GB2034012A

GB2034012A - Method and Apparatus for Producing Process Steam

Info

Publication number: GB2034012A
Application number: GB7936560A
Authority: GB
Original assignee: Thermo Electron Corp
Current assignee: Thermo Fisher Scientific Inc
Priority date: 1978-10-25
Filing date: 1979-10-22
Publication date: 1980-05-29
Also published as: FR2439870B1; JPS5585802A; GB2034012B; FR2439870A1; DE2943204A1

Abstract

Process steam is produced from a feed of low pressure steam by setting the water content of a feed of water and steam at a selected water-steam ratio, injecting the two-phase mixture into a helical screw compressor in which the mixture is compressed, and vaporising 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 temperature and pressure of the feed, process steam at a predetermined temperature and pressure is produced. The compression proceeds along a direct, energy-efficient thermodynamic path. The apparatus used for producing the process steam comprises a screw compressor 20 having a steam and water inlet 18 connected through a water content control device 16 to a steam source 10. Further steam may be supplied from a waste heat boiler 14. <IMAGE>

Description

SPECIFICATION Method and Apparatus for Producing Process Steam This invention relates to an energy-efficient method of producing useful process steam having a pressure in the range of from 50 to 1 00 psig from low grade, otherwise waste, steam or steam produced from industrial waste heat having a pressure of up to about 1 5 psig.

Currently in the United States, industry accounts for nearly 40 per cent of the nation's energy consumption. Among the various forms of industrial energy, about 45 per cent is consumed as process steam, and half of this is used at pressures of less than about 100 psig. Process steam is used in iron and steel production, petroleum refining, in the paper, aluminium, copper, and cement producing industries, and for space heating. In these and other uses, it is a requirement that the steam be at a useful pressure, usually above at least about two atmospheres. A pressure of 55 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 previously economically uncompetitive methods of conserving and producing useful energy. An example of such a method is disclosed in U.S.

Patent No. 1,066,348 according to which industrially useful steam is generated from the low pressure exhaust of a steam engine. In the method disclosed in U.S. Patent No. 1,066,348 the steam is compressed so that it again attains a thermodynamic condition of temperature and pressure suitable for exploitation. However, to be economically feasible, this method must have a high coefficient of performance, i.e. a high ratio of heat output to work (or its thermal equivalent) put into the compressor. Only under these circumstances will the additional capital investment in a compressor (or compressors) and accessories be justified.

An example of an improvement over the method described in U.S. Patent No. 1,066,348, wherein high coefficients of performance can be achieved is disclosed in U.S. Patent No.

3,962,873. In this system, low pressure steam is generated in a solar collector and is thereafter upgraded by the action of a compressor. The compressor is driven by an engine which produces a shaft power output, relatively high temperature exhaust gas, and low pressure steam generated by its cooling system. The heat of the exhaust gas which would otherwise be lost is used to generate process steam which may be added to that produced by the compressor itself.

Furthermore, the engine drives 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 system of U.S.

Patent No. 3,962,873, the system can produce approximately three times the amount of steam that would be produced under comparable conditions from an equal amount of fuel if the steam was generated directly in a conventional boiler.

The present invention is concerned with a further improvement in the generation of higher pressure process steam from low grade steam.

According to one aspect of the invention, a method of producing process steam at a useful temperature and pressure by compressing a feed of low grade steam comprises feeding a twophase mixture of water and steam into the inlet of a helical screw compressor having a pair of interfitting screws driven through timing gears and evaporating the water component of the mixture within the compressor to produce substantially saturated process steam.

With this method, the vapours leaving the compressor can have a selected temperature and pressure, dependent on its water content, the compression ratio of the compressors, and the initial temperature of the feed. In a typical situation, water of from about 10% to 20% of the mass of steam is added by means of an injector.

Conducting the compression in this manner has many advantages. Thus, of all the thermodynamic paths available the path followed in the method of the invention requires the least work input and thereby optimizes energy saving. Operation of the method is simple since interstage cooling is unnecessary. Also, thermal stress and distortion problems are minimal because the steam discharge temperature and the temperature rise across the compressor are low.

Preferably, the prime mover which drives the compressor is a fuel-consuming engine, such as a diesel engine or gas turbine, which ejects an exhaust gas having a temperature above the temperature of the process steam. In these circumstances, as in the system disclosed in U.S.

Patent No. 3,962,873, the otherwise wasted heat content of the exhaust gas may be used to generate additional process steam.

The combination of these features results in amethod having an attractive overall coefficient of performance, i.e., the ratio of the heat content of the product to the heat content of the fuel used in the performance of the method is high. Also, since the equipment necessary to conduct staged compression with intercooling is not needed, capital costs are relatively low.

The invention thus provides an improved, more energy-efficient method of generating useful process steam from low grade, otherwise waste, steam, to render the generation of low grade steam produced with waste heat from stacks, exhausts, and various industrial processes more economically attractive.

The invention also consists, according to another of its aspects, in apparatus for carrying out the method in accordance with the invention, the apparatus comprising a screw compressor having an inlet, an outlet, and a pair of interfitting helical screws driven by timing gears, a conduit for supplying 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 which is arranged to drive the compressor and, in operation, produces exhaust gas of high temperature, and process steam generating means heated by the heat of exhaust gas from the prime mover.

