GB2166529A - A system and method for the recovery of waste heat from exhaust steam - Google Patents

Abstract

Waste heat is recovered from the exhaust steam of a steam engine by passing the steam through cooler 2 from which it is delivered at or slightly above atmospheric temperature and pressure as mixed steam and condensate to a compressor that delivers the mixture as water at about 245/250 DEG F to receiver surge tank 4. Feed pump 6 returns the water from receiver 4 to the boiler. The compressor, receiver and feed pump are grouped together in a casing through which flue gas is circulated at about 270 DEG F by a fan. Air is admixed with the gas under control of a valve. Cooler 2 may contain water cooling tubes providing a path to an outlet or it may consist of a jet spray cooler. The compressor may be rotary, particularly a turbo-compressor. <IMAGE>

Description

SPECIFICATION A system and method for the recovery of waste heat from exhaust steam This invention relates to a system and method for the recovery of waste heat from the exhaust steam of a steam engine or other prime mover or apparatus from which steam is exhausted at a temperature and pressure suitable for recovery.

This system has been disclosed in the specification of my earlier Patent No. 1,080,933 and results in useful heat economy compared, for example, with an ordinary steam condensing system. A vacuum condenser is, as is well known, efficient but the result has to be paid for by the use of much extra equipment, i.e., bled steam feed heaters, vacuum producing plant, cooling towers and so forth. The system of patent No. 1,080,933 is less complicated and therefore cheaper to install.

As a result of considerable investigation it has been found that the economy can be enhanced to a considerable extent by the present invention.

According to the invention, a system for recovering waste heat from the exhaust steam of apparatus operated by a steam raising boiler, comprises a cooler into which the exhaust steam is directed and partially cooled and partially condensed, a compressor connected to receive the mixture of partially cooled and partially condensed steam for compressing it to a pressure lower than that at which the boiler operates so as to liquify the mixture, means for returning the water so produced to the boiler, the cooler and the compressor being grouped together and enclosed within a casing and means for circulating boiler flue gas, with or without admixture of air, or air preheated by flue gas through the casing to reduce loss of latent heat.

Advantageously the means for returning the water so produced from the compressor to the boiler comprises a tank for receiving the water from the compressor and a feed pump, the receiver tank and feed pump being contained within the casing.

The cooler may take a variety of forms. For example, it may be a water tube cooler or a spray jet cooler.

Similarly the compressor may be one of a variety of forms, e.g., a rotary compressor, or a turbo-compressor.

The invention also includes a method of recovering waste heat from the exhaust steam of apparatus operated by a steam raising boiler comprising partially cooling the exhaust steam by means of a cooler, compressing, by means of a compressor, the partially cooled steam to a pressure lower than that at which the steam raising boiler operates so as to liquify the steam by compression, feeding the water so obtained back to the boiler and maintaining a surrounding atmosphere consisting of flue gas from the boiler, or a mixture of such flue gas and air or plain air preheated by flue gas about the cooler, and compressor, to reduce the loss of latent heat.

In order that the invention may be clearly understood and readily carried into effect a system and method in accordance therewith will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a specific line diagram of a waste heat recovery system; Figure 1a shows a line diagram of a section of a fluid receiver; Figure 2 is a diagrammatic sectional elevation of one form of steam cooler; Figure 3 is a diagrammatic sectional elevation of another form of steam cooler; Figure 4 is an elevation of an assembly of parts for use in a practical application of the system of Figure 1.

Referring to Figure 1 which is derived from the specification of Patent No. 1,080,933, waste heat from the exhaust steam of a reciprocating steam engine 1 (or steam turbine or any other apparatus operated by steam raising plant) is recovered by passing the steam through a cooler 2 to a compressor 3 in which the steam is liquified prior to passing through a combined receiver surge tank and oil separator 4, the outlet from which is controlled by a pressure control valve 5 connected to a boiler feed pump 6 that delivers the water in the receiver 4 to a boiler 8. The receiver 4 has a safety valve 7.

The duty of the cooler 2 is to take the exhaust steam and extract a minimal amount of latent heat therefrom to maintain the steam at, or only just above, atmospheric pressure and temperature, thereby reducing its volume and increasing its density while leaving a large fraction of the latent heat in the resulting mixture of steam and condensate which can be liquified easily by compression in the compressor 3 and ultimately pumped back into the boiler 8 in a hot enough condition effectively to reduce fuel consumption.

