Heat Exchangers
Heat exchanger
LT Cooler
Corroded end cover of freshwater cooler
Heat transfer through a heat exchanger
To determine the amount of heat transferred in a heat exchanger, it is necessary to know the overall heat transfer coefficient, U, for the wall and fluid boundary layers and also the logarithmic mean temperature difference.
Heat transferred Q = U A t θm
Where, θm = (θ1 - θ2) /In (θ1/θ2)
U = Overall heat transfer coefficient
Heat transfer
a. Conduction is the flow of heat energy through a body, or from one body to another in contact with each other, due to difference in temperature.
b. The natural flow of heat takes place from a region of high temperature to a regien of lower temperature.
c. Metals are good Conductors of heat. Air and materials such as asbestos, cork, glass and wool are very bad conductors, called Insulators and are used to minimise heat loss.
d. Special plastic base compositions are used to lag boilers, pipes, casings to reduce loss of heat energy to the colder outside surroundings. Cork and fibre glass are common insulating materials to pack the hollow walls of refrigerating chambers to reduce heat flow into the cold chambers from the warmer outside surroundings.
Quantity of heat
The quantity of heat conducted through a material, in a given time
a. Depends upon the thermal conductivity of the material.
b. Is proportional to the surface are exposed to the source of heat,
c. Is proportional to the temperature difference between the hot and cold ends, and
d. Is inversely proportional to the distance or thickness through which the heat is conducted.
Quantity of heat ∝ area x time x temperature difference/ thickness
Flow paths in a multi-tubular heat exchanger
a. The flow paths are fixed by the division plates in the heat exchanger end-covers and the internal baffles or tube support sheets within the body of the heat exchanger.
b. The Division plates (in the end-corers) fix the positions of the inlet and outlet branches for the fluid passing through the tubes.
c. The Internal baffles fix the position of the inlet and outlet branches for the fluid passing though the body of the heater, on the outside of the tubes.
1. If the heat exchanger end-covers do not contain any division plates, the fluid passing through the tubes enters at one end of the heat exchanger and leaves at the other. This arrangement is referred to as a single-pass heat exchanger.
2. If the heat exchanger is a double-pass type, a division plate is fitted in one head.
3. The inlet and outlet connections for the fluid passing through the tubes are fitted on this head. The division plate prevents the fluid bypassing the tubes and causes it to pass through half the tubes in the heat exchanger, which is referred to as the inlet bank.
4. After the fluid passes through the inlet bank, it enters the other header. The direction of the fluicw is reversed in this header- and it passes back through the outlet bank of tubes and leaves at the outlet branch.
5. The fluid has passed throgh the tubes in two separate paths from which it gets the name two-pass, or double-pass type.
6. By dividing the total number of tubes into three banks, fitting a division plate in each head, and fitting the inlet branch on one head and the outlet branch on the other, a three- or triple-pass arrangement can be, formed.
Pate type heat exchangers
Plate type heat exchangers are being used for Jacket water coolers, Distillation plant heat exchangers, and consist of parallel plates fitted with sealing material like rubber, which alternately have coolant, and the fluid to be cooled, flowing through them.
The plates are corrugated to improve heat transfer rates, and are made of 70:30 copper nickel, aluminium brass, or titanium. Care has to be taken while overhauling the plate type coolers that the plates are correctly assembled, and the tightening procedure is correctly followed, to prevent leaks. The advantage here is, that cleaning the plates is a relatively easier task, since the heat transfer surface is readily accessible, unlike a tube, which is very difficult to clear, in case of chockage, and cannot be easily accessed, especially if the length is considerable.
When a cold fluid is passing through the tubes of a heat exchanger and a warm fluid is circulating outside the tubes, the shell expands more than the tubes.
