Thursday, January 14, 2010

Cooling Water System - 2

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Radiators

In a radiator the heat is rejected to the air which passes through it. In automotive engines the air is sucked through by a fan, assisted by the movement of the car. From the previous discussion of heat transfer between water and another liquid divided by a metal wall it is clear that the performance of a radiator depends upon the velocities of air and water. The increase of air velocity, in the first place, increases the quantity of air passing the radiator fins and, in the second place, by removing the inert air film sticking to the metal surface, it increases the outside surface heat transfer coefficient h2. The water velocity acts in the same way and increases the inner surface coefficient. Since the specific heat of air is less than one fourth that of water and the surface coefficient between metal and air is many times lower than between metal and water, the air cooled surface should be considerably greater than the inner surface in contact with the water. This difference is obtained by adding thin metal fins to the water tubes which form the water passages between the upper and lower radiator tanks. Radiators are used generally with mobile or portable engines and in temporary installations.

Vapour Phase Cooling

The advantages of the high temperature jacket, as explained before, apply particularly to a system, called vapor phase cooling figure 4.6. The water is circulated by the pump; when it is delivered to the overhead tank b, part of it boils out. The vapour rises over the partition and, because of the condensing action of the radiator tubes, flows down into the tank by the small pump.

Vapour phase cooling with air cooled condensor

The vertical pipe is a communication with the outside atmosphere to prevent the collapse of tanks b and e when the pressure inside them, owing to condensation, falls below atmospheric pressure.

For larger engines the condensation of vapor formed in the overhead tank b occurs in the heat exchanger, cooled by a secondary water circuit, and the water returns by gravity.

Direct Air Cooling: Because of the low value of the heat transfer coefficient between the metal and air, the wall temperature of air cooled cylinders is considerably higher than that of the water cooled type. In order to lower the cylinder wall temperature, the outside surface must be increased by fins.

Experiments have shown that for a satisfactory operation the cylinder head temperature of most engines should not exceed 570 to 600 0F. Air cooled cylinders are used chiefly in aircraft engines and in some automobile and small stationary diesel engines. In automobile and stationary engines the cooling air is furnished by a blower. The blower is either of the centrifugal type of conventional design driven by a belt from a pulley keyed to the engine shaft or is of the axial type with blades formed by the spokes of the engine
flywheel.

Heat Dissipation: The amount of heat that must be dissipated by an air cooled cylinder at full load is from I 500 to 2300 Bin per hp-br; in addition, the lubricating oil gives up in the oil cooler 200 to 1200 Btu per hp-br, the amount decreasing with an increase of the heat dissipated by the fins. Thus the total amount of heat that is extracted is about 2500 to 2700 Btu per hp-hr.

Cooling Equipment

The term cooling equipment covers accessories required for an effective cooling of a diesel engine. As explained earlier, the majority of diesel engines use a closed cooling system. Therefore the accessories comprising a closed water system will be taken as a basis for the discussion.

A complete system consists of

1. A soft water circulating pump.

2. Pipelines for soft water circulation.

3. An expansion tank for soft water.

4. A soft water cooler.

5. Thermometers for inlet and outlet water.

6. A temperature regulator for maintaining a desired outlet water temperature.

7. Safety devices for protecting the engine against excessive jacket water temperature or stoppage of water circulation.

8. A raw water softener.

9. A raw water circulating pump.

10. Pipelines for raw water circulation.

11. A raw water cooler.

In some cases, the last three items may be absent. Thus in pipeline plants, pumping oil or in water works engines, no raw water is circulated. The soft water is re cooled by putting it through a heat exchanger through which the pumped oil or water passes and thus carries away the jacket water heat. The same is true when a radiator is used. However, in this last case the place of the raw water pump is taken by a fan. In other cases the raw water cooler may be omitted, when the supply of water is abundant, such as when the engine is located on the bank of the river or lake or on board a boat. An open cooling system uses raw water in the jackets and the necessary equipment is reduced to

1. A water circulating pump.

2. Pipelines for water circulation.

3. A water cooler.

4. Thermometers for inlet and outlet temperatures.

5. A thermostat for maintaining a desired outlet water temperature.

6. Safety devices, the same as in a closed system.

Double circuit closed cooling system

Figure 4.7 Shows schematically a closed system used in a stationary diesel plant with item 4.1 to 4.4 and from 4.7 to 4.10 clearly indicated. The water from the cooling tower flows over and open coil type heat exchanger and thus cools the jacket water in the closed system. Figure 4.8 shows schematically a closed system used with a Marine Engine, with item 4.1 to 4.7.

