US2389452A - Drying - Google Patents

Description

NOV. 20, 1945 v, c PATTERSON 2,389,452

DRYING Filed July 10, 1943 3 Sheets-Sheet 1 VA CUUM PUMP LOW 5771 GE COMPEEJJO/Q.

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(Ittornegs Nov. 20, 1945. v. c. PATTERSON DRYING Filed July 10, 1943 3 Sheets-Sheet 2 T Q I I III '///////////////////l 3nventor (Ittomegs Nov. 20, 1945.

v. c. PATTERSON DRYING Filed July 10, 1943 3 Sheets-Sheet 3 Gttomcgs Patented Nov. 20, 1945 UNITED STATES PATENT OFFICE DRYING Volt 0. Patterson, York, Pa., assignor to York Corporation, a corporation of Delaware Application July 10, 1943, Serial No. 494,229

Claims.

This invention relates to refrigerating equip ment for use in drying systems in which the material being dried is maintained at sub-freezing temperatures. Such systems are used in drying sera, blood plasma. and certain animal and vegetable products, including solids, fruit and other juices, etc. They have the advantage of accomplishing the desired complete drying without harmful effect, or with less harmful effect than is caused by other drying processes.

If drying is to be carried out at such low temperatures. the'material being dried must be maintained under an absolute pressure lower than the vapor pressure at which evaporation of the removed moisture occurs. As a practical matter the pressure is of the order of 0.2 millimeter of mercury, which for water corresponds to a temperature of approximately minus 28 C.

While such low pressures (approximating a perfect vacuum) could be maintained by vacuum pumps or stage ejector systems, the operation. of these devices involves substantial technical difficulties, and recourse is had to refrigerated surfaces maintained at temperatures substantially below minus 28 C. to collapse and freeze the moisture. A vacuum pump is used, but only to remove noncondensible gases, initially present in the material being dried, or entering the dryer by leakage. This procedure, properly carried out. is more economical and gives better control.

The extremely low pressures at which dryin occurs necessarily connote the existence of freezing temperatures. In order to maintain the desired vacuum within the dryer without employing a vacuum pump of enormous size, it is essential thatthe vapor entering the cooling chambers pass immediately from the vapor state to the solid state as it deposits upon the refrigerated surface. To accomplish this result it is necessary that the freezing surface be maintained at a temperature slightly lower than that which for water corresponds to the absolute pressure within the dryer. Thus the temperature of the cold surface within the dryer must be so'low that on the basis of the thermodynamic properties of water, the temperature is lower than that corresponding to the absolute pressure in the dryer when the latter is evacuated. If such a temperature is maintained, the moisture sublimed off in the oven from the substance being dried passes immediately to the solid state on the cold surface, and it is this action that makes it possible to use vacuum pumps of reasonablesize.

The problem of developing a satisfactory refrigerating unit for such use is substantial. The

sure difference.

Obstructed flow imposes a heavy penalty on performance, so that short free paths are needed. Such paths however are likely to permit the passage of radiant heat directly from the heater or from the substance being dried to the refrigerated surface, and this, to the extent that it occurs, represents a direct loss. Drying requires an input of heat to supp y the latent heat needed for sublimation, and efficient application of this heat requires careful attention to details of design.

Moreover the drying process cannot be continuous but must be carried out on a batch basis, because the low pressure entails need for an absolutely tight closure, and because the ice which forms on the refrigerated surface must be removed and can only be removed periodically. The formation of ice requires the use of excess cooling surface, or operation at lower and lower suction pressures, to compensate for the increasing insulating effect of the ice. The former expedient is simpler from the operating standpoint.

The present invention, therefore, is concerned with .means to protect the refrigerated surface from short-circuiting flow of radiant heat from the drying substances, while permitting as nearly unobstructed vapor flow as practicable; efficient means to apply usefully the refrigerative effect accumulated in the ice coating which forms on the refrigerated surface; means to protect the device against inleakage of heat; and a refrigerative circuit especially adapted to use with a battery of dryers including means to de-. ice the refrigerative surfaces of successive dryers usually one at a time while the other dryers of the battery are maintained in operation.

