US2859809A - Compression evaporator control and method therefor - Google Patents

Description

Nov; 11, 1958 H. 6. EHRISMAN 2,859,309

COMPRESSION EVAPORATOR CONTROL AND METHOD THEREFOR Filed Feb. 2, 1955 INVENTOR Hen/7 0. E/zrvsmam ATTO GONDENSATE United States Patent I COMPRESSION EVAPURATOR CONTROL AND METHOD THEREFOR Application February 2, 1955, Serial No. 485,697

' 5 Claims. (Cl. 159--44) This invention relates to compression evaporation and more particularly to a novel method and apparatus for controlling the addition of auxiliary heat to such an evaporator. Since most evaporation processes are concerned with the evaporation of aqueous solutions the present invention will be described in connection with its use for evaporating such .a solution, although as the description proceeds it will be evident that the invention can also be used with advantage in connection with the evaporation of non-aqueous liquors as well.

In recent years there has been an increasing interest in compression evaporation as .a method of increasing the thermal efiiciency of various evaporation processes. In accordance with this method vapors distilled from the liquid being evaporated are removed from the evaporator body, compressed by a suitable compressor, and returned to the evaporator steam chest to be used as a heating medium for evaporating further quantities of the liquor to be evaporated. In such a compression evaporation process it is' customary to preheat the feed liquor with condensate from the evaporator steam chest, and if this heat exchange between feed liquor and condensate is carried out efficiently, an evaporative process is achieved which has exceptionally high thermal efficiency, i. e. a

process which will vaporize a relatively large quantity of water per unit of heat input.

It is evident however that even with the high efficiencies obtainable with such a compression evaporator some heat losses inevitably occur, and accordingly it is necessary that auxiliary or makeup heat be added in order to carry out the evaporation on a continuous basis. It is accordingly an object of the present invention to provide an improved method and apparatus for supplying auxiliary heat to a compression evaporator. It is another object of the invention to control the supply of heat to a compression evaporator in such manner that the evaporation proceeds in a more nearly uniform manner than has heretof-ore been achieved. It is still another object of the invention to provide a method and apparatus for controlling the supply of heat to the steam chest of a compression evaporator in such manner as to maintain the pressure in the vapor space of the evaporator substantially constant. Other objects of the invention will be in part obvious and in part pointed out hereafter.

The many objects and advantages of the present invention can best be understood and appreciated by reference to the accompanying drawing which illustrates diagrammatically apparatus incorporating a preferred embodiment of the present invention and capable of carrying out the method of the invention.

Referring to the drawing, the numeral generally designates an evaporator body having a heat exchange section 12 and vapor body 14. The evaporator body 10 is a conventional vertical tube, natural convection evaporator wherein the heat exchange section 12 contains a bundle of vertical tubes 16. Although only two tubes 16 are shown for purposes of illustration, it will be understood that the tube bundle comprises a large number of tubes largely filling the space within the heat exchange section 12. The space around tubes 16 is supplied with steam through a pipe 18 and will be referred to herein for convenience as the steam chest 20 of the evaporator.

The liquor to be evaporated is supplied to the base of the heat exchange section 12 through a pipe 22 and flows up into the tubes 16. Within the tubes it is heated by a steam in the steam chest 20 and partially vaporized. As a result of this vaporization process a mixture of vapor and hot liquor is delivered from the upper ends of the tubes 16 and impinges against a baflie 24 in the vapor body 14. The liquor and vapor are separated in the vapor body 14 and the liquor flows downwardlythrough a pipe 26 and is recycled through feed pipe 22 to the bottom of heat exchange section 12. Concentrated liquor is continuously withdrawn from vapor body 14 through a pipe 6% and pumped by .a pump 62 through a product pipe 64 to a suitable point of storage or use.

