Cooling Water System - 1

Learning Objectives

Most of the all process industry requires the cooling tower for certain application depends
upon industry and uses size of cooling tower may varry. For bigger and huge capacity
cooling tower high capital investment and high operation of cost involve. To select proper
cooling tower and execution and maintenance understand the cooling tower and its piping
is very important in this chapter we will study consideration of piping routing and layout of
cooling water piping.

The cooling water systems which are commonly used in practice according to the availability of the water are listed below:

Once — through or open cooling water system.

Re circulating or closed cooling water system.

Combination of Once * through and re circulating or mixed cooling water system.

Types of Cooling Water Systems

• Open System

In the open cooling water system, all the required quantity of cooling water is withdrawn from the source (sea, river, lake or well water) for entire plant cooling duties. In this system, the water is drawn directly from the upstream side of the river, pumped through the condensers and then discharged to the down ward side of the river at temperature 5 to 100c in excess of the inlet temperature. The temperature of the discharged water should be kept within safe limits to prevent harm to fishes. The limit of discharge water temperature is specified by the Fisheries Board. The arrangement of the open system is
shown in the figure 4.1.

The position of inlet and outlet should be chosen in such a way that there should not be any re circulation of hot water, which impairs the efficiency of the cooling water users. Therefore, the distance between the inlet and discharge points should be as large as one km or more. This type of cooling water system can be used only when required quantity of water is available throughout the year.

Plants on tidal waters (using water from the tides or sea) have a special problem in the construction of the cooling water system and in avoiding recirculation of the cooling water from the outlet back to the intake. In many plants, the intakes and outlets are separated by as much as 3 kilometres.

In tidal waters, the water flows in one direction for some time and in another direction for some time, therefore it requires special arrangements for the change in flow direction. Stations taking water from river or tidal sources usually have the ends of both section and discharge pipes submerged below the lowest recorded tidal level. The water circulation system will operate at all times and at all states of tide if the distance between the lowest level and highest point of the circulation water system does not exceed 10 metres.

• Closed System

The re circulating or closed cooling water systems operate in a closed loop with fresh water make-up which constitutes only a small fraction of the cooling water circulation.

Closed systems are of two types- Water or Air-cooled type. Water cooling type employ cooling towers or cooling ponds. Air cooling type employ direct dry type cooling tower system or indirect air cooling system.

When enough quantity of water is not available for cooling requirements from rivers, the closed type system is universally adopted.

In the closed system, the hot water coming out of the cooling water users is cooled either by sparging in the pond or passing through the cooling tower instead of discharging to the downward flow of the river.

The quantity of water required is from the river during flood period or when sufficient water is available with required purity and same water is used again and again for condenser by passing through the cooling towers. Such arrangement of cooling system is shown in figure 4.2.

With this system of cooling, an external source of water is needed to replace tower evaporation and carry over varies between 2 to 5% of that circulated, depending upon the design of the tower.

• Mixed System

A combination of the once-through and re-circulating or mixed cooling water system is sometimes employed to carry-out process cooling via the closed cooling water, which in turn is cooled by a open cooling water circuit in a suitable heat-exchanger. The arrangement of system is shown in figure 4.3 (a).

The main advantage in this system is the flexibility of operation it gives to the process side which is on the closed loop.

Another type of mixed system uses river water as well as the cooling tower simultaneously. This system overcomes the difficulty of re-circulation and meets the requirements of the Fisheries Board on a fairly small river. The arrangement of system is shown in figure 4.3 (b).

A part of the water from the cooling water users is discharged directly into the down stream of the river, and part of the water is pumped to the cooling tower, where it is further cooled and then discharged to the downward side of the river. In this way the water discharged to the river is maintained at a  suitable temperature and re-circulation troubles are eliminated. The advantages of this system are: -

(i) The size of the cooling towers can be reduced where the site area is limited.

(ii) The quantity of cooling water required is reduced as re-circulation is eliminated.

(iii) The turbine plant efficiency is increased.

(iv) This system should be adopted only when there is a possibility of recirculation
and it is necessary to meet the requirements of frsheries board.