To develop the utility and operative characteristics of the invention it is necessary to describe the relation of the drying ovens to the refrigerated chambers, but the novel features reside in the refrigerating units, the refrigerating circuit and the relations of the refrigerating units to the circuit, and to corresponding ovens.

No novelty is here claimed as to the ovens or the general method of drying. Theinvention is susceptible of use with various types of oven. y the term oven is meant any suitable means to enclose and support the substance being dried while supplying to the substance (and only to the substance, so far as is practicable) heat at a suitably controlled rate and at a temperature low enough to avoid thawing or other harmful effect.

Since water is the liquid normally removed in drying operations the description, including stated temperatures and pressures will be based on drying to remove water. While the same broad principles apply to volatile liquids generally, the exceptionally large specific volumes characteristic of water vapor at low pressures introduce special considerations of great practical importance.

The invention will now be described as embodied in a plant having six drying ovens, reference being made to the accompanying drawings, in which Fig. 1 is a diagrammatic elevation 01 the complete plant. I

Fig. 2 is a vertical axial section through one of the vapor condensers, the refrigerated surface therein being an evaporator as to the refrigerating circuit.

Fig. 3 is a detail of the tube connections shown in Fig. 2, drawn on an enlarged scale.

Fig. 4 is a section on the line 4-4 of Fig. 2.

Fig. 5 is a perspective view of an oven with the access door open. to indicate the position of the trays which hold the material being dried and the heated shelves on which the trays rest. No attempt is made to illustrate details of construction.

The diagrammatic view of Figure 1 shows an installation comprising a battery of six drying units, indicated generally by the reference letters A to F inclusive. Each unit comprises a shell i which encloses the oven and a pendant shell l2 which encloses the refrigerating unit for collapsing and freezing the moisture evolved in the drying operation. These two shells are in free communication with each other.

The present invention is not concerned with the construction of the oven proper, and since such ovens are known, it will suflice to say that the oven is proportioned to withstand the pressure of the atmosphere, is provided with a tight sealing access door and contains shelves, trays or other means for supporting the substances to be dried. These supporting means may take any form known in the art. There must also be within the oven some means to supply to the substances being dried, and on a low temperature plane, the heat necessary to evaporate the moisture to be removed in the dryer. The heat so supplied is latent heat. It passes on with the vapor and in effect is the aggregate of the latent heat of fusion and the latent heat of vaporization, both of which must be supplied if sublimation is to occur.

Reference should now be made to Figures 2 to 5.

In Figure 5 the oven 6 is illustrated. The oven 6 has hollow shelves 1' with supply connection 6 and discharge connection 9 for supplying heating medium. The shelves are spaced from the walls of oven 6 to permit circulation. The substance to be dried is held in trays Ill. An access door appears at II and is of a type which can be sealed when closed.

The oven 6 is shown as opening freely at its bottom into the interior of the shell [2. The lower end of the shell I2 is closed by a tube sheet ii. A cover It with refrigerant discharge connection I6 encloses a chamber It with which the tubes communicate. These tubes are indicated at H. They make tight Joints at IS with the tube sheet and are tied together at their upper ends by the cross braces l9.

The main header for supplying liquid refrigerant comprises a pipe 2| which extends diametrically across the upper end of the shell l2 and makes a tight welded joint with the shell at 22. Its inner end is closed at 23.

Projecting in opposite directions from the header 2| are a series of parallel branch headers 24. These are graduated in length as indicated in Figure 4 and are connected with the upper ends of the tubes H by the connectors 25 as best shown in Figure 3. The connectors 25 extend through and are welded into plugs 26 which close the upper ends of tubes l'l. Their lower ends are closed as indicated in Fig. 3, and each has a plurality of fine apertures 21 which spray liquid refrigerant against the inner surface of the corresponding tube Il. Thus the'liquid tends to fiow down the tube wall leaving a central passage for the flow of vapor evolved.