One satisfactory form of cooler is shown in Figure 2. This has a casing 9 of oval crosssection with the lower part of smaller radius than the upper part. The steam enters the casing through a port 10 and the mixture is delivered through ports 11. Water cooling tubes 12 are arranged in parallel rows or groups extending between end walls 13 of the casing and communicating at opposite ends with water boxes 14 at one end of the casing and water boxes 15 at the other end.

These boxes 14, 15 are arranged so that the coolant can pass through the rows of groups of tubes 12 in series as shown by the arrows. In practice each set of water boxes 14 or 15 is pressed against a gasket interposed between the set of water boxes and the asso ciated end wall 13.

No part of the mixture of steam and condensed water should be ailowed to fall below atmospheric temperature and atmospheric pressure. The cooling water is delivered through a header 17 to which the uppermost water box 14 is connected through a control valve 18.

The outlet pipe 16 extends above the highest water tubes 12 to ensure that none of the tubes are empty. A thermometer 19 is connected to the entry end of the outlet pipe 16.

The outlets 11 for the mixture may be replaced by a single outlet.

An alternative and very compact form of cooler is shown in Figure 3, in which a jet spray nozzle 20, is mounted on the centre line of a vertical cylindrical casing 21. The exhaust steam enters through a pipe 22 at the top of the casing and is distributed radially by a baffle 23 to pass downwards through a fine conical water spray extending peripherally to the casing wall from the nozzle 20. The water runs down the casing wall to a sump 24 from which it is withdrawn through a cooling radiator 25 by a pump 26 which returns the water to the nozzle 20.

A valve 27 controlling a passage through which the water can by-pass the pump 27 in its passage from the radiator to the nozzle, can be adjusted to suit the amount of steam entering the cooler. The cooled steam is removed through a pipe 28 having a central iniet 29 shielded, as by shaping it to point downwards as shown, to prevent ingress of falling droplets.

The cooler of Figure 3 may comprise more than one jet arranged coaxially.

To avoid heat loss the shortest possible path must be provided between the cooler 2 and compressor 3 which should be located at the base of the cooler and preferably directly beneath it. The object of the compressor is to compress the water-steam emulsion at a moderate pressure, e.g. 20-30 psi (1.41-2.11 kgf.cm2) to a comparatively small volume and a comparatively high density. A rotary compressor would be suitable. A rotary multistage compressor will be required for a large plant. To accommodate different loads, the compressor is preferably arranged to be driven at different speeds. For example, it may be driven by an exhaust steam turbine which automatically adapts the compressor speed to the output of exhaust steam from the engine 1.

On being discharged from the compressor the emulsion is taken to the receiver 4 through a non-return valve iOa (Figure la). The receiver comprises a small chamber to absorb surging in the fiow from the compressor 3 (shown in Figure 4 as a rotary compressor) and also in the delivery to the boiler feed pump 6. The receiver constitutes the demarcation between the low pressure stage and a high pressure delivery stage to the boiler. The receiver incorporates an oil separating baffle 1 1a and a blow down cock (not shown). The safety valve 7 is set at 30 psi (2.11 kgf/cm2) to relieve any excessive build up of air or vapour pressure, especially when starting up.

The outlet at the pressure control valve 5 from the receiver is set at a higher level than the inlet from the compressor 3. The valve 5 incorporates an adjustable spring loaded valve member which can be set to various pressures, e.g., 25 psi (1.76 kgf/cm2).

The receiver 4 provides self-adjustment without the necessity for by-pass because excess boiler feed pump capacity over the delivery from the compressor simply causes the water level to fall below the outlet at the pressure control valve 5 until further supplies from the compressor raise the level again to the outlet 5 to enable the feed pump 6 to resume pumping to the boiler.

The boiler feed pump 6 has a gross capacity somewhat greater than the rate of supply required by the engine. The feed pump is maintained at a temperature as closely as possible to that of the saturated temperature of the steam in the boiler plus the temperature generated by the work done in feeding the liquid at the pressure required to feed it into the boiler. The reciprocating boiler feed pump may be replaced by a centrifugal pump.