Arrangements to take care of differences of expansion.
a. In some types at heat exchangers, the body is made of mild steel plates and in order to cater for the differential expansion between the body and tubes, a bellows ring is welded circumferentially round the body of the heat exchanger. Expansion and contraction of the tubes are then catered for by the bellows, which deforms slightly to accommodate the changing length of the tubes. This type of expansion arrangement can be used for any number of fluid, passes through the tubes. The tube plates are bolted directly on to the flanges of the body and the tubes can be roller-expanded at both ends.
b. In heat exchangers with cast bodies, one tube plate is fastened to the flange on one end of the body.
The other tube plate is made to slide plate to make a seal. When there is a difference, in expansion between the tubes and the body, the differences are accommodated by the tube plate, which slides relative to the body. With this arrangement, the tubes can be arranged only for single or double-pass flow. This arrangement is most commonly used for engine cooling water and lubricating oil coolers. It also has the advantage of allowing very easy removal of the tube stack from the cooler body for chemical cleaning or repair. This is further facilitated if the cooler is mounted vertically and the sliding tube is at the lower end.
c. A third arrangement is to fasten the tube plates to the body of the exchanger and roller-expand and bell-mouth the inlet ends of the tubes. The outlet end of the tubes is fitted with a cotton cord packing with a threaded gland. Caulking fibre or foil is also used for sealing.
Air-lock and steps are taken to prevent this
a. An air-lock is caused by the ingress of air in a cooling system, when the removal of air is difficult, as in a U-bend. Air ingress can occur at pump glands, especially if fouling of sea chests and sea suction valves occurs, since this causes the suction pressure to fall, and air to be drawn in.
b. Some pumps have a lantern ring fitted, which is connected with a pipe from the discharge side, and supplies sufficient pressure to effect good sealing at the gland. If this pipe should get blocked or damaged, sealing is affected which causes air ingress.;
c. When a ship is light, in ballast or pitching in heavy weather, air is drawn under the bow of the ship and passes along the bottom just and leaves at the aft end. Sometimes this air finds its way into the sea suction chests, passes through the pumps and causes air locks in the sea-water sides of coolers. There is usually an air vent pipe fitted on the top of the cooler water boxes and sea chests. The pipe leads up to a goose-neck at the top of the engine room. d. After a vessel comes out of dry-dock the valves on the vent pipes should be checked to be open.
e. Joints on sea water strainers or filters are also a common point of air ingress. If the sea chests are fitted with compressed air deicing and weed-clearing connections, leakage of air from the compressed air line through the valve into the sea chest is another cause of air locking problems.
f. Air leakage into lubricating oil systems on engines may occur at glands on pumps, and suction filter cover joints. Another cause of air leakage into lubricating systems may be due to the sucking in of air at the tail end of the suction pipe in the drain tank. This condition can easily come about if foreign matter such as rag, old jointing or similar material is left in the crankcase. This material finds its way into the crankcase strainers at the rundown point into the sump tank and causes an accumulation of oil to occur in the bottom of the crankcase. This in turn causes a low level in the drain tank and allows the pump to pull in air. A similar condition occurs if the drain holes within the tank structure become blocked. The flow of oil to the pump suction becomes reduced and air is drawn into the pump. Cooling systems are of the pressurised type, where the cooling water is under a pressure head from an expansion tank. Problems with air locking do not normally arise. In ships with a limited height in the machinery space, the height of the expansion tank may be less than the static head in the cooling water at the engine cylinder tops. In such cases a circulation of water may come about from the de-aerating cocks on the uppermost part of the engine cooling system. When the cooling water circulates in this manner it goes up to at the top of the deaerating pipe and goes into the expansion tank via the save all funnel. The water in passing into the expansion tank pulls in air with it.
g. If the expansion tank is not fitted with air separation plates or if they have become corroded, air cannot be removed from the cooling system.
Thermal conductivity (k)
The thermal conductivity depends upon the nature of the material and its ability to conduct heat. This varies for different materials and sometimes varies slightly for the same material depending upon the temperature range.