All of the tubes ll are finned from end to end as indicated at 28 in Figure 2. For clarity the fins are not illustrated on most of the tubes. Spiral fins are shown, but the specific form of the fins is notmaterial except that they should be nearly horizontal or otherwise so arranged that they will retain ice or frost until completely melted but will not retain water in quantity. The purpose is to retain the frost or ice in contact with the tube I 1 and fins 28 but only until its refrigerative effect is recovered.

The vacuum pump connection above mentioned is indicated at 29, and communicates with the interior of shell l2.

The drain pipe ll communicates with the interior of the shell 12 at the level of the tube sheet 13. This is the connection for removin water flowing from the finned tubes during the de-icing operation. The upper end of pipe 3| is welded to the tube sheet I3 at 32, and there is a packed joint 33 with the cover I. A valve 34 is provided and is closed except while draining off water after depositing.

The shell is internally insulated as indicated at 35. This insulation is or a type which reflects or arrests radiant heat. Insulation sold under the trade name Ferrotherm, or reflecting foil insulation, or even aluminum paint may beused. External insulation is not justified for two reasons: first, because the high vacuum condition in the shell during the drying operation provides an ample and efllcient insulation of the cooling coil from all external heat except that which is transmitted by radiation, and second, because external insulation would in effect add heat storing mass to the metal of the shell which becomes warm during defrosting and from which heat must be removed when the apparatus is put into operation.

Supporting brackets 31 are provided as indicated.

By using finned tubes and presenting th ends of the tubes toward the opening between the oven 6 and the vapor condensing shell I2, a considerable degree of protection is obtained for the tubes from heat radiating from the substances in the oven 6. It can be seen in Figure 4 that the top fins on the tubes constitute a large area or umbrella for reflecting radiant heat back to the oven thereby greatly reducing the amount of radiant heat that might otherwise be absorbed by the cooling tubes. This use of fins with direct vapor flow from the drying oven in prefer ence to a solid sheet-type reflector with indirect vapor flow keeps the obstruction oife'red to the flow of vapor to the practicable minimum.

Reference should now be made to Figure 1 in which the connections between the units above described and the refrigerating circuit are diagrammatically illustrated. The vacuum connection 29 of each unit leads through a normally open stop valve 38 to the header 3! which connects with the intake of the vacuum pump 40. This pump is diagrammatically illustrated. It would necessarily be of a type suited to maintain the very low absolute pressure above described. Since it need' withdraw only noncondensable gases, its volumetric capacity need not be very great. For the first phase of the drying cycle it should have capacity suflicient to withdraw air and other noncondensable gases occluded or adsorbed in the material to be dried. After this first de-gassing operation is completed, the pump need be sufficient merely to take care of leakage. Consequently it is a desirable expedient to operate the pump at two rates, the lower of the two rates being used when only leakage is to be withdrawn. V

The volatile refrigerant which is to be evaporated in the tubes I! of the various drying units is drawn from a low pressure receiver 4| by a circulating pump 42 and delivered to the low pressure liquid line 43. From the low pressure liquid line It the liquid is delivered through valves ll to the headers 1| of such of the units A to Fas are in operation. Any unevaporated liquid refrigerant and vapor evolved by evaporation in the tubes ll. pass through the chamber l6 and flow by way of valve 45 and return line 48 to the low pressure receiver. The above presupposes that valves II and it are open so that the unit affected is cut into the circuit and is effective for refrigerating purposes. Any unit may be cut out and de-iced as will be later described.

The vapor and liquid separate in receiver H, and the vapor is drawn oil. by the low pressure suction line 50 to the low stage compressor-41. Compressor ll discharges through the line 48 into the intermediate cooler 49 and after being cooled therein the cooled refrigerant vapor flows through the suction line ii to the high stage compressor 52. This discharges through the high pressure line I! to the condenser 54.

The condenser 54 is illustrated onlyin diagram and may be of any suitable type. In it the refrigerant is liquified, and from it the refrigerant discharges through the liquid line 55 into the-high pressure receiver It.

