By Grant Laidlaw
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If you picture the operation of a condenser, you will see that the liquid seals at the condenser outlet. This is a seal of pure liquid refrigerant. Air cannot be condensed in a normal refrigerant plant. Air is commonly termed a â€˜noncondensableâ€™.
Arthur asks: Grant we all know we need to draw vacuum and I understand that moisture can block a system but can you explain what air in a refrigeration system does. Looking at it I cannot see why air is a problem. Hi Arthur, Air is foreign in a refrigeration system. It may have been left by improper evacuation techniques (Either by a proper vacuum not having been drawn, or by parts of the system under evacuation having been isolated during evacuation through there being closed solenoid valves, gauge valve or check valves). Or it could be introduced by poor service techniques. Daltonâ€™s Law tells us that when gases share a space, each behaves independently of the other, but their partial pressure are summed to provide the total pressure of the system. In simple terms, this means that the condensing pressure will be increased for any combination of load and condenser water or air temperature. The compressor has to continuously work against a higher discharge pressure that is necessary. This means that it will continuously draw more electrical power than is necessary, and its working temperature will be higher than necessary. The higher working temperature is damaging to refrigerant and oil, as it can cause carbonisation of oil, and can start chemical reaction and breakdowns between these fluids.
As result of this, air moves into the condenser with the hot gas, but it becomes trapped there. Therefore the result of all air which may be present in the system is concentrated in the condenser. As condenser operation determines discharge pressure, this concentration increases the size of the problem. The discharge temperature increases possibly causing carbonisation of oil on the discharge valve reeds or discs. This carbon layer stops valves from closing properly, therefore reducing compressor efficiency. This increases the compression ratio and in turn increases the amperage drawn by the compressor. This in turn has the effect of adding additional heat to the system causing even higher temperatures and deduced C.O.P.
OTHER NON-CONDENSIBLES While we are discussing air, remember that gases such as dry nitrogen might have been used for pressure testing, and they equally will affect the working pressure of the system. SUBCOOLING In the case of an air-cooled condenser. The return bends are sometimes accessible, and refrigerant temperature at various spots across the condenser can be measured, using a surface contact thermometer. It is easy thus to identify the de-superheating, condensing and sub cooling zones of the condenser. The repeat temperatures coming up across the main part of the condenser are for example 45˚C. This is the temperature at which condensing is taking place. If the refrigerant is R22 and the system is free of air and other non-condensable, then a pressure gauge would indicate that condensing pressure is 1640 KPA(gauge). The other direct reading we can take is a liquid line temperature. Should we get for example 43˚C. Our temperature reading show 45˚C-43˚C=2K of sub cooling. But, let’s say that we measure a condensing pressure of 1770 KPA (gauge) at the same time as the above temperatures. That would tell us that we have non-condensable in the system. Reading from a R22 temperature pressure chart, the pressure is higher than indicated for our given temperatures. Jan asks: We have a service contract on a building which includes cooling towers, during an inspection by the client. Coming across the water bleed off asked me about it. My reply was met with unpleasant questions as to the cost of water and do I realise that this is Africa where water is scarce and impossibly expensive. As I do not know much on the subject I could not really explain, could you enlighten us on this subject. Hi Jan, This all revolves around keep the Total Dissolved Solids in the water to an acceptable level. If this is not controlled we begin to develop scale deposits. Scale is deposits of crystallized chemical salts, generally intimately bound up with dirt and other particulate matter into a matrix which adheres very strongly to wetted surfaces. The origin of scaling is the chemical salts present in the supply water fed to cooling towers to replenish the water lost by evaporation. Sometimes, the quality of the supply water contains only very small amounts of the types of dissolved salts which result in scale formation. In these cases, the nature of the supply water often tends to be corrosive to metal surfaces. Therefore, to anticipate future problems arising in a cooling water system, the