Bellows control of by-pass

Closed cooling system, Marine Engine, Sea water circulation

Water Pumps

Some marine diesel engines use reciprocating plunger pumps for water circulation and drive them by gears from the crank shaft . Some small diesel engines have water circulating pumps of the gear type. However, the majority of diesel engines use centrifugal pumps for circulating both jacket water and the secondary cooling water furnished to the heat exchanger.

The conventional centrifugal pump consists of an impeller, with varies curved in the direction opposite to the direction of rotation, and a spiral housing or scroll, with the cross section increasing toward the outlet. The water inlet is at the center axial the outlet is tangential. The pressure necessary to push the water through the engine jackets and the heat exchanger is produced by the centrifugal force which, during the rotation of the impeller, throws the water toward the tip ends of the vanes at high velocity. When the water passes through the expanding spiral housing, its velocity is reduced and the corresponding kinetic energy is transformed into pressure. The pumps operate at speeds from 1,200 to 3,500 rpm, depending on the size and design.

It should be remembered that, when a centrifugal pump is not running, water will leak back through it, sometimes even if a check valve is put in the suction line. Centrifugal pumps are not self priming. The water level in the sump or other source of supply must therefore be higher than the top of the pump, and water should flow into the suction end of the pump by gravity or under pressure.

Centrifugal water pumps are driven from the engine crankshaft by means of gears or chains. In power plants with large diesel engines, the pumps are driven by electric motors.

Piping

Flow Resistance: Every pipe presents a certain resistance to the flow of the fluid, in this case water, which it conducts. The flow resistance in a pipe increases in direct proportion to the length and approximately as the square of the velocity of the fluid. Since the water velocity is inversely proportional to the cross section of the pipe, a reduction in the cross area or size of the pipe will increase the flow resistance and, with a given pressure head created by the pump, will decrease the flow rate. The resistance also increases with
every elbow and valve through which the water must pass. The resistance of a valve depends upon its construction; thus the resistance of a globe valve is higher than that of a gate valve.

In renewing a pipe line, one must be careful not to increase the flow resistance by making changes in the original installation.

The following data may serve as a guide for piping layout and installation; suitable water velocities, on the suction side 60 to 200 ft. per mm and on the discharge side 120 to 240 ft per min. the smaller the pipe diameter, the lower should be the velocity. Velocities higher than those indicated give excessive resistance; lower velocities require excessively large pipe diameters and mean an unnecessarily high first cost. The resistance of fittings is usually considered to be equal to a certain additional length of the pipe: an elbow is equivalent to three pipe diameters, a gate valve to about five diameters and a globe valve to ten diameters, at most.

Expansion Tank

The water in a cooling system expands as the water temperature goes up and the excess water goes into a so called expansion tank. This tank is located at the highest point of the pipe line, maintains a constant pressure in the system, prevents formation of air or steam pockets in it, and serves to add make up water to take care of unavoidable leaks in the system.

The size of the expansion tank depends upon the water capacity of the whole system, including the water space in the engine jackets. The volume of the tank should be not less than 5 per cent of the total water capacity in order to allow for the expansion from room temperature to the temperature of the water leaving the engine. A greater volume, up to 10 percent , is advisable in order to take care of the unavoidable losses through leaks, such as through pump glands, and evaporation.

The tank must be of well galvanised steel in order to prevent rusting occasioned by the fluctuating water level. Sometimes the expansion tank serves also as a soft water supply tank; it is then made considerably larger.

Soft Water Cooler

In stationary installations, the cooler is usually a pipe coil placed either fiat in the sump of the cooling tower which is used for recooling the raw water or vertically and raw water from the cooling tower runs over it.

The advantages of these coolers, which are of the open or atmospheric type, are the following : (1) the evaporation of the cooling water running over the coils helps heat dissipation; (2) there is no danger of raw water leakage into the closed soft water circuit, because the pressure inside the coils in higher; (3) good accessibility for cleaning scale and mud deposits off the coils; (4) low first cost.