An example of a method and of apparatus in accordance with the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a block diagram illustrating the method and the apparatus; Figure 2 is an idealized temperature-entropy diagram showing several possible paths along which low pressure steam can be compressed to high pressure steam; and Figure 3 is a somewhat diagrammatic crosssection, through a dry helical screw compressor with water injection means shown in simplified form for clarity.

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, wherein low pressure steam or a mixture of water and steam at temperature T1 is to be upgraded to the condition represented by point X on the saturation dome. The point designated by the letter A represents pure (saturated) steam at temperature T,. The point designated by the letter B represents water at temperature T1. Points between A and B, e.g. point C, represent various two-phase mixtures of steam and water. Pure steam at temperature T1 (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 compressions and water additions are repeated in stages (e.g.

through points G, H and I) until point X is reached.

Both of these paths require equipment 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 adding 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, heat of compression being used to vaporize liquid droplets and to raise the temperature of the ultimately saturated vapor.

A review of Figure 2 indicates that path ACX requires the least amount of work, as long as the fluid means substantially homogeneous and at thermal equilibrium during compression.

However, it would be expected that during a conventional compression the liquid droplets would not be completely vaporized and thus would not be in thermal equilibrium with surrounding superheated vapor because of the known low heating transfercoefficient of steam and the normally short residence time of fluid in compressors. If some means could be found to effect the compression of a two-phase mixture along line C-X, no interstage cooling would be necessary (as required, for example, for path AEFGHIX). Further, the discharge temperature of the saturated vapour would be minimal, and the thermodynamics of the change would be optimized.

In accordance with the invention, it has been discovered that helical screw compressors of a type currently available from, for example, Ingersoll-Rand, Beloit Power Systems and Kobe Steel are capable of efficiently compressing twophase water-steam mixtures. The operable screw compressors have unlubricated or water lubricated interfitting male and female helical rotors which rotate in a stationary housing and are controlled by timing gears. These types of machine are available in a wide range of sizes and have been used with many different gases. They are characterized by good efficiency over the range of 50 to 100 percent of maximum capacity.

Flow and horsepower are proportional to speed, and speed variation is the most efficient method of capacity control. Oil flooded units which do not employ timing gears but rather depend on an oil film between the rotors to prevent rotor contact cannot be used in the method and system of the invention. Water flood machines and dry screw machines, both of which employ timing gears and can run with at least small amounts of water in the feed work well. The main differences between the latter two types of machines is operating speed; the water flooded machine runs at about one-third the speed of the dry machines. Water in the feed steam provides a sealing action of the inlet of the tip of such screw compressors and reduces leakage.

In contrast, conventional centrifugal and recprocating compressors cannot be efficiently used for this type of compression. A reciprocating compressor is unsuitable because liquid in the inlet causes excessive wear or breakage of valves, pistons, and rings. Further, compressors with nonlubricated cylinder would be required, and these are rather expensive. Centrifugal compressors have the capacity to handle very large flow rates, but are sensitive to water droplets which cause blade erosion. Also, centrifugal compressors suffer from surge when operated in off-design conditions.

In order to produce steam in accordance with the invention at a selected temperature and pressure, (e.g., point X of Figure 2) three factors must be considered: the mass ratio of water to steam in the feed; the equilibrium temperature of the feed; and the compression ratio of the compressor. 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 feed 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 upstream of the compressor inlet, or a higher capacity compressor should be used. In the former case, any of the well-known techniques for lowering the water content of steam (such as cyclone separation) may be employed, or the wet steam can be heated at constant pressure to vaporize some of the available water. In cases where the temperature and pressure of 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 preferred method of controlling water content is to inject a fine spray of water directly into the inlet of the compressor together with saturated steam. This mode of operation allows precise control of the steam-water mass ratio.

Where the fuel steam contains some water, it will often be advisable to remove it prior to injecting the spray.

Referring to Figure 1, a preferred embodiment of the invention is shown. Low grade steam generally having a pressure an the order of O to 1 5 psig. either saturated or wet, is transported from a source 10 by a conduit 12. In an appropriate situation, industrial waste heat, e.g., on the order of 3500 F, can be used to generate low grade steam in waste heat boiler 14, and the resulting steam may be added to the conduit 12.

Of course, source 10 may itself comprise a waste heat boiler. The low grade steam in conduit 12 is next subjected to the action of a water content control means 1 6 which, as required, either increases or decreases the water content of the feed, and preferably produces a homogeneous mixture. In the usual case, it will be necessary to add water, typically in an amount equal to about 10% to 20% by weight of the saturated component of the two-phase mixture, thereby fixing the liquid-gas ratio at, e.g., point C (Figure 2). In this case, control means 1 6 can comprise means for injecting a spray of water into the inlet 1 8 of the compressor 20. Preferably, a fine, homogeneous water spray is introduced.Because of the high surface area mass ratios of the water and of the turbulence in the inlet, high heat transfer and operation at close to thermodynamic equilibrium results. Thus, within the compressor 20, the water component of 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 34, and is transported for use through conduit 36. The pressure of the steam can be controlled within narrow limits, and either slightly unsaturated or superheated steam can be produced.

