CA1180240A - Energy efficient method of producing process steam - Google Patents

Abstract

ENERGY EFFICIENT METHOD OF PRODUCING PROCESS STEAM

ABSTRACT OF THE DISCLOSURE
Process steam is produced from a feed of low pressure steam by fix-ing the water content of the feed at a selected water-steam ratio, injecting the two-phase mixture into a helical screw compressor, and vaporizing the water in the feed during compression. The water-steam ratio is set so that, given the compression ratio characteristic of the compressor and the tempera-ture and pressure of the feed, process steam at the selected temperature and pressure is produced. The compression proceeds along a direct, energy-effici-ent thermodynamic path.

Description

BACKGROUND OF THE INVENTION
This invention relates to an energy-efficient method of producing useful process steam having a pressure in the range of 50-100 psig from low grade otherwise expendable steam or steam produced -Erom industrial waste heat having a pressure on the order of 0-15 psig.
Currently, U.S. in~ustry accounts for nearly 40 percent of the nation's energy consumption. Among the various forms of industrial energy, ~5 percent is consumed as process steam, and half of this is used at pres-sures less than about 100 psig. Process steam is used in iron and steel pro-ductionJ petroleum refining, in the paper, alumlnum, copper, and cement pro-ducing industries, and for space heating. In these and other similar uses, it is a requirement that the steam be at a useful pressure, usually above at least abou* two atmospheres. A pressure of 65 psig may be taken as a typical process steam pressure.
The rising cost of fuel and the increase in demand for energy is making more attractive heretofore economically ~mcompetitive methods of con serving and producing useful energy. An example of such a process ls dis-closed in U.S. Patent No. 1,066,3~8 to G. T. Voorhees, which discloses gener-ating industrially useful s-team ~rom the low pressure e~haus-t o-~ a st&am ?O &ngine. In the Voorhees process, the st&am is compress&d so that i-t again attains a thermodynamic condition of ~&mperatur& and pressur& suitabl& ~or e~ploitat:ion. llowever, -to b& &conomically f&asibl&~ this typ& oE system must have a high coeffic-lent of p&r~ormanc&~ i.e.~ a high ratio o~ h&at output to work ~or its thermal equivalent) put into *he compressor. Only in this cir-cumstance ~Yill the additional capital investment in a compressor ~or compres-sors) and accessories be justified.
An e~ample of an improvemen* over the Voorhees system wherein high coeficients of performance can be achieved is disclosed in U.S. Patent No.
3,~62,873 to J. P. Davis. In the Davis syst&m, low pressure steam is gener-ated in a solar collector and thereafter upgraded by the action of a compres-sor. The compressor is driven by an engine which produces shaft work, rela-tively high temperature exhaust gas, and low pressure steam generated by its cooling system. The heat of the exhaust gas which would othelwise be lost is used to generate process steam which may be added to that produced by the compressor itsel~. Furthermore, the engine powers a second compressor which upgrades the low pressure steam generated by the engine's cooling system to provide still more process steam. Because of these and other features of the Davis system, it ccm produce approximately three times the amount of steam that would be produced ~mder comparable conditions from an equal amount of fuel iE the steam were generated directly in a conventional boiler.
SUMMARY OF THE INVENTION
The instant invention relates to a further improvement in -the gene-ration of high pressure process steam from low grade steam. In its broadest aspects, it involves a process and means for eleva-ting low grade steam to process steam of an elevated temperature and pressure via a thermodynamic path which is highly energy efficient. In accordance with the invention, a two-phase mixture of water and steam is :Eed to the inlet of a helical screw compressor having a pair oE inter:Eitting helical screw rotors driven through timing gears. The total aqueous content of the wa~er-steam mix-ture, when vaporized and compressed ~y the compressor, is sufficient to result in use-ul process steam. Thus when the mixture is fed into the inle-t o-E the com-pressor, the liquid oomponent oE the mixture is evaporated therewithln. The exiting vapor can have a selected -tempera-ture and pressure> dependent on its ~ater content~ the compression ratio of the compressor, and the initial temperature of the feed. In a typical situation, water on the order of 10%-20~ of the mass of stecam is added by means of a~n injector. Conducting the compression in this manner has many advantages. Thl15J of all the thermo-dynamic paths available, the path followed in the process oE the invention requires minimal work input and thereby optimizes energy saving. Operation of the system is simple since interstage cooling is unnecessa~y. Also, thermal stress and distortion problems are minimal because the steam dis-charge temperature and the temperature rise across the compressor are low.
According to another aspect of the invention, there is provided che method of producing substantially saturated process steam from low grade steam comprising: feeding a two-phase mixture of water and steam into the inlet of a screw compressor having a pair of interfitting helical screw rotors driven through timing gears, said mixture having a water-steam mass ratio controlled so as to yield saturated steam upon compression to a desired pressure; and compressing said mixture along an essentially isentropic thermo-dynamic path such that the water evaporates and substantially saturated pro-cess steam is produced at the outlet of said compressor.
The apparatus of the invention comprises:
a screw compressor having an inlet, an outlet~ and a pair of inter-fitting helical screws driven by timing gears;
a conduit or transporting low grade steam Erom a source thereof to the inlet of the compressor;
water content control means in communication with the compressor inlet for providing a steam-water mixture thereto;
a conduit :Eor transporting process steam from the compressor outlet;
a prime mover Eor powering said compressor and having an exhaus~
gas of high temperature; and process steam genera-ting means powered by tile heat of the e~haust Oe said prime mover.
Pre-Eerably, the prime mover which drives -the compressor is a fuel--consuming engine such as a diesel or gas turbine which ejects an exhaust gas having a temperature above the temperature of the process steam. In this circumstance, as in the Davis patent, the otherwise wasted heat content of the exhaust gas may be used to generate additional process steam.
The combination of these Eeatures results in a process having an ~0 attrac-tive overall coefficient of performc~lce, i.e., the ratio oE the hea-t ~1 ~J~