As indicated above, the present evaporator is a compression evaporator wherein the vapor is compressed and used as a heating medium in the steam chest of the evapo rator. Still referring to the drawing, the vapor leaves the vapor body 14 through a pipe 28 and flows to the inlet of a compressor 30 that is driven by a steam turbine 32. The compressed vapor is delivered by the compressor 30 to the pipe 18 through which it flows to the steam chest 20 of the evaporator body.

In order to improve the thermal efliciency of the unit the feed liquor is preheated by being caused to flow in heat exchange relationship with the condensate formed in the steam chest 20. More particularly, condensate is withdrawn from steam chest 20 near the bottom thereof through a pipe 34 and flows successively to the shell sides of the heat exchangers 36 and 38 and thence to a suitable point of disposal through the pipe 40. Feed liquor to be evaporated enters the :system through a pipe 42 and flows through the tube sides of heat exchangers 38 and 36 in sequence to the feed pipe 22 referred to above. It is thus apparent that the feed liquor and condensate are caused to flow in generally countercurrent heat exchange relation to cause the liquor to be heated by the sensible heat in the condensate leaving the system.

The rate of introduction of feed liquor into the evaporator body is determined by the liquor level in the heat exchange portion 12 of the evaporator body 10 and control mechanism is provided for maintaining this liquor level at the desired point. Thus a gauge pipe 44 is provided which is connected at its upper end to vapor body 14 and at its lower end to the lower portion of heat exchange section 12 below the bottom of the tube bundle 16. Mounted in the lower portion of gauge pipe 44 there is a differential pressure responsive device 46 which may be of conventional construction, and for example of the specific construction shown in Cook Patent 2,539,892. Thus if a device such as that shown in the Cook patent is used the passage 111 of the Cook patent is connected to the vertical portion of the pipe 44 and the passage 98 of the Cook patent is connected to the liquid space at the bottom of evaporator 10.

The diiferential pressure responsive device 46 is eflectively responsive to the height of the liquor level in the tubes of tube bundle 16 and operates to generate a pneumatic pressure proportional to the liquor level. This pneumatic pressure is transmitted by a pipe 48 to a controller 50 which is in turn connected by a pipe 52 to a pneumatically operated regulating valve 54 located in the feed liquor pipe 22. (If a device like the Cook device is used, pipe 30 of the Cook patent corresponds with pipe 48 of the present application.) The controller 50 operates to generate a pneumatic pressure which is a function of the liquor level in the tubes 16 and supplies this pneumatic pressure to the valve 54 to so regulate the feed liquor rate as to maintain the desired liquor level. in. the evaporator. Pneumatically operated controllers capable of'operating. inthisway are conventional and well-knownin the art andhence the construction of controller 50 need not be described in detail herein.

In like manner the rate of withdrawal of condensate from steam chest 20 is regulated so as to maintain the steam chest substantially free from accumulated condensate. More particularly the pipe 34 has a U-section 35 to the right-hand leg. of which a differential pressure responsive device 56 is connected by pipes 57 and 59. The device 56 may be similar to the device 46 and generates. a pneumatic pressure that is supplied through pipe 61 to a regulating valve 58, also located in the pipe 3 3. (If a device such as that shown in Cook Patent 2,539,892 is used the passage 111 of the Cook patent is connected to pipe 57 and the passage. 98 of the Cook patent is connected-to pipe h.) The differential pressure responsive device 56 operates to supply to valve 58 a pneumatic pressure which positions the regulating valve 58 to maintain the flow of. condensate through the pipe 34 at such a value that the condensate level in the right-hand leg of U-section 35. remains between the points at which pipes 57 and 59 are connected to pipe 34. Thus condensate formed in steam chest 20 immediately flows into pipe 34 and no substantial amount of condensate accumulates in the steam chest.