The thermal conductivity (k) of a material expresses the quantity of heat energy conducted through unit area in a unit time for unit temperature difference between two opposite faces of a material of unit distance apart. Considering the heat flow through a flat wall
Q = kAt(T1- T2)/S
Where, Q=Quantity of heat conducted, in J.
A = Area through which heat flows, in m2.
t = Time of heat flow, in seconds.
T1- T2 = Temperature difference between the two faces, K.
S = Thickness of wall, in m.
k = QS/At (T1 - T2)
Tube removal
Tubes are normally expanded into the tube plates, to seal the opening around the tube and fix the tube firm.
To remove the damaged tube, you need to first remove the flare, which is created by the expander. Chisel off the flare flush with the tube plates, and then split the tube with a diamond point chisel, taking care not to damage the tube plate.
Tubes can be cut with a Tube cutter, to drop them inside the shell. Collapse and remove the ends.
Fitting of new tubes is done by 'Flaring'. Flaring can be done by hand or with a pneumatic or motor operated machine (used where large number of tubes need to be done).
Tube replacement
Tubes need to be replaced if more than 10% have been plugged, which greatly affects the efficiency of the heat exchanger. Before fitting a new tube, check that the tube plate hole is not damaged.
Cut the tube to the correct size with a tube cutter. The other end of the tube may shift, while expansion is being done at one end. To prevent the tube from rolling in its position, drive in a thin tapper wedge between the tube and the tube plate hole:
Over-rolling the tubes is bad, as it forces the tube plates apart, forcing long tubes to bow.
Over-rolling will cause 'work hardening' of the tube. This makes the tube brittle, causing an early failure. It also causes surface tears inside the rolled part. The surface tears are also a cause of early tube failure.
While rolling the tube, do it slowly, and stop when tiny flakes start leaving the tube plate around the tube. Over-rolling beyond this point will unnecessarily deform the tube plate. The tube is also thinned and weakened inside the tube plate to a dangerous point.
LT Cooler
Corroded end cover of freshwater cooler
Heat Exchangers
Ships require heat exchangers, to permit the transfer of heat in various systems — heat from the engine to the jacket water, from jacket water to sea water, from lubricating oil to sea water and so on.Heat transfer through a heat exchanger
To determine the amount of heat transferred in a heat exchanger, it is necessary to know the overall heat transfer coefficient, U, for the wall and fluid boundary layers and also the logarithmic mean temperature difference.
Heat transferred Q = U A t θm
Where, θm = (θ1 - θ2) /In (θ1/θ2)
U = Overall heat transfer coefficient
Heat transfer
a. Conduction is the flow of heat energy through a body, or from one body to another in contact with each other, due to difference in temperature.
b. The natural flow of heat takes place from a region of high temperature to a regien of lower temperature.
c. Metals are good Conductors of heat. Air and materials such as asbestos, cork, glass and wool are very bad conductors, called Insulators and are used to minimise heat loss.
d. Special plastic base compositions are used to lag boilers, pipes, casings to reduce loss of heat energy to the colder outside surroundings. Cork and fibre glass are common insulating materials to pack the hollow walls of refrigerating chambers to reduce heat flow into the cold chambers from the warmer outside surroundings.
Quantity of heat
The quantity of heat conducted through a material, in a given time
a. Depends upon the thermal conductivity of the material.
b. Is proportional to the surface are exposed to the source of heat,
c. Is proportional to the temperature difference between the hot and cold ends, and
d. Is inversely proportional to the distance or thickness through which the heat is conducted.
Quantity of heat ∝ area x time x temperature difference/ thickness
Flow paths in a multi-tubular heat exchanger
a. The flow paths are fixed by the division plates in the heat exchanger end-covers and the internal baffles or tube support sheets within the body of the heat exchanger.
b. The Division plates (in the end-corers) fix the positions of the inlet and outlet branches for the fluid passing through the tubes.
c. The Internal baffles fix the position of the inlet and outlet branches for the fluid passing though the body of the heater, on the outside of the tubes.