content oE the product to the heat content of the fuel used to drive the system is high. Also, since the equipment necessary to conduct staged com-pression with intercooling is not needed, capital costs are relatively low.
~ ccordingly, objects of the invention include the provision of an improved, more energy-efficient method of generating useful process steam from low grade, otherwise expendable steam) to render the generation oE low grade steam produced with waste heat Erom stacks, exhausts) and various industrial processes more economically attractive by providing all efEicient method fox upgrading low pressure steam to a useful condition, and to pro-vide a means and process for eEficiently increasing the pressure and tempera-ture of low pressure steam.
These and other objects and features of the invention will be apparent from the following description of a preferred embodiment and from the drawing wherein like reference characters in the respective figures indi-cate corresponding parts.
BRIEF DESCRIPTION OF T~IE DRAWING
Fig. 1 is a block diagram oE an important embodiment of the inven-tion;
Fig. 2 is an idealized temperature-entropy diagram showing several 2a possible paths along which low pressure steam can be compressed to high pres-sure steami and Fig. 3 i5 a cross-sectional, par-tly schematic view o:E a dry helical scre~ compressor with injection means shown in simplified Eorm for clarity.
DESCRIPTION OF A PREFERRED EM~ODIMENT
Low pressure steam can be compressed to a higher pressure and temperature by way of several thermodynamic paths. Three possible paths are shown in Figure 2~ ~herein lo!~ pressure steam or a mixture oE water and steam at temperature Tl is to be upgraded to the condition represented by 3~ point X on the saturation dome. The point designated by the letter A repre-sents pure (saturated) steam at temperature Tl. The point designated by the letter B represents water at temperature Tl Points between A and B, e.g., point C~ represent various two-phase mixtures of steam and water. Pure steam at temperature Tl (point A) may be raised to point X, via path ADX
wherein dry saturated steam is directly compressed to its final pressure ~point D), and the final temperature is obtained by adding water to the superheated vapor. Alternatively, the same general approach may be taken, but in stages. In this case, saturated steam is directly compressed to point E, water is added to reduce its temperature to point F, and the compres-sions and water additions are repeated in stages ~e.g. through points G~ il culd I) until point X is reached. Both of these paths require equipmen-t in addition to the compressor for effecting the constant pressure temperature reduction accomplished by adding water. The third path by which saturated steam could be upgraded to the condition represented by point X involves add~
ing water to the steam prior to compression so that the compressor inlet -feed consists of a mixture of steam and water as exemplified by point C.
Thereafter, the two-phase mixture is compressed, hea-t of compression being used to vaporize liquid droplets and to raise the temperature o:E the ulti-mately saturated vapor.
A review o:E F;gure 2 ;.ndlcates that path ACX requires the leas-t amount of work, as long as the :Eluid remains substantially homogeneous and at tllermal equilibrium during compression. ~lowever~ it would be expected that during a conventional compression the liquid droplets would not be com-pletely vaporized and thus woulcl not be in tllermal equilibriwn with surrouncl-ing superheated vapor because of the known low heat transfer coefficient of steam and the normally short residence time of fluid in compressors. If some means could be found to e:Efect the compression of a two-phase mixture along line C-~, no interstage.cooling would be necessary ~as required, :Eor example, for path A~F~II~. Further, the discharge temperature of the satu-~0 rated vapor would bc minimal, and the thermodynamics o:f the change would be optimized.