Engine cooling

After the above discussion on cooling water systems and cooling system principles, we now come to the specifics of cooling of engines. The necessity of cooling: Part of the heat developed during the combustion in engines flows from the gases to the cylinder walls, raising their temperature. If, with an un-cooled piston, the wall temperature is allowed to rise above a certain limit, about 300 0F, the oil that lubricates the piston begins to evaporate rapidly, and both piston and cylinder may be injured. Warping of valves and pistons takes place. The proper cooling of the engine is absolutely necessary
to extend the life of the plant. At the same time high local temperatures in certain parts of the engine, such as the cylinder head and piston, may cause excessive stresses and cracking of these parts. Additional heat is developed through friction between various rubbing surfaces, chiefly between the piston and piston rings and the cylinder walls. With oil-cooled pistons the limit for a safe cylinder wall temperature is considerably higher.

The heat generated in an engine cylinder by the combustion of the fuel varies from about 6,000 to 10,000 Btu per hp-hr. Tests show that from 25 to 35 percent of this heat in water-cooled and about 15 to 25 percent in air-cooled engines finds its way into the cylinder walls and must be carried away. If some means were not provided for the removal of this heat, the temperature of the metal would begin to approach that of the combustion gases as they leave the engine cylinder, or about 800 to 1200 0F. Therefore, this heat removal, or cooling, problem is so vital that, if not taken care of properly, it can cause more engine trouble than any other phase of engine operation. The exit temperature of the cooling water must also be controlled. If it is too low, lubricating oil will not spread properly and wearing of piston and cylinder takes place. If it is too high, the lubricating oil burns. Therefore, the maximum exit temperature of the water is limited to 70 0C. Constant cooling water flow rate rises the exit water temperature with the increase in load or vice versa, when inlet water temperature is constant. Therefore, a control on the flow of the cooling water is necessary according to the load conditions on the plant.

Heat Transfer

The three means of heat transfer conduction, convection, and radiation are used in cooling engine cylinders. Conduction plays an important part in carrying the heat through the metal walls and the thin layers of stagnant gas and water in contact with the walls; the rest of the heat is exchanged partly by radiation but chiefly by convection.

The heat flow between the two fluids separated by a metal wall can be best explained using the figure 4.4. The temperature ta of the gas at a point in the interior of the cylinder gradually falls to the value tb at the surface of the inert gas film. The thermal resistance of this film is very great and a great temperature head, tb - tc, is required for conduction. The temperature head required to cause heat flow through the metal wall is tc-td The temperature head required for conduction through the outside film is td - te. Its value is comparatively small if the cooling fluid is water and large if it is air. Finally, the temperature of the cooling liquid drops to tf at some distance away from the wall.

The heat flow per unit area of surface in contact with hot gases on one side and the cooling medium on the other side thus depends upon the inside film coefficient h1, the conductivity k of the metal, thickness L of the cylinder wall, on the outside film coefficient h2 between the metal and cooling medium, and the difference between the gas temperature and the cooling-medium temperature.

The value of the outside-film coefficient depends in the first place on whether the outside surface of the cylinder is cooled directly by air or by liquid. The  value of Fit is rather small if the cylinder is cooled by air and considerably higher when it is cooled by water.

For reference the thermal conductivity k of metals is given in Table — 1. However, the coefficient of conductivity k affects the heat flow to every small extent, much less than the film coefficient hi and h2.

Table - 4.1 gives also the specific heats and coefficients of linear expansion of metals used for pistons, cylinders, and other engine parts. The temperature t2 of the inside surface of the cylinder wall is not constant during a cycle but fluctuates following the variation of the gas temperature. The temperature of the inner surface goes

up to tmax during combustion, and drops to tmin toward the end of the suction stroke. However, these fluctuations are not great: for a two stroke oil engine at full load the fluctuation above the average value, tmax - t2 is about 25 0F and that below it, t2 - tmax is about 15 0F. In a four- stroke engine the downward fluctuation will be about the same as the upward one owing to cooling during the suction stroke. This gives a temperature range of about 50 0F. The temperature fluctuation does not penetrate deeply; 3/8 in from the surface the range of fluctuation is less than 1 0F.

Heat Flow

When the rotary speed of an engine increases, the duration in seconds of all events of each cycle decreases. However, the increased piston speed creates a greater turbulence, slightly increasing the heat flow, and as a result the percentage of heat of the fuel rejected to the jacket increases slightly with the engine speed.

Tests have shown that the percentage of jacket loss is nearly independent of the engine load and decreases slightly with an increase in the cylinder diameter.