1. If the heat exchanger end-covers do not contain any division plates, the fluid passing through the tubes enters at one end of the heat exchanger and leaves at the other. This arrangement is referred to as a single-pass heat exchanger.
2. If the heat exchanger is a double-pass type, a division plate is fitted in one head.
3. The inlet and outlet connections for the fluid passing through the tubes are fitted on this head. The division plate prevents the fluid bypassing the tubes and causes it to pass through half the tubes in the heat exchanger, which is referred to as the inlet bank.
4. After the fluid passes through the inlet bank, it enters the other header. The direction of the fluicw is reversed in this header- and it passes back through the outlet bank of tubes and leaves at the outlet branch.
5. The fluid has passed throgh the tubes in two separate paths from which it gets the name two-pass, or double-pass type.
6. By dividing the total number of tubes into three banks, fitting a division plate in each head, and fitting the inlet branch on one head and the outlet branch on the other, a three- or triple-pass arrangement can be, formed.
Pate type heat exchangers
Plate type heat exchangers are being used for Jacket water coolers, Distillation plant heat exchangers, and consist of parallel plates fitted with sealing material like rubber, which alternately have coolant, and the fluid to be cooled, flowing through them.
The plates are corrugated to improve heat transfer rates, and are made of 70:30 copper nickel, aluminium brass, or titanium. Care has to be taken while overhauling the plate type coolers that the plates are correctly assembled, and the tightening procedure is correctly followed, to prevent leaks. The advantage here is, that cleaning the plates is a relatively easier task, since the heat transfer surface is readily accessible, unlike a tube, which is very difficult to clear, in case of chockage, and cannot be easily accessed, especially if the length is considerable.
When a cold fluid is passing through the tubes of a heat exchanger and a warm fluid is circulating outside the tubes, the shell expands more than the tubes.
Arrangements to take care of differences of expansion.
a. In some types at heat exchangers, the body is made of mild steel plates and in order to cater for the differential expansion between the body and tubes, a bellows ring is welded circumferentially round the body of the heat exchanger. Expansion and contraction of the tubes are then catered for by the bellows, which deforms slightly to accommodate the changing length of the tubes. This type of expansion arrangement can be used for any number of fluid, passes through the tubes. The tube plates are bolted directly on to the flanges of the body and the tubes can be roller-expanded at both ends.
b. In heat exchangers with cast bodies, one tube plate is fastened to the flange on one end of the body.
The other tube plate is made to slide plate to make a seal. When there is a difference, in expansion between the tubes and the body, the differences are accommodated by the tube plate, which slides relative to the body. With this arrangement, the tubes can be arranged only for single or double-pass flow. This arrangement is most commonly used for engine cooling water and lubricating oil coolers. It also has the advantage of allowing very easy removal of the tube stack from the cooler body for chemical cleaning or repair. This is further facilitated if the cooler is mounted vertically and the sliding tube is at the lower end.
c. A third arrangement is to fasten the tube plates to the body of the exchanger and roller-expand and bell-mouth the inlet ends of the tubes. The outlet end of the tubes is fitted with a cotton cord packing with a threaded gland. Caulking fibre or foil is also used for sealing.
Air-lock and steps are taken to prevent this
a. An air-lock is caused by the ingress of air in a cooling system, when the removal of air is difficult, as in a U-bend. Air ingress can occur at pump glands, especially if fouling of sea chests and sea suction valves occurs, since this causes the suction pressure to fall, and air to be drawn in.
b. Some pumps have a lantern ring fitted, which is connected with a pipe from the discharge side, and supplies sufficient pressure to effect good sealing at the gland. If this pipe should get blocked or damaged, sealing is affected which causes air ingress.;
c. When a ship is light, in ballast or pitching in heavy weather, air is drawn under the bow of the ship and passes along the bottom just and leaves at the aft end. Sometimes this air finds its way into the sea suction chests, passes through the pumps and causes air locks in the sea-water sides of coolers. There is usually an air vent pipe fitted on the top of the cooler water boxes and sea chests. The pipe leads up to a goose-neck at the top of the engine room. d. After a vessel comes out of dry-dock the valves on the vent pipes should be checked to be open.