In accordance with the invention, it has been discovered that helical screw compressors oE a type currently available from, for example, ~rngersoll-Rand, Beloit Power Systems and Kobe Steel are capable of effi-ciently compressing two-phase water-steam mixtures. The operable screw com-pressors have unlubricated or water lubricated inter~itting male and female helical rotors which rotate in a s~ationary housing and are controlled by timing gears. These types of machine are available in a wicle range of sizes and have been used with many different gases. They are characterized by good eiciency over the range of 50 to 100 percent of maximum capacity.
Flow and horsepower are proportional to speed, and speed variation is the most eficient method of capacity control. Oil Elooded units which do not employ timing gears but rather depend on an oil Eilm between the rotors to prevent rotor contact cannot be used in the method and system of the inven-tion. Water flooded machines and dry screw machines, both o which employ timing gears and can run with at least small amounts of water in the Eeed work well. The main diferences between the latter two types of machines is operating speed; th~ water Elooded machine r~ms at about one-third the speed o-E the dry machines. Water in the feed steam provides a sealing action at the inlet oE the tip o such screw compressors and reduces leakage.
In contrast, convent;onal centrifugal and reciprocating compressors cannot be ef-iciently used Eor this type oE co~pression. A reciprocating compressor is ~msuitable because liquid in the inle-t causes excessive wear or breakage oE valves, pistons, and rings~ Fur-ther~ compressors w;th non-lubricated cylinders would be required, and these are rather expensive.
Centriugal compressors have the capacity to handle very large Elow rates, but are sensitive to water droplets which cause blade erosion. Also, centri-fu~al compressors suEer rom surge when operated in off-design conditions.
In order to procluce steam in accordance with the invention at a selected ~emperatura and pressure, (e.g., point ~ of Figure 2) -three factors must be considered the mass ra-tio oE water to steam in the Eeed; the .~

equilibrium tempera-ture of the feed; and the compression ratio of the com-pressor. Often, low grade steam will already contain some water, and thus less additional water need be added than would otherwise be necessary if low grade saturated steam were the Eeed material. Some low pressure steam, as available, may be characterized by a water content which places it somewhere on line B-C of Figure 2. In this case, unless unsaturated process steam can be tolerated, means for changing the liquid/gas ratio of the feed such that it lies at a desired point, e.g~, point C on line A-B, should be included up-stream of the compressor inle-t, or a higller capacity compressor should be used. In the former case~ any oE the well-known techniques for lowering the water content oE steam ~such as cyclone separation) may be employed9 or -the wet steam can be heated at constant pressure to vaporize some of the avail-able water. In cases where the temperature and pressure o-E the process steam is not critical and the low pressure steam already contains some water, it will be possible to simply compress the available two-phase mixture.
The pre:Eerred method oE controlling water content is to inject a fine spray of water directly into the inlet of the compressor toge~her with saturated steam. This mode oE operation allows precise control oE the steam--water mass ratio. Where the Euel steam contains some water, it will often be advisable to remove it prior to injecting the spray.
Referring to Figure 1, a preEerrecl embodiment oE the inven-tion is shown. Low grade steam generally having a pressurc~ on the order oE O to 15 psig, either satura-ted or we-t, is transpor-tecl Erom a source 10 by a conduit 12. In an appropriate situation, industrial waste heat, e.g., on the order of 350F, can be used to generate low grade steam in waste heat boiler 14, and the resulting steam may be aclded to the conduit 12. Of course, source 10 may itself comprise a waste heat boiler. The low grade stc~am in conduit 12 is ne~t subjected to t~e action oE a water content control means 16 which, as required, either increases or decreases the water conten-t of -the feed, and ~0 preferably produces homogeneous mi~ture. In the usual case, it ~ill be neces-q~