From the bottom of the high pressure receiver a liquid line 51 leads and serves to deliver refrigerant for de-icing purposes through various branches each controlled by a stop valve 58.

Each of these branches leads to the refrigerant manifold II of a corresponding one of the units A to I", as the case may be. Gages 59 are-connected to the manifolds II to show the pressure existing in the manifolds, and in the connected tubes II.

In ordinary practice one of the drying units is cut out for unloading, recharging and defrostin while the others remain in action. During defrosting the line II serves to deliver relatively warm liquid refrigerant from the high pressure receiver through the valve II thence through the tubes ll of the unit being de-iced, cooling the liquid refrigerant and melting the ice off the tubes. The liquid refrigerant so cooled is coileeted by a cooled liquid line 6| which communicates through respective stop valves 82 with the discharge connection [5 of corresponding units A to F respectively. The line 8| communicates through a valve 63 with the liquid supply line 84. This line 84 leads to two float valves. The first and more important of these is the float valve 65 which responds to the level of liquid in the.low pressure receiver ll and controls the supply of liquid refrigerant to that receiver through the connection 66.

The second float valve fed by line 84 is the float valve 81 which responds to the level of liquid in the evaporative cooler 49 and controls the supply of liquid from line 84 through branch 68 to the connection 48. Thus the float valve 61 functions to maintain a fixed liquid level in th shell of the intercooler 49. The liquid which it delivers mixes with hot gas discharged by the low stage compressor 41 and the mixture is delivered below the liquid level in the shell 49. Thus some liquid refrigerant is evaporated at interstage pressure cooling the remainder to the corresponding temperature. Other types of intercooler could be used, but this evaporative intercooler is preferred for this service.

The line 64 is fed from the line 6| whenever deicing operations are in progress. At other times the line 64 is fed directly from the high pressure receiver 55 by way of the stop valve H.

Thermometers l2 and 13 are inserted adjacent the valves II and 63, the former to show the temperature 'of liquid refrigerant. leaving the high pressure receiver. 56,, and, the latter to show the temperature of refrigerant discharging from the cooled liquid line Bl, i. e. liquid refrigerant after it has been cooled byflowing through tubes 11 of an idle dryer to-de-ice the same and beforethe liquid has been allowed to expand into the low pressure-receiver. These thermometers are used in checkingthe performance of the de-icing operation.

Operation The general scheme of operation of a plant such as that diagrammed in Figure 1 is to operate the plant continuously so that at all times at least five of the six drying units ar'eunder vacuum I and operating while the sixth may be shut down and opened to discharge dried material, defrost the refrigerative circuit and introduce new material to be dried. It is possible by adopting a suitable number of units to shut them down seriatim and have just about a convenient time for the discharging, defrosting and recharging operations. It is desirable so to arrange matters because the refrigerative circuit operates somewhat more emciently during defrosting of a unit.

Assume that all six units are in operation. Under these conditions all the valves 38 are open as are all the valves 44 and 45. All the valves 50 are closed as are all the valves 62. The valve H is open and the valve 63 is closed. Assume now that the time to shut down unit F and recharge it has arrived. The first step is to close its valve 38 disconnecting unit F from the vacuum pump whereupon its valves 45 and 44 are closed 7 must be opened very slowly to allow pressure in the coils to build up gradually to 45 lbs. gage as indicated on the gage 59. This is the pressure corresponding to the melting point of ice. Slow opening is necessary to prevent temperature shock.

When the desired pressure has been attained, the valve 82 in unit F is opened. whereupon the valve 83 is opened and the valve II is closed. This interposes the tubes I! in unit F in the path of flow of liquid refrigerant from the high pressure receiver It to the low pressure receiver 4| and to the intercoolerfl. The drain valve 34 is then opened. This condition is allowed to continue until the thermometers l2 and" indicate that the ice on the tubes ll of unit 1'' has all melted and that no further refrigerative effect can be recovered. When this has occurred, the valve 34 and the valve 63 are closed and the valve II is' opened, whereupon the valve 02 for unit 1' is closed as isthe valve 58 for that unit. The oven is then charged with new material to be dried and the oven is sealed.

