GB2226962A

GB2226962A - Steam condensing apparatus

Info

Publication number: GB2226962A
Application number: GB9000071A
Authority: GB
Inventors: Leslie Michael Frase Coulthard
Original assignee: Birwelco Ltd
Current assignee: Birwelco Ltd
Priority date: 1989-01-06
Filing date: 1990-01-03
Publication date: 1990-07-18
Anticipated expiration: 2010-01-03
Also published as: WO1990007633A1; GB9000071D0; GB2226962B

Abstract

In a system in which exhaust steam is condensed by an air-cooled condenser (13, 15) and the condensate held in a receiver (17), a small portion of the exhaust steam is led to the receiver and brought into condensing contact with the condensate entering the receiver, so that the temperature of the condensate is raised to the equilibrium temperature corresponding to the pressure at the condenser inlet. Inefficiency due to over-cooling of the condensate is avoided. <IMAGE>

Description

STEAM CONDENSING APPARATUS This invention relates generally to steam condensing apparatus. More particularly, it relates to apparatus including an air cooled condenser of the kind in which the steam to be condensed flows through tubes whose external surface area is extended by being provided with fins over which air flows to remove the heat energy given up by the steam as it condenses. The invention is applicable primarily to apparatus wherein the pressure in the condenser is maintained at a sub-atmospheric value, to obtain high efficiency in, for example, a power generation system using steam.

The usual arrangement of an air cooled condenser comprises an array of a large number of individual finned tubular elements. Steam is led to such elements by a large capacity duct connected to the exhaust steam outlet of a turbine or other steam-expanding device. In a first set of finned tubular elements, the steam travels generally downwardly condensing as it does so, the condensate being collected by a manifold or manifolds at the lower ends of such tubular elements. The condenser may have a further array of tubular elements which complete the condensation of substantially all steam exhausted from the turbine, leaving only a trace quantity of steam together with any non-condensible substances such as entrapped air to be extracted by an appropriate extraction system so that the pressure in the condenser as a whole is maintained at the required sub-atmospheric value.Condensate collected in the manifold or manifolds is led to a receiving tank from which it is extracted and ultimately returned to the boiler of the system.

The nature of an air cooled condenser is that the array of finned tubular elements presents a certain resistance to flow of steam, so that a pressure drop exists through the condenser. The pressure at the inlet side of the condenser, i.e. in the exhaust steam duct leading from the steam expanding device, is higher than the pressure maintained at the outlet of the condenser, i.e. the condensate collection manifold. The pressure drop means that there is a difference between the water-steam equilibrium temperatures at the inlet and outlet of the condenser.Therefore as the steam condenses in the finned tubular elements and runs down such elements to the manifold or manifolds where it is collected, the temperature of the condensate is reduced to a value corresponding to the equilibrium temperature at the outlet of the condenser, below the temperature of the steam in the condenser inlet duct. This over-cooling of the condensate represents a small but significant loss of energy, which energy has to be put back into the condensate when it is returned to the boiler of the system.

In comparison, condensate from a conventional surface condenser is generally at a temperature which is not significantly lower than the temperature of steam at the condenser inlet.

It is broadly the object of the present invention to provide for an improvement in the efficiency of a system including an air cooled condenser, and more particularly to overcome the above described problem of over-cooling of condensate in the condenser.

According to the invention, we provide steam condensing apparatus comprising a duct for receiving exhaust steam from an expander; a number of air cooled condensing elements connected to said duct to receive steam therefrom; a manifold for collecting condensate from said condensing elements; a receiver for said collected condensate; means for taking a-portion of said exhaust steam to said receiver; means for steam sealing and developing a pressure head in the condensate to cause it to enter the receiver, and means for causing condensing contact between the condensate entering the receiver and said exhaust steam portion, to condense said steam portion and raise the temperature of the condensate in the receiver.

The present invention, by taking some of the exhaust steam to the receiver, establishes pressure and temperature conditions in the receiver substantially equal to those in the exhaust steam duct. The temperature of the condensate led from the manifold to the receiver is raised to a temperature which is not significantly lower than the water-steam equilibrium temperature at the pressure of the exhaust steam in the duct leading to the inlet of the condenser, substantially eliminating the above described loss of energy due to overcooling of the condensate in the air cooled condensing elements.

