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.
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
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.