sary to add water, typically in an amount equal to about 10% to 20% byweight of the saturated component of the two-phase mixture, thereby fixing Lhe liquid-gas ratio atJ e.g., point C (Figure 2). In this caseJ control means 16 can comprise means for injecting a spray of water into the inlet 18 of the compressor 20. Preferably, a fine, homogeneous water spray is intro-duced. Because of the high suTface area mass ratios of the water and of the turbulance in the inlet, high heat transfer and operation at close to -thermo-dynamic equilibrium results. Thus, within the compressor 20, the water com-ponent o:E the feed is vaporized, and -the resulting, substantially saturated steam is compressed and heated. Process steam of the selected temperature and pressure exits the compressor at outlet 3~, and is transported for use through conduit 36. The pressure of the steam can be controlled within nar-row limits, and ei.ther slightly ~msaturated or superheated steam can be pro-duced.
The compressor is powered by the prime mover 22 whic}l receives fuel at 2~ and emits exhaust a-t 26. The work outyut shaft 28 of prime mover 22 is operatively connected through appropriate gearing 30 and driven shaft 31 to compressor 22. With a high rpm prime mover such as a gas turbine, gea.ring 30 comprises a speed reduction system. wit}l a relatively low rpnl prime mover such as a diesel engine, gearlng 30 ls such as -to increase the speed o:E the sha:Et 31.
~ r~O factors influence the cho.ice o:E -the pr.ime mover: hrake speciic uel consumption ~S~C~ and exhaust gas tempera~ure. The impor-tance o ~SFC is obvious; the exhaust gas temperature determines how much energy can be recovered as process steam from a boiler, indicated at 32, utilizing prime mover exhaust. Gas turbines are believed to be the most suit-able compressor drive and are commercially available in a continuous range o~
capacities Erom about 1,000 to 7,000 horsepower. Diesel or gas powered inter-nal combustion engines m~y also be used. Diesel engines have a per:Eormance advantage up to about 13,000 horsepower. Turbines up to this size range have a poor BSFC. In large turbines, BSFC is improved but is not as favorable as the figures for diesel engines. However, a system emplo~ing a large gas tur-oine is attractive despite the more favorable fuel consumption oE the diesel, because turbine ine~ficiencies are in the orm oE high temperature e~haust gases that can be used directly to generate high pressure steam. In con-trast, about one third of the diesel waste heat is dissipated into the cool-ing jacket, which typically is at about 200~, a temperature too low Eor economical heat recovery. As a result, the gas turbine exhaust will gener-ate about three times more steam than diesel exhaust for a comparable power rating. Because oE the higll temperature oE the exhaust, which is typically well above khe final desired temperature oE the process steam, it can be used to produce s*eam at substantially the same temperature and pressure as that from the outlet side 34 of compressor 20, and thus can be added through conduit 33 to conduit 36.
~ dry helical screw compressor useEul in the system and process of the invention is illustrated in Figure 3. It comprises a drive shaf-t 31 and drive gearing 3S whlch actuate driven gear 40 of a first stage 44 of the com-pressor, and driven gear 4~ o:E a second stage 46 oE-the compressor. Each stage comprises a housillg 48 containin~ a palr oE dry lnterEl-ttlng hellcal serew rotors S0 and 52. The ro-~ors are fixed -to a~les 54, 56 whlch are mounted or rotation throllgh journals 58 an~l bearings 60. Rotors 50 drive rotors 52 through timing gears 51~ 53. ~ince -the tr~msEer of motion Erom rotor 50 to rotor 52 does nok require mating contact between the helical screws themselves, a lubricating oil fllm on the screws is not re~ulred.
Each stage has an inlet 18 and an outlet 34. Inlet 1~ oE first stage 44 features a nozzle 6~ which is fed with water and injects a ine spray dlrectly lnto the first stage 44. Crossover pipe 64 communicates between the outlet 34 of the flrs-t stage and the inlet lS of the second stage. Pro-cess steam is *ransported rom the compressor through conduit 36. Optlon-ally, a second water injection nozzle ~not shown) can be provided at -tlle ln-let 18 o second stage ~6.