Water Circulation

Quantity - The quantity of water that must be circulated depends upon the initial temperature and the desired temperature rise of the water. The initial temperature depends upon the atmospheric conditions, either directly, as in marine engines, or indirectly, if a re-cooling system is used and the water is re-circulated over and over. In order to avoid excessive heat stresses, the temperature difference between the incoming and outgoing water should be about 20 0F in small and medium sized engines and slightly less in large engines. The temperature of the outgoing water was usually not allowed to go above 140 0F. For engines with a closed system a maximum temperature of 160 to 180 0F was allowed. In automotive engines the cooling water often reaches the boiling point, about 212 0F, without damage to the engine, but thermostats are usually set for 180 0F. The results of investigations of cooling by evaporation discussed below with jacket water temperatures from 215 to 250 0F, will probably change the above limits.

If an engine is cooled by untreated water, which always contains dissolved salts and other foreign matter, the temperature should be kept low enough to prevent the precipitation of impurities and the formation of scale. If an engine uses salt water in the cylinder jackets, the temperature of the outgoing water should not exceed 110 to 115 0F.

The water is usually circulated through the lubricating oil cooler, through the cylinder jackets, then to the cylinder heads; after this, in large engines, a branch line leads water to the exhaust valve cages. Pistons are usually cooled from a separate pipeline.

The quantity of water G that must be circulated, gallons per hour, is

Where Q = the amount of heat rejected to the cooling water, Btu per hr

t1 = the temperature of the incoming cooling water, degrees F

t2 = the temperature of the outgoing water.

For average conditions, the heat flow to the water jacket, in unsupercharged engines, is about 2600 Btu per hp-hr for large engines, increasing to about 3000 and to 3500 Btu per hp-hr for small and less efficient engines. In a supercharged engine the total heat flow, Btu per hr, is about the same as in an engine with natural aspiration of the same dimensions and speed. However, since a supercharged engine develops from 35 to 50 percent more power, the specific heat flow, or heat flow referred to I hp-hr. is correspondingly smaller, about 1850 to 2300 Btu per hp-hr. The heat flow to lubricating oil coolers, where these are used, is about 100 to 200 Btu per hp-hi, depending upon the
amount of oil circulated and friction losses in the bearings.

Excessive water circulation resulting in low final water temperature is not desirable, since it will increase the fuel consumption and decrease the useful power.

A low cooling water temperature increases the viscosity of the lubricating oil and, consequently, the piston friction. The difference between friction loss at high and low jacket temperature may amount to as much as 8 percent of the power, if the piston is large and heavy, and drops to about 4 per cent, if the piston has small bearing area and weight.

Temperatures

Formerly it was considered good practice to operate all engines so as to maintain a moderate outlet water temperature of some 120 to 1400F and not over 160 or at the most 180 0F, when using an enclosed cooling system. The object in using low water temperatures was mainly to reduce the formation of scale in the cylinder jackets. Scale is particularly dangerous in horizontal engines.

The dew point of the water vapour in the exhaust gases depends upon the pressure and hydrogen content of the fuel. A considerable condensation of water is bound to occur on the cylinder walls. The water causes corrosion, which seems to be one of the main causes of cylinder wear.

Numerous tests conducted since 1937 and careful observation of a number of
installations have shown that permitting the water temperature to rise above the boiling temperature, to about 220 to 250 0F, gives very important and far reaching advantages:

1. It eliminates the condensation of the water vapour contained in the products of combustion, thereby

(a) preventing, or at least reducing materially, the washing off of the lubricating oil film from the cylinder and piston ring surfaces and prevents the formation of sulphuric acid from sulphuric dioxide, often contained in the products of combustion; these two factors reduce the wear of the cylinder, piston rings, and valves
considerably under certain conditions down to one eighth of the usual amount;

(b) eliminating crankcase condensation and sludging of the lubricating oil.

2. It lowers the viscosity of the cylinder lubricating oil; this reduces the mechanical losses and raises the mechanical efficiency of the engine, thereby permitting a lower fuel consumption per horsepower hour.

3. It reduces the amount of water which must be circulated, because part of the water is evaporated in the jackets and the cooling effect of each pound of evaporated water is about 970 Btu per lb, instead of the 10 to 20 Btu per lb absorbed by the water due to temperature difference; this fact reduces the fuel consumption still further.