Conveniently the condensing contact between the condensate and exhaust steam portion may take place in the condensate receiver. The necessary intimate contact between the steam and condensate for effective condensation to occur when there is only a small temperature difference between the liquid and vapour is conveniently achieved by causing the condensate to enter the receiver as a jet or spray.

A conventional surface condenser is generally disposed immediately adjacent the exhaust of a turbine, so that there are no losses involved in transmitting the exhaust steam to the condenser. On the other hand, an air cooled condenser is generally a fairly large piece of equipment, requiring to be disposed outside the building in which the rest of the steam plant is accommodated and necessitating a long duct leading from the turbine exhaust to the condenser. A small but significant pressure drop takes place in such a duct, leading to a further drop in condensate temperature and thus in the efficiency of the system in which the condenser is incorporated.

To maximise recovery of this temperature drop, the portion of the exhaust steam is preferably taken from the duct at a position as far upstream therein as possible.

The invention also provides a method of condensing exhaust steam from an expander by use of an air cooled condenser, wherein collected condensate from the condenser is brought into condensing contact with a portion of the exhaust steam to condense the latter and raise the condensate temperature.

In some previously known air cooled condenser installations a relief pipe may be provided from the condensate receiving tank to the inlet of the condenser, to permit escape from the tank of flashed steam which might be generated under certain transient conditions.

However, there is no flow of steam through such pipe into the receiving tank, because the receiving tank is liquid sealed in this case to prevent condenser by-pass.

Therefore such an arrangement has no significant effect on the temperature of the condensate.

These and other features of the invention will now be described by way of example with reference to the accompanying drawings, of which: Figure 1 shows diagrammatically a condenser system embodying the invention; Figure 2 shows a modification of part of the apparatus.

The illustrated system comprises a duct 10 which is arranged to be connected at 11 to a steam expanding device such as a turbine, to receive exhaust steam therefrom. An air cooled condenser arrangement is indicated generally at 12, comprising a first series of condenser elements 13 which receive steam from the duct 10. The elements 13 comprise arrays of tubes provided with external fins to increase their surface area and provide for effective heat transfer to the surrounding air. The condenser elements 13 connect at their lower ends to a manifold 14 to receive condensate therefrom, and any uncondensed steam and non-condensible gases. The air cooled condenser 12 further comprises condensing elements 15 which extend from the manifold 14 and are together connected at 16 to an air extraction system, usually a pump or ejector. The condenser elements 15, called the dephlegmation section of the condenser, substantially complete the condensation of the steam leaving only non-condensible gases with some trace steam to be extracted by the air pump or ejector to ensure that the condenser pressure is maintained below atmospheric pressure.

A receiver 17 is provided to receive condensate from the manifold 14, and hold such condensate ready for ultimate return to the boiler of the steam system, not shown, by a condensate extraction pump 18. Condensate enters the receiver 17 by gravity from the manifold 14, through a pipe 19.

According to the invention, a pipe 20 is provided to take a small proportion of the steam flowing in duct 10 to the receiver 17. The condensate entering the receiver from pipe 19 is delivered into the receiver from a spray arrangement 21, so that the condensate is brought into intimate contact with the steam in the receiver, thereby condensing the steam and bringing the condensate up to the equilibrium temperature corresponding to the pressure prevailing in the receiver 17. A pipe 22 leads from the receiver 17 to the manifold 14, for any excess of the steam and the non-condensible gases carried thereby.

Such excess steam, and non-condensibles, are dealt with by the dephlegmation condenser elements 15 and air extraction system, as are such components entering the manifold 14 from the condenser elements 13. It will be appreciated that flow of steam through the pipe 20 into the receiver will occur because of the prevailing pressure in the duct will be higher than that in the manifold 14, by reason of the pressure drop occurring in the condenser elements 13. The receiver pressure is held at a value close to that in the duct 10 by suitably restricting the pipe 22: it may be advantageous to provide an adjustable valve in the pipe 22 for this purpose.