composition of the supply water must be examined. For practical purposes, supply water for cooling systems can be divided into three categories, measured by the amounts of dissolved chemical salts they contain. A further simplification is to consider only the dissolved calcium and magnesium carbonates [or carbonate equivalents] which provide the character commonly called hardness to water. The levels of dissolved salts [or solids] are measured in parts per million (ppm) or milligrams per litre (mg/l) which mean exactly the same thing for cooling water technology. The three categories are:Ca + Mg carbonates: Dissolved solids level: Less than 50 mg per liter, Water description:Fairly soft Potential for scale formation: Very little Less than 50 mg per liter Ca + Mg carbonates Dissolved solids level: 50 to 80 mg per litre Water description: Neutral Potential for scale formation Medium Ca + Mg carbonates Above 80 mg per liter Water description: Hard Potential for scale formation: High
For a more accurate prediction of scaling potential, other water measurements such as pH, alkalinity and temperature must be included. Problems caused by Scale Deposits A scale deposit build-up:
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Reduces heat transfer on critical heat-exchange surfaces Affects designed water flow rates Blocks up spray nozzles, screens, pumps, valves, pipes and tubes Reduces designed airflow through cooling towers. Promotes electro-galvanic corrosion sites on steel surfaces Fouls up measuring instrument sensors
Scale deposits, especially thick ones, are difficult and costly to remove. Using acid to dissolve scale always results in more or less corrosion of metal surfaces and other
machinery, thereby reducing the life of the cooling system. Even under experienced supervision, there is always a risk that acid cleaning can cause severe damage which is very costly in time and money to rectify. Dirt and scaling are two important contributors to inorganic fouling in cooling water systems. Another source of inorganic fouling can arise from the corrosion products of steel and other metal surfaces. Bleed-off is not used in any closed cooling water system unless there is something very unusual which causes progressive contamination of the circulating water. Evaporative cooling water systems, however, all need some form of bleed-off primarily to prevent the TDS. in the circulating water rising beyond manageable levels. Bleed-off also helps to keep dirt and dust levels from becoming a problem, although by itself it is seldom sufficient, and so regular cleaning out is almost always needed. The primary function of bleed-off is control of dissolved solids which cannot be removed by any form of filtration. These solids will continue to increase in the circulating water unless a sufficient amount is bled off to compensate for the TDS continuously introduced by the supply make-up water. Consider a simple example of evaporating 1 litre of water containing 100 ppm TDS in a container at an evaporation rate of 1 litre/hour. At the start there is 1000 ml of water at a TDS of 100 ppm. After 30 minutes 500 ml of water remain. The TDS has risen to 200 ppm After 45 minutes 250 ml of water remain. The TDS has risen to 400 ppm. After 52.5 minutes 125 ml of water remain. The TDS has risen to 800 ppm In this case the TDS rises increasingly rapidly with time (curve (a), chart B01a).
No top-up; no bleed-off
Top-up, then bleed-off Now suppose the container was continuously topped up with 100 ppm TDS make-up water to compensate for the evaporative loss of water, maintaining the volume at a constant level of 1000 mls. The TDS would then increase at a steady rate of 100 ppm every hour as shown by curve (b) see Chart B01b. If after 7 hours, a continuous bleed-off of water from the container is started that will get rid of the same amount of TDS that is coming in with the top-up water, then the TDS will remain constant, in this case at 800 ppm. Curve (b) now becomes curve (c). This is exactly what happens in an evaporative cooling water circuit where the bleed-off not only stops the TDS from going too high but also automatically holds the TDS at the optimum level for effective water treatment and minimum use of make-up water supply. This level of 800 ppm TDS is only an example to demonstrate why bleed-off is necessary and how it works. The actual required operating TDS in the circulating water of an evaporative cooling water circuit will depend on the nature of the make-up water available and the operating conditions inside the cooling system. Most normal evaporative cooling water circuits control TDS at approximately 1000 ppm. Bleed-off Methods Continuous bleed-off can only be used on evaporative cooling water systems which run at a constant loading or heat rejection, otherwise if the loading increases, the bleed-off will not be enough and the TDS will rise too high. Conversely, if the loading decreases then there will be too much bleed-off which is a waste of water and water treatment chemicals. Also, the TDS will drop which might cause the circulating water to become corrosive. In practice, continuous bleed-off should only be applied to small evaporative cooling water systems where a small amount of water wastage is not costly and where the circulating water at reduced TDS will not