4. It increase the temperature difference between the cooling water and the air to which the heat is rejected and, if a radiator is used, a considerably smaller radiator surface and a smaller fan will do, and fuel will thus be saved.

5. At a full load, the total fuel saving may reach 10 percent. Quite naturally, with an increase of the jacket temperature, the heat absorbed by the jacket from the gases in the cylinder decreases because of a smaller temperature difference, for a typical engine. The heat not transferred to the jacket water increases the heat carried away by the
exhaust gases and raises their temperature.

The use of higher jacket temperatures, up to 250 0F, does not require a change in the construction of the engine or in the lubricating-oil specifications. However, it is desirable to have wide water passages and to eliminate possible vapor pockets.

Other Liquids

The use of ethylene glycol, or Prestone, which at atmospheric pressure has a boiling temperature of 387 0F, instead of water, gives the same advantages, except No. 3, if the jacket temperature is maintained at the same level. The specific heat of Prestone is 0.675 Btu per lb at 212 0F and about 0.775 Btu per lb at the boiling temperature.

The pistons of big double-acting engines are sometimes cooled by circulating oil instead of water through them. Pistons of high-speed single acting engines are sometimes cooled by a jet of oil directed toward the underside of the piston top.

 

Construction Features of Engine Parts Requiring Cooling

• Cylinders

In some small and medium sized engines the water jacket is cast together with the cylinder. In many automotive and in all larger engines the cylinder is formed by a cast-iron liner inserted into a cast iron jacket.

The water space between the cylinder proper or liner and the water jacket is made such as to obtain a fair velocity of water circulation, at least 5 ft per mm in stationary engines and up to 60 ft per mm in automotive engines.

• Cylinder Heads

In a four stroke engine the heat carried away by the water that cools the head comes from two places: from the bottom plate, which forms the upper wall of the combustion space, and from the exhaust passage and exhaust valve, if the latter is not water cooled.

In cylinder heads good cooling is obtained by (1) eliminating air and steam pockets, (2) maintaining, as far as possible, uniform water velocities in all parts of the water space, and (3) avoiding narrow water passages that are apt to become close due to formation of scale and thus disturb proper circulation. Exhaust valves- need cooling only in large engines. With the use of heat resisting steels or special cast iron for valve heads, even large engines are built with uncooled exhaust valves but then have water cooled valve cages or valve seats.

• Trunk Pistons

Dissipate heat to the cylinder walls and to the lubricating oil quite satisfactory and some engine builders therefore dispense with special cooling with pistons up to 22 in. in diameter. However, most engines from 6 in up have oil cooled pistons.

Pistons of many small and medium sized diesel engines at present are cooled by lubricating oil delivered in comparatively large quantities through the rifle bored connecting rods. In some engines the oil is admitted to one side of an enclosed space under the piston crown and discharged on the opposite side and allowed to flow down into the oil sump. In other engines the oil is discharged in the form of a jet from the top of the connecting rod and impinges on cooling ribs on the inside of the piston crown.

• Barrel Pistons

With the improved design of the water circulating system some large engines now use water for piston cooling. However, the majority uses oil. Piston rods in cross head engines are cooled by water or oil that is admitted through the cross head to the pistons.

There are two possible cooling mediums available for engine either water or air-cooling.  Air-cooling is reserved for small engines which do not develop large amounts of power. Only engines designed and properly equipped for air cooling can be used as such they generally incorporate ducting and finned flywheels to promote a rapid flow of air around the hot spots of the block. In areas of heavy water pollution, the simple air-cooled engine has big advantage in turns of ease of maintenance and lack of corrosion. The vast majority of engines nowadays are water cooled using heat exchanges or keel cooling. There can be two types of cooling water circuits.

• Direct or Raw Water Cooling

The ultimate short term money saver is the direct cooling system which takes raw water directly out of the sea or river, circulates it around the engine block, and finally discharges back to the source. There are, however, many drawbacks to this system. It is not possible to use a standard thermostat to allow the engine to run at its correct temperature, of around 80 — 85 0c as this will eventually cause severe blockages to the water passages from impurities building up on the walls.

The usual recommended workship temperature for a direct cooled engine is around 54 0c which causes some sluding of the oil as it can never achieve its optimum working temperature; the result of which is increased engine wear. But the most important point is that corrosion products from the hot raw water will continually attack the engine internals as it is impossible to add inhibitors to the water. The engine cooling system
must also be drained during the winter months to prevent damage from freezing as antifreeze cannot be added.

