Equipment Performance Condenser Performance

The vacuum at which a turbine exhausts is determined by the condenser. In the condenser the large quantity of heat remaining in the turbine exhaust steam has to be transferred to the circulating water. To make heat flow across a heat transfer surface a temperature gradient must exist, in a condenser the temperature of the condensing steam must be higher than the temperature of tlie circulating water. To obtain he highest operating efficiency this temperature difference between the exhaust steam and the circulating water must be kept as small as possible thus ensuring the lowest temperature of condensation and hence the best vacuum for any circulating water temperature. On the outside of the condenser tubes the temperature of condensation remains almost the same over the whole of the steam space except for the air cooling portion of the condenser where the air and vapour mixture is flowing towards the air extraction pipes. On older condensers there might be a pressure drop between the exhaust steam inlet and the air cooling section, but more modern condensers have wide steam lanes arranged to give the steam easy access to all parts of the tube surfaces and the whole steam space is virtually at one pressure and temperature. Inside the tubes the circulating water temperature rises throughout its passage through the condenser, the rise depending on the amount of h-at being rejected and the amount of circulating water being used. The all important temperature difference between steam and circulating water will therefore vary, being relatively large at the circulating water inlet and small at the circulating water outlet. The very simple diagram given at Fig. 38.

depicts the temperature gradients in a condenser, the vertical scale is tempe- rature and the horizontal scale represents the length of the circulating water path through the condenser. The vacuum in the condenser of Figure-17 has been assumed 7 to be 0.90 Kg/cm so that the temperature on the steam side of all the tubes would be about WC. On these diagrams the vacuum temperature is usually given the symbol. The circulating water enters the condenser at 25° C (t.) and leaves the condenser at 33°C (tJ. The rise in temperature of the Circulating water is not uniform throughout its traverse of the condenser because of the diminishing temperature gradient between steam and water. At the circulating water inlet the temperature difference is 15°C. The amount of heat transferred per square meter of tube surface is proportional to the temperature difference, so that at the circulating water inlet where the temperature gradient is at its highest the heat transfer will also be at its highest and the rise in circulating water inlet (where the temperature gradient is at its highest the heat transfer will also be at its highest and the rise in circulating water) temperature per meter length of tube will be at its highest as well. As the circulating water is approaching the condenser outlet at 33° the temperature gradient is now only 7° so the heat transferred per square meter of tube surface will be reduced correspondingly, in fact it will be only a little more than one third of what it was at the circulating water inlet. As a

consequence the circulating water temperature rise per meter run of tube gets smaller and smaller from inlet to outlet and results in the curve of circulating water temperature of Figure-17. This phenomenon makes it rather difficult to measure directly, or visualize, an average temperature gradient. However, the three temperatures which are available on the instrumentation, the circulating water inlet and outlet temperatures and the condensing temperature corresponding to the vacuum, can be combined to give the required average or log mean tempe- rature difference and is normally given in the following form:

Writing down the numerator in full shows that it is simply the temperature rise the circulating water

Q1, is sometimes called the initial temperature difference and (L is called the ter-

minal temperature difference. The terminal temperature difference can be used as a guide to condenser performance, particularly if there is little variation in circulating water quantity, but the log mean temperature difference is the real driving force behind the heat transfer and as previously stated this must be kept as low as possible in order to achieve the lowest heat rejection temperature, which is the same as saying that the heat transfer across the tube surfaces must be as high as possible for best results.

Heat transfer is affected by the cleanliness of the tube surfaces and by the amount of air present in the steam space. These are the items which should be carefully controlled to obtain best performance. In addition heat transfer is affected bv the velocity of the circulating water through the tubes and also by its

temperature. These two effects rather complicate the checking of condenser performance because unless they are taken into account it is difficult to assess the cleanliness of the condenser. The effects the normal variations in circulating water quantity and temperature will have on heat transfer rate, log mean temperature difference and vacuum will therefore be considered. If these are understood it should be possible to assess the cleanliness of a condenser under various operating conditions.

Causes of Poor Condenser Performance

Should the continuous checking of condenser performance indicate an increasing terminal temperature difference, then there must be some corresponding worsening of heat transfer rate. The main causes of poor heat transfer are usually:

1. Contamination of the circulating waterside of the tubes by slime and dirt. 2. Excessive air in the steam space.

Every effort should be made to keep the tube surfaces as clean as possible and the methods will depend on the nature of the circulating water itself. The source of most tube fouling is organic slime which often also helps mud and acale to adhere to the tubes. The most effective method of dealing with the slime is by intermittent chlorination of the circulating water to kill the algae. The frequency of chlorine injection will depend on the rate at which the slime is forming, which depends on the composition and temperature of the circulating water. Other sub- stances besides the algae will absorb the chlorine and the dose should allow for this. Chlorination can control organic contamination but not inorganic deposits and if a condenser is prone of fouling which can only be removed by mechanical means, such as 'bulleting', then it is necessary to have some sort of programme of condenser cleaning to maintain an optimum vacuum. This programme will depend on whether the condenser has divided water boxes or not so that part of

it can be isolated and cleaned while on load, otherwise an off-load period will be required. If scale forms in the tubes, which it might in cooling tower systems, it may be necessary to resort to acid cleaning. Condenser cleaning can be very costly in manpower and it is obviously preferable to prevent tube fouling and obviate cleaning. However, should this prove impossible, a well thought out cleaning programme will probably pay handsomely in reduced fuel consumption owing to better vacuum.