Heat Exchangers

Sometimes the soft water is run through some kind of heat exchanger, usually of the shell and tube type. The soft water flows inside the tubes, the raw water from the outside of the tubes, directed in its flow by baffles. The baffles give better contact with all parts of the tube and increase the water velocity; these two conditions increase the heat transfer. Sometimes raw water is passed inside the tubes in order to make their cleaning easier. In heat exchangers used in pipeline plants the cooling oil is passed through the tubes, in order to reduce the flow resistance, and the tensile stress in the tubes produced by the big temperature difference is taken up by special tie rods and braces.

Shell and tube heat exchangers are used sometimes in other than marine and pumping plants because of the following advantages: (1) good heat transfer due to the use of small diameter thin walled tubes and relatively high water velocities; (2) compactness the exchanger takes up little space and may be placed in any position; (3) ease of cleaning the tubes from the inside and, in exchangers with an expansion joint or floating head, also from the outside.

Pressure

In order to prevent leakage of raw water into a closed system if the tube ends eventually become loose in the tube plates, the pressure of the raw water should be always less than that of the soft water; this precaution necessitates larger raw water piping and smaller water velocities.

Zinc Electrodes: Cooling systems using sea water must have zinc electrodes inserted in the sea water inlet line. This is necessary in order to control electrolysis which takes place in the sea water lines of the cooling system from stray electric currents. The zinc provides a terminal which attracts the stray current and thus restricts the electrolytic action to corroding the zinc and leaves the other parts of the system intact. The zinc electrodes are corroded rather fast; they must therefore be inspected at regular intervals
and replaced before they become too small, in about three to six months. In shell and tube heat exchangers the zinc electrodes are made of plates fastened inside the shell if sea water is circulated through the tubes.

Radiator Units

Sometimes soft water is cooled by circulating it through a radiator unit, which consists of a radiator similar to one used in automobiles, tractors, and trucks mounted on a common base with the water circulating pump and fan, both driven from an electric motor or from an extension shaft of the diesel engine. Such units are light and compact and are suitable for temporary or portable installations. However, they are not economical, because of the relatively large amount of power required to drive the fan. They are therefore seldom used in stationary power plants, except where their compactness is of particular importance, such as in a diesel power plant installed in the basement of an office building.

Water Softeners

Except where distilled water is available, the treating of cooling water, i.e., elimination of its temporary and permanent hardness, is done in so called water softeners. A typical water softener consists of a metal shell or tank containing zeolite material which abstracts the hardness from the water as it flows through the tank. The zeolite exchanges its sodium for the calcium and magnesium in the water, leaving only soluble sodium salts in the water, which do not form scale. After a certain amount of hard water is run through the softener, its charge must be regenerated with common salt.

The proper size and type of water softener depends upon the raw water analysis; this analysis should be made by the concern furnishing the softener. The softener works practically automatically and all necessary instructions in its proper operation are furnished by the softener manufacturer.

Raw water Cooling

Raw water is cooled by evaporation in cooling towers. A good estimate of the amount of water that must go over the tower is twice as much as is circulated through the engine jacket.

Atmospheric Cooling Towers

It consists essentially of a system of distributing gutters, or troughs, which allow the water to trickle down through the successive decks of the tower, eventually collecting in a basin or sump at the bottom after having been exposed to the cooling effect of the air; this effect is due chiefly to evaporation of a certain part of the water. The various decks of the tower are protected by louvers to prevent the wind from carrying away the falling streams of water. The water is admitted through sprinklers at the top and flows by gravity into the sump.

For service in cold weather a secondary distributing system is sometimes located nearer to the bottom: the sprinklers on the top are shut off and the water is passed only over a small portion of the tower when the atmospheric temperature makes it unnecessary to use all decks.

Some towers, particularly in smaller sizes, are made without troughs inside and the water is broken up by spray nozzles to which the water is delivered under a pressure of 3 to 5 psi. The above described cooling towers are called atmospheric towers, because
evaporation is assisted by the natural movement of atmospheric air, or natural draft.

Mechanical draft cooling towers are made in the form of a tall steel box with spray nozzles near the top to break up the water and S-shaped steel baffles above the nozzles to prevent water drops from being carried away. Either the air is forced through the tower by a fan located near the bottom, or the draft is induced by a fan on top of the tower.

Mechanical draft cooling towers made of steel are more expensive than wooden towers. However, they are much lighter and therefore are used for temporary and semiportabie installations and on the roofs of buildings.

Open Cooling System: The danger of scale formation and , therefore, of impaired cooling in an open cooling system can be materially reduced if soft water is used for jacket cooling and the make up for evaporation is also treated in a water softener.