In order to prevent the loss of latent heat, the compressor 3, receiver tank 4 and feed pump 6 should be heated at least to the same temperature as the saturated steam but as the steam is liquified at 25 psi (1.76 kgf/cm2) and fed into the receiver 4 at this temperature the components should be at the corresponding temperature which is 267oF (130.5 C), i.e. approx 270"F (132.2"C). To avoid the necessity of separate jacketing for each component for this purpose, the cooler casing 9 (Figure 2), compressor, receiver and feed pump are grouped closely together and enclosed in a sheet metal casing 69 (Figures 4 to 7). In the arrangement of Figure 4 flue gas diluted with a controlled amount of air is delivered through a duct 70 by a fan 71. The flue gas is fed to the fan 71 through a pipe 72 containing a control valve 73 and the diluting air is received through a branch pipe containing a thermostatically controlled valve 74. The assembly in the casing 69 is placed as close to the boiler as possible. The casing 69 is heat insulated as also are the boiler and all external pipework. The flue gas is returned to the flue through a pipe 75 from the top of the casing.

It will be seen that only the effective part of the cooler 9 is inside the casing 69, the water boxes 14, 15 being outside. If there is some fear of corrosion due to the flue gas, plain air heated by a heater may be fed through the flue gas duct in association with a suitable flue control. If necessary the arrangement may be such that only clean air, preheated by the flue gas, is fed through the casing.

The system is such that the feed pump can pump direct to the boiler against a pressure of 1500 psi (105.46 kgf/cm2) or more.

Claims (12)

1. A system for recovering waste heat from the exhaust steam of apparatus operated by a steam raising boiler, the system comprising a cooler into which the exhaust steam is directed and partially cooled and partially condensed, a compressor connected to receive the mixture of partially cooled and partially condensed steam for compressing it to a pressure lower than that at which the boiler operates so as to liquify the mixture, means for returning the water so produced to the boiler, the cooler and the compressor being grouped together and enclosed within a casing, and means for circulating boiler flue gas, with or without admixture of air, or air preheated by flue gas through the casing to reduce loss of latent heat.

2. A system according to Claim 1, in which the means of returning the water from the compressor to the boiler comprise a tank for receiving the water from the compressor and a feed pump, the receiver tank and feed pump being contained within the casing.

3. A system according to Claim 1 or Claim 2 in which the said means for circulating the flue gas comprise a fan for delivering the flue gas to the base of the casing, supply pipes for delivering the flue gas and cold admixing air to the fan, a valve for controlling the quantity of admixing air fed to the flue gas and a pipe for discharging the flue gas from the top of the casing.

4. A system according to any one of the preceding claims, in which the cooler comprises a casing containing parallel water tubes arranged for cooling water to pass in series therethrough, or in series through rows or groups of the tubes, to an outlet from the cooler, valve means being provided for the controlled admission of cold water to the tubes, the cooler casing being provided with an inlet for the steam and an outlet for the mixture arranged so that the steam passes transversely over the cooling tubes.

5. A system according to any one of Claims 1 to 3, in which the cooler comprises a vertical casing, a jet spray nozzle mounted centrally in the casing and formed so as to deliver a fine water cooling spray peripherally to encounter the casing wall circumferentially, means at the bottom of the casing for collecting the cooling water that runs down the casing wall, means for cooling the water so collected and returning it to the nozzle, a pipe for delivering the steam to the top of the cooler casing so that it passes through the cooling spray and means for collecting the mixture of steam and condensate beneath the spray for delivery to the compressor.

6. A system according to any one of the preceding claims, in which the compressor is a rotary compressor.

7. A system according to Claim 6, in which the compressor is a turbo-compressor.

8. A system for recovering waste heat substantially as hereinbefore described with reference to the accompanying drawings.

9. A method of recovering waste heat from the exhaust steam of apparatus operated by a steam raising boiler, the method comprising partially cooling the exhaust steam by means of a cooler, compressing by means of a compressor the paritally cooled steam to a pressure lower than that at which the steam raising boiler operates so as to liquify the steam by compression, feeding the water so obtained back to the boiler and maintaining a surrounding atmosphere consisting of flue gas from the boiler, or a mixture of such flue gas and air or plain air preheated by flue gas about the cooler and compressor, to reduce loss of latent heat.

10. A method according to Claim 9, in which the said water from the compressor is delivered back to the boiler by way of a tank for receiving the water from the compressor and a feed pump both maintained in the said surrounding atmosphere.

11. A method according to Claim 9, in which the cooler delivers the steam in the form of a mixture of steam and condensate at substantially atmospheric temperature and atmospheric pressure and in which the compressor increases the mixture temperature converting it substantially into water at approximately 270"F (132.2"C), the flue gas or admixture of flue gas and air being maintained at a temperature at least as high as the temperature of the saturated steam in the boiler, to prevent the loss of latent heat by radiation.

12. A method substantially as hereinbefore described with reference to the accompanying drawings.