As pointed out above, in a compression evaporation system of the type shown certain heat losses occur and hence auxiliary heat must be supplied to permit the evaporator to operate on a continuous basis. This auxiliary heat can manifestly be obtained from any of various sources. However in the preferred embodiment of the invention here being described the compressor 30 is driven by a steam turbine 32 and the exhaust steam from the steam turbine is used as a source of this auxiliary heat. More particularly the steam turbine 32 is driven by high pressure steam supplied through a pipe 66 which contains a regulating valve 68. Exhaust steam from turbine 32 flows through a pipe 70 to a low pressure steam system (not shown) which is maintained at a pressure of say 50 lbs. per square inch. A portion of the exhaust steam flows through a branch pipe 72 containing a regulating valve 74 to the steam supply pipe 18 that supplies steam to the steam chest 20 of the evaporator. The flow of auxiliary steam from the exhaust of steam turbine 32 to steam chest 20 is controlled by a controller 76 in a manner that is further described hereafter.

As an incident of the normal operation of evaporators, the steam supply to the evaporator commonly contains small quantities of fixed gases and these gases accumulate over a period of time in the evaporator steam chest. To

effect removal of thesefixed gases the present evaporator system is provided with a vent system comprising a pipe 78 connected to steam chest 20 just above the condensate level. The pipe 78 contains a pneumatically operated regulating valve 80 and is provided with a by-pass 82 around the valve 80 which contains an orifice 84. The orifice 84 is so selected as to withdraw the vent gas and admixed steam from steam chest 20 at such a rate as to maintain the vent gas concentration within the steam chest at a relatively low value with the valve $0 closed. The valve 80 forms part of the control system for the evaporator which will now be described.

In the operation of an evaporator it is important in securing efiicient evaporation that the evaporation rate be maintained relatively uniform and that vaporization occur smoothly and continuously. In accordance with the present invention such uniform continuous operation is obtained by maintaining the pressure in. the vapbispace of vapor body 14 substantially constant, and this constant pressure in the vapor space within body 14 is achieved by conjoint regulation of the steam supply through pipe 72 and of steam withdrawn from steam chest 20 through pipe 78. More particularly the gauge pipe 44 is provided near its upper end with a pressure responsive device 38 which may be like the devices 46 and 55 previously referred to but which is exposed to atmosphere on one side thereof and hence measures the pressure in the vapor space 14. (If a device such as that shown in Cook Patent 2,539,892 is used the passage 111 of the Cook patent is connected to the upper end of gauge pipe 44 and the passage 98 of the Cook patent is left open to the atmosphere.) The device 88 generates a pneumatic pressure proportional to the pressure in vapor space 14 which pneumatic pressure is supplied as a control pressure to the controller 76. The controller 76 in response to the pressure values received from device 88 generates a pneumatic pressure that is applied through the pipe 9i) to the valve 74 and through the pipe 92 to the valve 8t). The regulating valves 74 and are so adjusted in relation to the pressure supplied by the controller 76 that the valve 74 is open only when the controlled pressure, i. e. the pressure within the vapor body 14, is below the desired value and the valve 80 is open only when the controlled pressure is above the desired value.

' The cooperative action of the valves 74 and 80 and controller 76 can best be clarified by a specific example.

Assume for example that:

(l) The desired pressure in the vapor space of the evaporator is 5 lbs. per square inch.

(2) When the pressure in vapor body 14 is stabilized at 5 lbs. per square inch the controller 76 generates a pneumatic pressure of 9 lbs. per square inch.

(3) As the pressure in vapor body 14 varies the output pressure controller 76 can vary between 3 lbs. per square inch and 15 lbs. per square inch.

(4) As the vapor body pressure increases the output pressure of controller 76 decreases and vice versa.

(5) A change of 1 lb. per square inch in vapor body pressure corresponds to a change of 2 lbs. per square inch in controller output pressure.

(6) At the time being considered the vapor pressure in the vapor body is 2 lbs. per square inch.

It is evident that under the conditions set forth above the controller 76 will be supplying its maximum air pressure of 15 lbs. per square inch to both valves 74 and 80. The valve 74 is an air-to-open valve and is so adjusted that it is wide open with 15 lbs. air pressure on its diaphragm. The valve 80 is an air-toclose valve and is adjusted so that it is fully closed with 15 lbs. air pressure on its diaphragm and does not begin to open until the pressure on its diaphragm drops to 9 lbs. per square inch.

