as illustrated in Figures 7 and 8 is common to three controlled units, it is preferred to partition off said manifold between said units as by the partitions 44 and 45. The manifold 39 is supplied with liquid refrigerant from a conduit 46 to which the individual expansion valves 41, 48 and 49 are connected. Said expansion valves also communicate with the individual sections of the manifold 39 defined between the partitions 44 and 45.

Thermostatic bulbs 50 individual to the suction manifolds 43 are immersed in said manifolds preferably at the top, each being connected by a tube 5|. Thus individual control of the evaporator coils which lie in one plane is accomplished, the dividing of the coils into groups being primarily in the interest of uniform distribution.

In Figures 3 and 4, another form of the invention is illustrated in which the evaporator tubes 22 emanate from a common manifold 23 which is divided into isolated sections by an insert consisting of a rod 24 having disks 25 and 28 secured in perpendicular relation thereto and having an end piece 21 secured in like manner. The disks 25 and 26 are properly spaced so that when the insert is in place they isolate the sections of the manifold commanded by the thermostatic devices 28, 29 and 30. This construction is particularly adapted to the conversion of existing evaporators into apparatus embodying the principles of the present invention and in adapting the several sections of the manifold to be connected to separate compressors if desired.

While I have in the above disclosure described what I believe to be preferred and practical embodiments of my invention, it will be understood to those skilled in the art that the specific embodiments as shown, and the details of construction as illustrated and described are merely exemplary and that the inventive principle may be applied to any grouping of tubes in an evaporator, no' matter how they may be arranged with respect to the liquid and suction headers.

What I claim is:

1. Refrigeration system including in combination with a multi-pass evaporator having tube loop units connected in multiple between the conduits which constitute the liquid and suction limbs of said system, the suction limb conduits constituting branches of a suction manifold, means for maintaining a maximum area of wetted surface in each tube loop unitregardless of variable loads on the several tube loop units, comprising an expansion valve and a control thermostat individual to each tube loop unit, the thermostats being located in said branches whereby they respond to the temperature of the eflluent gas adjacent the suction ends of the respective tube loop units.

2. Refrigeration system including in combination with a multi-pass evaporator having tube loop units connected in multiple between the conduits which constitute the liquid and suction limbs of said system, the suction limb conduits constituting branches of a suction manifold. means for maintaining a maximum area of wetted surface in the several tube loop units of the evaporator regardless of variable loads on the several tube loop units, comprising expansion valves individual to the several tube loop units, and thermostats for controlling said expansion valves individual to the several tube loop units, said thermostats being immersed in the efiiucnt gas adjacent the suction ends of the respective tube loop units, whereby they are substantially isolated from the thermal conductivity of proximate metal masses.

3. Refrigeration system including in combination a multi-pass evaporator comprising a plurality of tube loop units, headers to which said tube loop units are connected in multiple, said headers being divided into sections individual to a tube loop unit and forming parts of such unit, conduits constituting the liquid and suction limbs of said system, branch conduits individual to said sections connecting the respective headers to the liquid and suction conduits, expansion valves in dividual to the header sections located in the branches to said liquid conduit and thermostats for controlling said expansion valves, individual to said header sections and locateddn the branches communicating with the suction conduit of said system, said thermostats being immersed in the ciiiuent gas within said branches.

4. Refrigeration system including in combination a multi-pass evaporator comprising a plurality of tube loop units, inlet and outlet headers to which the opposite ends of the loops of said units are respectively connected, conduits constituting the liquid and suction limbs of said system, expansion valves individual to the tube loop units communicating with the liquid conduit and with the header on the high side ofsaid units, branch conduits individual to said tube loop units communicating with said suction conduit and with the header on the suction side of said evaporator and thermostatic bulb elements individual to said tube loop units within the branches of said suction conduit and operatively related to said expansion valves.

JOHN L. SHRODE.

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