Cooling Controls

Temperature Measurement: Water temperatures are conveniently measured with ordinary glass mercury thermometers, usually of the industrial type, in a metal case protecting the glass from easy breakage. Dial thermometers are also used. In some installations, in order to centralize the observation of various temperatures by using Thermocouples, the latter are used also for water temperature reading.

Temperature regulators are automatic valves operated by thermostatic elements which are set to open or close at a certain temperature. The several types of thermostatic elements in use consist of the following elements: (1) a corrugated metal pipe, called a bellows; (2) a bimetallic coil; and (3) a cylinder with a readily evaporating substance and a piston.

Bellows: Such a thermostat consists of a brass or monel metal thin wall pipe with deep corrugations. The inside of the bellows is filled with a volatile liquid, such as alcohol or ether, which evaporates readily with a rise of the temperature of the water in which the element is immersed. The resulting vapour pressure pushes the lower, free end of the bellows in respect to the fixed one. This motion is increased by the large number of corrugations, which make the element more sensitive to a change of pressure inside it,
leading to a change of temperature outside it. In small engines the bellows are fastened directly to the regulating valve; as the water temperature goes up, the by-pass is gradually closed, and more water is forced to the cooler or heat exchanger, until at maximum load the by-pass is closed entirely.

In large engines, the valve regulating the rate of flow to the cooler and operated by the bellows is placed in the soft water piping where it is most convenient and the action of the bellows is transmitted by remote control. A temperature regulator with remote control consists of a steel bulb filled with a volatile liquid, inserted in the water outlet from the engine and connected to the bellows by a fine tube which transmits the pressure change from the bulb to the bellows.

Bimetallic Coil

This element consists of a strip made up of two metals, which have different coefficients of heat expansion, formed into a flat spiral coil. The outer end of the coil is fastened solidly to the perforated housing, and the inner end is fastened to a shaft. When the temperature of the bimetallic coil changes, its free end moves, rotating the shaft and thus opening or closing a flat hinge type valve to the by-pass pipe.

Cylinder and Piston

This thermostatic element consists of a cylinder with a well- fitting piston and some readily evaporating substance between them. When, with an increase in temperature, the substance melts and begins to evaporate, it pushes the piston, which in turn operates a disk valve and at the same time compresses a spring. When the temperature decreases, the pressure under the piston begins to drop and the spring begins to return the piston and the disk valve toward its seat.

Safety Devices

These devices may be divided into two groups: (1) instruments which sound an alarm when the water temperature reaches a certain height and (2) instruments which stop the engine automatically when the water temperature reaches a predetermined value.

Alarm

One of the alarm devices consists of an electric bell or siren whose circuit is closed by a mercury glass thermometer with a contact embedded in the glass tube at a certain point. Another scheme uses a switch in the circuit which is operated by vapor pressure through a capillary tube from a bulb immersed in the water discharge pipe. Still another scheme uses a switch operated by a shaft fastened to a bimetallic coil similar to the one used as thermostat element and also located in the discharge pipe.

The water level alarm is sometimes installed in addition to the float indicator to call the attention of the operator when the water level in the soft water supply tank becomes too low.

Automatic Stop Device

This device consists of a bulb with a volatile liquid, which is inserted in the water discharge pipe and connected by a capillary tube to a special control box; the box contains a valve which normally closes tightly a by-pass in the fuel line leading from the service pump to the injection pump. If the water temperature exceeds a certain maximum, the pressure in the bulb acts on the control box and opens the by-pass valve, thus shutting off the fuel supply to the engine and allowing any oil left in the fuel tine between the control box and the injection pump to drain back into the fuel supply tank. An engine stopping device usually is combined with an audible alarm signal, which begins
to sound a warning slightly before the engine is stopped.

Summing Up

Cooling water piping is plant piping as well as part of yard piping. It may be small piping or the biggest bore piping in the plant. So routing, supporting and execution of cooling tower piping is important since this is also a part of yard piping and the pipe rack design is also affected by this piping. Since it is one close loop network containing the cooling tower, pump, supply line and return line head as well as quantity balancing is important. Piping engineering plays an important role in keeping network always safe and economical.


Cooling Water System - 1

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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.

Through or open  system

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.

Reculation of closed system

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).

(a) Mixed type combined river and cooling water

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).

(b) Mixed type combined river and cooling water

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.

Heat flow from hot gases to water

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

Heat-Properties of metals

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.

Gravity a circulation cooling

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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