Effect of Air in Condenser Steam Space :

Air is one of the main causes of poor vacuum. As parts of the turbine itself and the low-pressure heaters are below atmospheric pressure during normal operation there are innumerable places where a leak would allow air to be sucked into the steam space. Air in the steam space reduces the heat transfer rate, which causes the condensing temperature to rise in order to transfer the heat across the tubes, and so results in a worse vacuum. In this respect air has just the same effect as tube fouling. If the performance of a condenser should deteriorate then the first job is to decide if the cause is excessive air leakage, some modern sets will have an air metering device on the air outlet from the air pumps, in which case it is an easy task to see if the air discharge has increased above normal. Older plants will not have this facility, but if the air ejector discharge is reasonably accessible for the installation of a simple metering orifice and manometer, it would probably be worthwhile making one for use when air leakage is suspected. The air ejector or air pump manufacturer would probably advice on the size of orifice best suited to his equipment, or even supply one. Failing the direct metering of air handled by the dir removal equipment a rough check on air quantity could be obtained by bringing into service the standby ejector. If there is an appreciable improvement in vacuum then it is apparent that the air leakage is too great for one ejector to handle efficiently. Alternatively, the air suction valve ot the running ejector could be closed and the rate of loss of vacuum quickly ascertained before re-opening. This pre-supposes that the normal rate of loss of vacuum without an ejector in service is known.

There are other methods of differentiating between poor vacuum caused by air leakage and by fouled tubes. Excessive air under cools the condensate, that is lowers the temperature of the condensate leaving the condenser. It also lowers the air suction temperature. To understand the reasons for these two effects it is necessary to think rather deeper into what happens on the steam side o^ a con denser.

Dalton's law of partial pressures states that if two gases are mixed in a vessel then the pressure of the mixture will be the sum of the pressures each gas would have if it occupied the vessel alone. For example, if a condenser shell filled with steam alone had a pressure of 0.1 Kg/cm and the same vessel tilled with air had a pressure of 0.15 Kg/cm then if those quantities of air and steam were mixed together and occupied the same vessel the total pressure would be 0.25 Kg/cm . The temperature of condensation depends on the partial pressure of the steam and not on the total pressure of air and steam together. However, at the inlet to the condenser the ratio of steam to air will be so great that the partial pressure of steam, would be about 0.9 percent of the total pressure so the air pressure can be ignored and the temperature can be said to correspond to the total pressure, which it does within a few hundredths of a degree. As steam flow through the condenser proceeds, the steam is condensed but the air remains intact, and at some point the mixture would have a ratio to say, "f air 50 Kg/hr. to 500 Kg/hr. of steam, and as the air suction pipe is approached in the mixture might be 50/5G and half the total pressure due to Air and half due to steam.

The main thing to realize is that when there is air iii a condenser, heat transfer is impeded and the temperature on the steam side has to go up in order to have sufficient temperature difference between it and the circulating water to get the heat across the insulating barrier of air. This effect makes the vacuum worse. Because of the partial pressure of the air at the bottom of the condenser the condensate temperature is lowered below that corresponding to the total pressure, since it corresponds to the partial pressure of the steam only. This phenomenon of under cooling of condensate but could have the same effect of

worsening the vacuum. The air suction temperature is an even better indicator of excessive air than the under cooling of the condensate. With a completely air free system the temperature in the air suction pipe would correspond to the absolute pressure existing at the same point. With a reasonably air tight plant the air suction temperature would be within a few degrees of the vacuum temperature. The more the air leakage the lower will be the air suction temperature until it approaches within a few degrees of the circulating water inlet temperature.

Once it is established that air leakage into the condenser is excessive and is causing loss of vacuum and increase in fuel consumption, there remains the diffi- cult task of locating the leak and stopping it. The time honoured method of finding air leak was by means of offering a lighted taper to any likely place to see if the flame would be sucked in. Probably a better method is to spray some innocuous detecting volatile fluid in the vicinity of the suspected leak and watch for its appe- arance at the air discharge from the air pump or ejector. The detector on the air discharge could be a blow lamp drawing its air from the air ejector discharge pipe. The volatile fluid should be something capable of turning the blow lamp flame a tell tale colour. Another method is to introduce nitrous oxide to the suspected leak and detect its presence in the air discharge pipe with an infra red analyser. If there is a noticeable difference in air leakage quantity between full load and a much lighter load, it probably indicates air leakage into some steam space which approaches or exceeds atmospheric pressure at full load, the steam pressure rising with load and cutting down the air leakage.

MILL PERFORMANCE