The compressor is powered by the prime mover 22 which receives fuel at 24 and emits exhaust at 26. The work output 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, gearing 30 comprises a speed reduction system. With a relatively low rpm prime mover such as a diesel engine, gearing 30 is such as to increase the speed of the shaft 31.

Two factors influence the choice of the prime mover brake specific fuel consumption (BSFC) and exhaust gas temperature. The importance of BSFC 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 suitable compressor drive and are commercially available in a continuous range of capacities from about 1,000 to 7,000 horsepower. Diesel or gas powered internal combustion engines may also be used. Diesel engines have a performance 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 employing a large gas turbine is attractive despite the more favorable fuel consumption of the diesel, because turbine inefficiencies are in the form of high temperature exhaust gases that can be used directly to generate high pressure steam. In contrast, about one third of the diesel waste heat is dissipated into the cooling jacket, which typically is at about 2000F, a temperature too low for economical heat recovery. As a result, the gas turbine exhaust will generate about three times more steam than diesel exhaust for a comparable power rating. Because of the high temperature of the exhaust, which is typically well above the final desired temperature of the process steam, it can be used to produce steam 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.

A dry helical screw compressor useful in the system and process for the invention is illustrated in Figure 3. It comprises a drive shaft 31 and drive gearing 38 which actuate drive gear 40 of a first stage 44 of the compressor, and driven gear 42 of a second stage 46 of the compressor. Each stage comprises a housing 48 containing a pair of dry interfitting helical screw rotors 50 and 52. The rotors are fixed to axles 54, 56 which are mounted for rotation through journals 58 and bearings 60. Rotors 50 drive rotors 52 through timing gears 51, 53. Since the transfer of motion from rotor 50 to rotor 52 does not require mating contact between the helical screws themselves, a lubricating oil film on the screws is not required.

Each stage has an inlet 18 and an outlet 34. Inlet 18 of first stage 44 features a nozzle 62 which is fed with water and injects a fine spray directly into the first stage 44. Crossover pipe 64 communicates between the outlet 34 of the first stage and the inlet 1 8 of the second stage.

Process steam is transported from the compressor through conduit 36. Optionally, a second water injection nozzle (not shown) can be provided at the inlet 1 8 of second stage 46.

In operation, saturated low grade steam enters the inlet 1 8 of compressor 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 turbulent 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 example 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 750/a efficiency, about 176.8 BTU of workilb. is required. This means that 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 input is 1.87+.47=2.34 pounds.

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

Claims

1. A method of producing process steam at a useful temperature and pressure by compressing a feed of low grade steam, the method comprising feeding a two-phase mixture of water and steam into the inlet of a helical screw compressor having a pair of interfitting screws driven through timing gears and evaporating the water component of the mixture within the compressor to produce substantially saturated process steam.

2. A method according to Claim 1, wherein the helical screw compressor is a dry screw compressor.

3. A method according to Claim 1 or Claim 2, further comprising controlling the water-steam mass ratio in the feed to produce process steam at a predetermined temperature and pressure.

4. A method according to any one of Claims 1 to 3, wherein the compressor is driven by a prime mover which ejects an exhaust gas having a temperature above that of the collected process steam, and the heat content of the exhaust gas is transferred to water to produce additional process steam.

5. A method according to Claim 4, wherein the prime mover is a gas turbine.

6. A method according to Claim 4, wherein the prime mover is a diesel engine or a gas engine.

7. A method according to any one of the preceding Claims, wherein the low grade steam is generated in a waste heat boiler.

8. A method according to any one of the preceding Claims, wherein the water content of the low grade steam is from 10% to 20% by weight.

9. A method according to any one of the preceding Claims, wherein the water content of the mixture is controlled by injecting a fine spray of water into the inlet of the compressor.

10. Apparatus for carrying out the method in accordance with Claim 1, the apparatus comprising a screw compressor having an inlet, an outlet, and a pair of interfitting helical screws driven by timing gears, a conduit for supplying 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 which is arranged to drive the compressor and, in operation, produces exhaust gas of high temperature, and process steam generating means heated by the heat of the exhaust gas from the prime mover.

11. Apparatus according to Claim 10, wherein the helical screw compressor is a dry screw compressor.

12. Apparatus according to Claim 10 or Claim 11, wherein the prime mover is a gas turbine.

13. Apparatus according to Claim 10 or Claim 11 , wherein the prime mover is a diesel engine or a gas engine.

14. Apparatus according to anyone of Claims 10 to 13, wherein the source is a boiler heated by industrial waste heat.

1 5. Apparatus according to any one of Claims 10 to 14, wherein the water content control means comprises injecting means for injecting a spray of water into the low grade steam.

1 6. Apparatus according to Claim 15, wherein the injecting means includes a nozzle positioned to inject a fine spray of water into the inlet of the compressor.

1 7. A method according to Claim 1, substantially as described with reference to the accompanying drawings.

1 8. Apparatus according to Claim 10, substantially as described with reference to the accompanying drawings.