o In operation, saturated low grade steam enters the inlet 18 of com-pressor stage 44 together with a fine spray of water provided by nozzle 62.
Because of the high surface area to mass ratio of the spray and the turbu-lent flow within the compressor, the water is vaporized within stage 44.
Steam or a steam-water mixture then passes along crossover pipe 64, enters second stage 46 through inlet 18, and is further compressed. Process steam of the selected temperature and pressure exits through conduit 36.
As an e~ample of the operation of the system, it will be assumed that low grade steam at 14.7 psia is to be upgraded to process steam at 75 psig. A gas turbine having an efficiency of 33% is coupled with a helical dry screw compressor having an efficiency of 75%. Per input of 1000 BTU as fuel to the turbine, there will be provided shaft work of 330 BTU and 670 BTU
of exhaust. A boiler powered by the exhaust can produce about 0.47 pounds of steam. It takes about 136.2 BTU to raise one pound of steam from 0-75 psig~
but since the compressor here operates at 75% efficiency, about 176.8 BTU of work/lb. is required. This means ~hat the 330 BTU of shaft work can produce about 1.87 pounds of steam, and the total mass of 75 psig steam produced with the 1000 BTU lnput is 1.87 ~ .~l7 - 2.34 pounds.

In eontrast, the same 1000 BTU, if used to fire a boiler of 80% efficiency, could produee only about 0.8 pound of steam. Thus, about three times as much steam is produced in t~e system described here as in a conventional boiler.

In view of the foregoing description, it will be apparent that various changes ean be made without departing fr~m the seope and spirit of the invention. Accordingly, other em~3-diments are within the following elaims.

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Claims (16)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. The method of producing substantially saturated process steam from low grade steam comprising: feeding a two-phase mixture of water and steam into the inlet of a screw compressor having a pair of interfitting helical screw rotors driven through timing gears, said mixture having a water-steam mass ratio controlled so as to yield saturated steam upon compression to a desired pressure; and compressing said mix-ture along an essentially isentropic thermodynamic path such that the water evaporates and substantially saturated process steam is produced at the outlet of said compressor.

2. The method of claim 1 wherein said screw compressor is a dry screw compressor.

3. The method of claim 1 wherein the compressor is powered by a prime mover which ejects an exhaust gas having a temperature above the temperature of the collected process steam, said method comprising the further step of transferring the heat content of said exhaust gas to water to produce addi-tional process steam.

4. The method of claim 3 wherein said prime mover is a gas turbine engine.

5. The method of claim 3 wherein said prime mover is a diesel engine or a gas engine.

6. The method of claim 1 wherein said low grade steam is generated in a waste heat boiler.

7. The method of claim 1 wherein the water content of the low grade steam is between about 10% and 20% by weight.

8. The method of claim 1 wherein the water content of the feed is con-trolled by injecting a fine spray of water into the inlet of the compressor.

9. A system for producing saturated process steam at a selected thermodynamic condition from low grade steam, said system comprising:
a screw compressor having an inlet, an outlet, and a pair of interfitting helical screws driven by timing gears, a conduit for transporting low grade steam from a source thereof to the inlet of the compressor;
water content control means in communication with the compressor inlet for providing a steam-water mixture thereto;
a conduit for transporting process steam from the compressor outlet;
a prime mover for powering said compressor and having an exhaust gas of high temperature; and process steam generating means powered by the heat of the exhaust of said prime mover.

10 . The system of claim 9 wherein said helical screw compressor is a dry screw compressor.

11. The system of claim 9 wherein said prime mover is a gas turbine.

12. The system of claim 9 wherein said prime mover is a diesel engine, as a gas engine.

13. The system of claim 9 wherein said source is a boiler powered by industrial waste heat.

14. The system of claim 9 wherein said water content control means comprises means for injecting a spray of water into the low grade steam.

15. The system of claim 14 wherein said means for injecting is a nozzle positioned to inject a fine spray of water into the inlet of said compressor.

16. An energy efficient method of producing process steam at a useful temperature and pressure from low grade stean, said method comprising the steps of:
A. providing a screw compressor having a pair of interfitting helical rotors driven by timing gears, and having an inlet, and an outlet;
B. providing low grade steam containing sufficient water such that the total aqueous content of the water-steam mix-ture, when vaporized and compressed by said compressor, is suf-ficient to result in useful process steam;
C. feeding the water-steam mixture of step B into the inlet of said compressor;
D. evaporating the liquid component of said mixture within the compressor; and E. collecting useful process steam from the outlet of said compressor.