The next step is to open the valve 45 slowly until the pressure in the tube I! as indicated by gage 59 has been reduced to the pressure in the low pressure receiver, whereupon the valve 44 for unit F is opened, returning the unit to its normal circulatory connections. The valve 38 is then opened, restoring the conditions originally assumed. At this time another unit such as unit A ought to be approaching the stage for discharging and de-icing, and this would be accomplished by valve manipulation as already described.

What is claimed is:

1. In a refrigerant circuit containing a volatile refrigerant, the combination of a low pressure receiver; a plurality of low temperature evaporators; means for circulating liquid refrigerant from the low pressure receiver through said evaporators and back to the low pressure receiver; a high pressure receiver; refrigerant liquefying means adapted to withdraw vaporous refrigerant from the vapor space of the low pressure receiver and deliver it in the liquid phase to the high pressure receiver; means for disconnecting any selected evaporator from said liquid circulating means; and connections operable to deliver liquid refrigerant from the high pressure receiver to the low pressure receiver, either directly. or through the selected evaporator while disconnected from the circulating means.

2. In a refrigerating circuit containing a volatile refrigerant, the combination of a low pressure receiver; a plurality of low temperature evaporators; means for circulating liquid refrigerant from the low pressure receiver through said evaporators and back to the low pressure receiver; a circuit leading from the vapor space of the low pressure receiver and comprising in the order stated, stage compressing means including at least one interstage cooler, a condenser and a high pressure receiver; means for disconnecting any selected evaporator from said circulating means; and means operabl to deliver liquid refrigerant from the high pressure receiver to the low pressure receiver, either directly or through the selected evaporator while disconnected from the circulating means.

3. In a refrigerating circuit containing a volatile refrigerant, the combination of a low pressure receiver; a plurality of lowtemperature evaporators; means for circulating liquid refrigerant from the low pressure receiver through said evaporators and back to the low pressure receiver; a circuit leading from the vapor space of the low pressure receiver and comprising in the order stated, stage compressing means including direct expansion interstage cooling means, a condenser and a high pressure receiver; means for disconnecting any selected evaporator from said circulating means; and means operable to deliver liquid refrigerant from the high pressure receiver to the low pressure receiver and to the direct expansion cooling means, either directly or through the selected evaporator while disconnected from the .circulating means.

.4. The combination of a plurality of airtightenclosures each having means for supporting material to be dried and for supplying thereto the latent heat necessary for sublimation of frozen moisture; evacuating means operable to withdraw noncondensable gases from the various enclosures at will; refrigerative coolers, one mounted in each of said enclosures and comprising hollow means enclosing flow passages for volatile liquid refrigerant, said hollow means having external fins adapted to retain an ice coating until melted; means for supplying volatile liquid refrigerant to the flow passages of selected coolers and for causing it to evaporate therein; and means for passingsaid liquid on its way to such selected coolers, through the flow passages of an inactive but recently active cooler to de-ice the latter and subcool the liquid.

5. In a plant for drying from the frozen state, the combination of a plurality of dryers, each ineluding an airtight enclosure with means to support material and supply thereto the latent heat necessary for sublimation of frozen moisture; means operable substantially to evacuate each of said enclosures at will and thus maintain therein an absolute pressure below 4.579 millimeters of mercury; a refrigerative evaporator in each of said enclosures; a refrigerative circuit for liquefying volatile refrigerant and for evaporating it in said evaporators at temperatures which on the basis of the thermodynamic properties of water are lower than that corresponding to the absolute pressure in the enclosure when so evacuated, wh'ereby moisture evaporated in drying forms an ice coating on any active evaporator; and means for disconnecting an enclosure from the evacuating means at the end of its drying cycle, and for interposing its evaporator in the path of liquid refrigerant flowing to other evaporators, whereby the liquid is sub-cooled and the evaporator is deiced by heat exchange between the liquid and g the ice previously formed upon the evaporator.

VELT C. PATTERSON.

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