With the valve 74 Wide open a full flow of auxiliary steam will occur through pipe 72 and thence through pipe 18 to steam chest 20 thus increasing the rate of evaporation and the pressure within the vapor space of the vapor body 14. This increase in vapor pressure will be reflected in a decrease in the output pneumatic pressure of controller 76 and the valve 74 will be gradually closed. When the vapor pressure in the evaporator reaches 5 lbs. per square inch both valves 74 and 80 will be fully closed. If thereafter the vapor pressure of the evaporator rises above its control value, that is 5 lbs. per square inch, controller 76 will generate a pressure below 9 lbs. per square inch and valve 80 will begin to open. As a result of the opening of the valve 81 steam will be vented from steam chest 20 thus decreasing the rate of evaporation of the liquor in the evaporator and tending to reduce the vapor pressure therein. t will thus be evident that the controller 76 operates to regulate the valves 74 and 80 conjointly to maintain the pressure in the evaporator vapor space substantially at a desired value.

From the foregoing description it should be apparent that the present invention provides apparatus capable of achieving the objectives set forth at the beginning of the present specification. It is of course to be understood that the embodiment of the invention described herein is illustrative only and that numerous changes can be made therein without departing from the spirit of the invention as set forth in the appended claims.

I claim:

1. In a compression evaporator of the type wherein water vapor is withdrawn from the vapor space of the evaporator, compressed by a compressor, and returned to the evaporator steam chest, apparatus for maintaining the pressure 'in said vapor space substantially constant, comprising, in combination, an auxiliary steam conduit connected to supply steam to said steam chest, a first regulating valve in said auxiliary steam conduit, a vent conduit connected to said steam chest for venting steam therefrom, a second regulating valve in said vent conduit, and control means responsive to the pressure in said vapor space and connected to. operate said first and second regulating valves, said control means operating said first regulating .yalve to close said valve with increasing'pressure in said vapor space and vice versa and operating said second regulating valve to open said valve with increasing pressure in said vapor space and vice versa, said control means conjointly operating said two valves to maintain the pressure in said vapor space substantially constant.

' 2. Apparatus according to claim 1 and wherein said control means upon an increase in pressure in said vapor space causes said auxiliary steam valve to close completely before permitting said vent valve to open and upon a decrease in pressure in said vapor space causes said vent valve to close completely before permitting said auxiliary steam valve to open.

3. Apparatus according to claim 1 and wherein said compressor is driven by a steam engine and said auxiliary steam conduit is connected to the exhaust of said steam engine, whereby exhaust steam from said engine is used in the auxiliary steam for said evaporator.

4. The method of operating a compression evaporator of the type wherein water vapor is'withdrawn from the vapor space of the evaporator, compressed by a steam engine-driven compressor, and returned to the evaporator steam chest to maintain the pressure in said vapor space substantially at a predetermined value, which comprises the steps of, supplying exhaust steam from the exhaust of said steam engine to said evaporator steam chest when the vapor space pressure is below said predetermined value and venting steam from said steam chest when the vapor space pressure is above said predetermined value.

5. The method of operating a compression evaporator of the type wherein water vapor is drawn from the vapor space of the evaporator, compressed by a compressor, and returned to the evaporator steam chest to maintain the pressure in said vapor space at substantially a predetermined, desired value, which comprises the steps of supplying auxiliary steam to said steam chest, venting steam from the steam chest of said evaporator and conjointly regulating in response to variations of the pressure in the vapor space of said evaporator the flow of auxiliary steam and the flow of vent steam to increase the auxiliary steam flow when the vapor space pressure is below said desired value and falling, and to increase the vent steam flow when the vapor space pressure is above said desired value and rising to maintain the vapor space pressure substantially at said desired value.

References Cited in the file of this patent UNITED STATES PATENTS

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