e. Joints on sea water strainers or filters are also a common point of air ingress. If the sea chests are fitted with compressed air deicing and weed-clearing connections, leakage of air from the compressed air line through the valve into the sea chest is another cause of air locking problems.
f. Air leakage into lubricating oil systems on engines may occur at glands on pumps, and suction filter cover joints. Another cause of air leakage into lubricating systems may be due to the sucking in of air at the tail end of the suction pipe in the drain tank. This condition can easily come about if foreign matter such as rag, old jointing or similar material is left in the crankcase. This material finds its way into the crankcase strainers at the rundown point into the sump tank and causes an accumulation of oil to occur in the bottom of the crankcase. This in turn causes a low level in the drain tank and allows the pump to pull in air. A similar condition occurs if the drain holes within the tank structure become blocked. The flow of oil to the pump suction becomes reduced and air is drawn into the pump. Cooling systems are of the pressurised type, where the cooling water is under a pressure head from an expansion tank. Problems with air locking do not normally arise. In ships with a limited height in the machinery space, the height of the expansion tank may be less than the static head in the cooling water at the engine cylinder tops. In such cases a circulation of water may come about from the de-aerating cocks on the uppermost part of the engine cooling system. When the cooling water circulates in this manner it goes up to at the top of the deaerating pipe and goes into the expansion tank via the save all funnel. The water in passing into the expansion tank pulls in air with it.
g. If the expansion tank is not fitted with air separation plates or if they have become corroded, air cannot be removed from the cooling system.
Thermal conductivity (k)
The thermal conductivity depends upon the nature of the material and its ability to conduct heat. This varies for different materials and sometimes varies slightly for the same material depending upon the temperature range.
The thermal conductivity (k) of a material expresses the quantity of heat energy conducted through unit area in a unit time for unit temperature difference between two opposite faces of a material of unit distance apart. Considering the heat flow through a flat wall
Q = kAt(T1- T2)/S
Where, Q=Quantity of heat conducted, in J.
A = Area through which heat flows, in m2.
t = Time of heat flow, in seconds.
T1- T2 = Temperature difference between the two faces, K.
S = Thickness of wall, in m.
k = QS/At (T1 - T2)
Tube removal
Tubes are normally expanded into the tube plates, to seal the opening around the tube and fix the tube firm.
To remove the damaged tube, you need to first remove the flare, which is created by the expander. Chisel off the flare flush with the tube plates, and then split the tube with a diamond point chisel, taking care not to damage the tube plate.
Tubes can be cut with a Tube cutter, to drop them inside the shell. Collapse and remove the ends.
Fitting of new tubes is done by 'Flaring'. Flaring can be done by hand or with a pneumatic or motor operated machine (used where large number of tubes need to be done).
Tube replacement
Tubes need to be replaced if more than 10% have been plugged, which greatly affects the efficiency of the heat exchanger. Before fitting a new tube, check that the tube plate hole is not damaged.
Cut the tube to the correct size with a tube cutter. The other end of the tube may shift, while expansion is being done at one end. To prevent the tube from rolling in its position, drive in a thin tapper wedge between the tube and the tube plate hole:
Over-rolling the tubes is bad, as it forces the tube plates apart, forcing long tubes to bow.
Over-rolling will cause 'work hardening' of the tube. This makes the tube brittle, causing an early failure. It also causes surface tears inside the rolled part. The surface tears are also a cause of early tube failure.
While rolling the tube, do it slowly, and stop when tiny flakes start leaving the tube plate around the tube. Over-rolling beyond this point will unnecessarily deform the tube plate. The tube is also thinned and weakened inside the tube plate to a dangerous point.