be corrosive. The previous example of the small refrigeration plant could fit into this category as follows:Assuming that the plant runs at a continuous duty rate using 300 litres/hour of make-up water, this water is only enough to replace the water lost by evaporation. Therefore the situation is as represented by curve (b) on the diagram where the TDS will continue to increase and begin to cause problems. Also assume that the target for operating the circulating water is 1000 ppm TDS; this will be achieved after the system has run under load for 90 hours. To hold the TDS steady at 1000 ppm by continuous bleedoff, the amount of TDS bled off must equal the amount of TDS coming in. TDS coming in = 300 litres at 100 ppm = 30 gms of dissolved solids /hour. Each litre of circulating water at 1000 ppm TDS holds 3 gms of dissolved solids. Therefore, to get rid of 30 gms /hour of dissolved solids, 30 litres/hour of circulating water must be bled off continuously. Note:- this increases the amount of makeup water to 330 litres/hour, i.e. a 10% increase in water usage. There are two very important points to bear in mind for continuous bleed-off:-
The bleed-off must be located at a point where water will only flow through the bleed-off line when the circulating pump of the cooling system is running. 2. The water pressure at the bleedoff point must be at least 50kPA, otherwise it is likely to become partially or completely blocked up by dirt and other impurities in the circulating water. Continuous bleed-off can be and is used on cooling water systems like this one. However, the reality is that most cooling water systems do not run under reasonably constant loading but have varying loadings which in turn mean varying make-up water demands from zero to maximum. Under these conditions, an automatically controlled bleed-off is required, of which there are two types commonly used:-
Conductivity controlled Proportional to make-up water demand. Conductivity controlled Bleed-off
The electrical conductivity of water is proportional to the amount of dissolved solids in the water. As a rule of thumb, TDS ppm = 0.7 x conductivity as microsiemens/cm. A conductivity probe is installed to sense the conductivity [and therefore the TDS] of the circulating water. The probe is connected to a control unit which is set to the desired TDS level. When the TDS reaches slightly above this level, the control unit actuates a solenoid bleed-off valve which bleeds water to drain at a fairly high flow rate. This in turn causes an increase in the make-up water demand which, when mixed in with the circulating water, starts to reduce the TDS. As soon as the TDS reduces to the pre-set desired level, the bleed controller switches off the solenoid bleed valve and the TDS begins its gradual climb until the controller switches the solenoid bleed valve on again. This cycle repeats as often is as necessary to maintain the TDS at a constant level within a very small range. Bleed-off in Proportion to Make-up Water Demand Proportional bleed-off uses a pulse output water meter installed in the make-up water supply line so that each time a set volume of make-up water passes through the water meter, a signal is sent to the bleed controller which opens a solenoid bleed valve through a timer which holds the bleed valve open for a pre-set time. The result is that a set volume of water is bled off to drain in proportion to the amount of make-up water demand. Both conductivity controlled and proportional bleed-off systems operate more frequently as the cooling system works harder at higher loadings. Also, the conductivity type will hold the bleed valve open for as long as is necessary to bring the TDS of the circulating water down to the desired level. A further advantage of conductivity over proportional bleed-off is that the conductivity type will maintain the pre-set TDS in the circulating water even if the TDS of the make-up water changes significantly, whereas the proportional type would have to be re-set for any changes in the make-up water. One disadvantage of the conductivity type is that regular cleaning of the conductivity probe is vital, because even small amounts of fouling on probes can result in malfunctions. Proportional bleed-off is normally less costly and less maintenance-intensive.
There is one other type of bleed-off system sometimes used, which is simply a solenoid bleed-off valve actuated by a timer at regular intervals. This is really another type of continuous bleed-off because there is no sensing of either TDS or make-up water demand to alter the amount of bleed-off in accordance with varying loading on the cooling system. It is however very much better than no bleedoff at all, and will operate well enough on those cooling water systems which have little variation in loading. And Jan, there you have it, hope you can explain the situation in more depth to satisfy your client. Thanks to everybody for the overwhelming response. I receive on average, over sixty questions per month and cannot publish all of them. But keep them coming, as I may answer you directly. Looking forward to hearing from you. Grant Laidlaw REFERENCES: SAIRAC merSETA training