The pipe 19 incorporates a water seal 19a. This prevents the possibility of flow of steam directly from the pipe 20 to the manifold 14 by way of the receiver 17, which could occur due to the difference between the pressure in the duct 10 and that in the manifold 14, and which if it occurred would tend to enter the condenser elements 13 and trap non-condensibles therein. Sufficient head in the condensate from the manifold 14 for it to enter the condensate receiver 17 and to ensure the satisfactory operation of spray arrangement 21 if fitted is provided by water present in pipe 19.

In use of the system described above, the temperature of the condensate stored in the receiver 17 is substantially equal to the equilibrium temperature corresponding to the pressure of the exhaust steam in the duct 10. Thus the energy loss and loss of efficiency in the overall system, resulting from overcooling of the condensate in the condenser elements 13 and 15, is avoided.

In the above described example, the condensate is brought into condensing contact with the portion of the exhaust steam which has entered the receiver through the pipe 20 by sprayers 21 in a part of the receiver 17.

Figure 2 shows a modification wherein, instead of a spray or jet arrangement, an arrangement comprising a cascade of perforated trays 23 is disposed in a part of the receiver 17. Condensate entering the receiver passes over the trays 23, which provide the necessary contact between steam and condensate over a large surface area.

In a further modification, instead of causing condensing contact between the steam from the pipe 20 and the condensate actually in the receiver 17, a separate device could be provided between the manifold 14 and receiver 17 for this purpose. Alternatively, a suitably enlarged section of pipework between the manifold 14 and receiver 17 could provide for the necessary condensing contact between the steam and condensate.

Figure 2 of the drawings shows part of the apparatus including a further modification from the apparatus shown in Figure 1. There is shown a pipe 24 leading from the receiver, which pipe is connected directly at 25 to the air extraction system of the apparatus instead of being connected to the manifold 14 in the manner of the pipe 22 in Figure 1. Trace steam and non-condensibles remaining from the steam which has entered the receiver through pipe 20 are removed by the air extraction system together with such substances from the manifold 14 and condenser elements 15. The pipe 24 would be so dimensioned that the air extraction system is able to remove the non-condensibles from the receiver whilst the receiver pressure remains substantially equal to that in the duct 10.

By way of example, the amount of steam passing through the pipe 20 may be of the order of only 1% of the steam passing through the duct 10.

A further advantage of the invention is that it helps ensure that the oxygen content of the condensate held in the receiver is minimised.

Claims

1. Steam condensing apparatus comprising a duct for receiving exhaust steam from an expander; a number of air cooled condensing elements connected to said duct to receive steam therefrom; a manifold for collecting condensate from said condensing elements; a receiver for said collected condensate; means for taking a portion of said exhaust steam to said receiver; means for steam sealing and developing a pressure head in the condensate to cause it to enter the receiver, and means for causing condensing contact between the condensate entering the receiver and said exhaust steam portion, to condense said steam portion and raise the temperature of the condensate in the receiver.

2. Apparatus according to Claim 1 wherein said condensing contact between the condensate and exhaust steam portion takes place in said receiver.

3. Apparatus according to Claim 2 comprising a jet, spray, or perforated plate arrangement for condensate entering the receiver.

4. Apparatus according to Claim 2 or Claim 3 comprising means for taking trace steam and non-condensibles from said receiver directly to an air-extraction system of the apparatus.

5. Apparatus according to Claim 2 or Claim 3 comprising means for taking trace steam and non-condensibles from said receiver to said manifold and thence to an air-extraction system of the apparatus.

6. A method of condensing exhaust steam from an expander by use of an air-cooled condenser, comprising collecting condensate from the condenser and developing a pressure head in the condensate to cause it to enter a condensate receiver, taking a portion of the exhaust steam to the receiver, and causing condensing contact between the condensate entering the receiver and the exhaust steam portion, to condense said steam portion and raise the temperature of the condensate in the receiver.

7. Apparatus substantially as hereinbefore described with reference to the accompanying drawing.

8. A method of condensing exhaust steam, substantially as hereinbefore described.