Indirect or fresh water cooling : None of the above mentioned problems occur with indirectly cooled engines which have a separate freshwater supply within the engine block in the manner for which the engine was designed. This means that antifreeze and corrosion inhibitors can be added to the freshwater supply preventing the problems which beset the raw water cooled engines. They can also use a standard thermostat and run at their correct designed temperature for maximum efficiency and long life. The modest extra cost is therefore well worth considering. Additional equipment required for indirect cooling includes a heat exchanger (often combined with the water cooled manifold ) and an engine oil cooler if required.

• Circulation

Two methods of water circulation are in use gravity circulation and forced circulation. Gravity circulation, also called thermo siphon circulation, is based on the fact that when water is heated its density decreases and it tends to rise, the colder particles sinking to take the place of the rising, warmer ones. Circulation is obtained if the water is heated at one point and cooled at another. Gravity circulation is used only in small engines - seldom in those of more than 30 hp. Figure 4.5 Shows the gravity a circulation arrangement for a small horizontal engine.

Water heated in the cylinder jacket flows to a tank where it is cooled by radiation and convection, gradually descends to the bottom and flows back to the engine. In an automotive engine to obtain proper water circulation the connections between the engine jacket and the radiator must present small resistance to the water flow and be wide, short and have as few bends as possible. Even under favorable conditions circulation is slow, especially when the temperature difference is small, as at light loads. At heavy loads the jacket heat may exceed the heat dissipated by the radiator and the water in the jacket is apt to boil.

This system is used only is smaller engines where simplicity is of importance. Most engines have forced circulation by pumps, of either the centrifugal or the plunger type. The advantage of the forced circulation is the ease of controlling the jacket water temperature. This may be accomplished either by regulating the opening of the valve between the pump and the engine or by regulating the water discharge valve of individual cylinders.

Evaporative Cooling

If the water in the cylinder jacket is allowed to boil, 1 lb of evaporated water will absorb heat equal to the latent heat of vaporisation, or about 970 Btu. This is from 24 to 48 times more than the heat carried away by 1 lb of circulating water with a temperature rise of 20 to 40 0F. Neither pump nor radiator being required, this system has the advantage of simplicity and is used for small stationary and tractor engines. The water jacket is made large at the top, forming a so called hopper. The quantity of water in the hopper must be sufficient to run the engine for several hours without the addition of
water. The evaporative system is not advisable if the water contains impurities which form scale on the cylinder walls.

The re cooling of water for continuous use can be effected by one of the following means

1) direct evaporation;

2) heat exchangers with secondary water circulation;

3) radiators with atmospheric air as a coolant.

By the first method, called an open cooling system, the water from the jacket is discharged either into a cooling pond or to the top of a cooling tower and is cooled by the latent heat of evaporation of the part carried away by the air. The advantage of this system is its simplicity and the small expenditure of power needed for circulation of the water. Its big drawback is a gradual contamination of the water by salts. As pure water evaporates, leaving salts behind, and make-up water is added, with salts of its own, the salt concentration gradually increases. When it reaches a certain limit, all water must be drained and fresh water added into the system. However, even if this is done regularly, a certain amount of sediment is deposited in the engine jackets and forms scale, which eventually may cause cracks, usually in the cylinder head. At the same time, this system requires low jacket temperatures, with the ensuing drawbacks mentioned before.

A closed system normally uses distilled or treated soft water. However, raw water is also occasionally used because the original small mineral content in the raw water is not increased and therefore little scale is deposited . The cooling water from the engine is passed through a heat exchanger where it is cooled and then led back to the jacket. The heat exchanger may be either simply a coil in the basin of a cooling tower or a shell and tube exchanger. In the latter the jacket water passes through the tubes and the cooling medium through the shell. In oil pipe line pumping stations, the pumped oil is used as a coolant. A closed system permits the use of any jacket temperature up to the highest desirable; if the amount circulated is large enough, the temperature difference
between the incoming and outgoing water can be kept low, 10 to 20 0F.

The drawbacks of the closed system are a slightly greater power requirement for the two pumps and a higher initial cost. However, the elimination of scale and the advantages of higher jacket temperatures are so important that the use of the closed system has become almost universal.

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