LT Cooler
Water in the high temperature circuit is circulated through the main engine and auxiliary diesels by the pumps to the left of the engine in the sketch. At the outlet, the cooling water taken to the fresh water distiller ( evaporator) where the heat is used for the evaporation of sea water. From the outlet of the evaporator, the cooling water is led back to the suction of the high temperature pump through a control value (c) which is governed by engine inlet temperature. The control value mixes the low and high temperature streams to produce the required inlet temperature, which is about 620 C. Engine outlet may be about 700 C.
For the low temperature circuit, the heat of the water leaving the central coolers is regulated by the control value (F). Components of the system are arranged in parallel or series groups as required. The pressure control value works on a bypass. The temperature of the water after the coolers may be 350C and at exit from the main engine lubricating oil coolers it is about 450C.
To improve the performance of the LT circuit, the main cooler needs thorough cleaning accompanied by testing to see if any tube is leaking and needs plugging. This is the first operation to be carried out.
- Subsequently the control valve C which mixes the high temperature and the low temperature side is to be checked for its proper operation.
- The control valve F which controls the low temperature circuit also needs to be checked.
For the main cooler to be cleaned and checked , this has to be done in port when the aux cooler will be used for the aux engine on load.
The Main cooler covers will be opened after ensuring that the cooler sea valves are shut tight and not leaking. If needed spoon blanks will be fitted on the sea inlet and outlet valves.
The tubes will be cleaned by passing a cane of suitable .length and correct diameter to clear all the tubes of accumulated mud.
- After this the cooler will be tested by running the fresh water through and detecting the leaky tubes by observing the flow of fresh water from tube ends. The leaky tubes will be suitably plugged by brass ptaper plugs turned by the ships lathe.
- A final test will be carried out on the cooler by running the fresh water pumps. The covers will be refitted after suitable application of the recommended epoxy paint and renewal of the zinc slabs in the water boxes
With these operations the LT side should perform satisfactorily.
Corroded end cover of freshwater cooler
Straight tube type heat-exchanger is mainly composed of inlet & outlet end covers, return cover, shell, tube plate, cooling tubes, bafflers and accessories, etc.
End covers of shell-tube type heat-exchanger are made up of casting or welding pieces. There are inlet & outlet connecting pipes, both of which are equipped with thermometers for cold medium and vent & drain plugs in inlet and outlet covers. For exchangers with the medium of S.W., the end covers must be equipped with zinc bars for the prevention from corrosion.
Various measures are being taken for repairing a marine diesel engine fresh cooler end cover which is badly corroded and holed. There are two general methods of repairing a hole: put something into it or put something over it. Choice of method depends upon the availability of repairing material and time to carryout the repair.
(A) While in operation,
When it is required to repair the heat exchanger end cover while it is in operation temporary cold repair is preferred. Cold repair methods such as using DEVCON require a curing period. But as the cooler is in operation the curing period is shall not be available. DEVCON emergency repair using underwater repair putty however is found to be an effective method to arrest the leakage and don't require any curing time.
Few other methods involve using a jubilee patch and clamp cannot be used because the shape of the end cover of cooler does not permit.
a. Hook bolt
It is a long bolt having the head end so shaped that the bolt can be hooked to plating through which it has been inserted. The common types are the T, the J and the L, so called because they resemble those letters. The long shanks are threaded and provided with nuts and washers. Steel or wooden strongbacks are used with them; generally the latter. A hookbolt has no regular head.
The head end of the bolt is inserted through a hole, and the bolt is rotated or adjusted until it cannot be pulled back through the hole. A pad or gasket, backed by a plank or a strong back, is then slid over the bolt and the patch is secured in place by taking up on the nut. It is generally necessary to use these bolts in pairs.
A variation of the hook bolt is the folding T. It resembles the T hook bolt, but it has a hinge where the shank joins the cross-piece, so that it is much like the tumble toggle bolt. This bolt may be folded and inserted through a small hole; when pulled back, the cross-piece catches on the hull plating.
b. Wooden plug:-
The simplest method of repairing a fairly small hole is to insert some kind of a plug. Plugs made of soft wood have been found rather effective under battle conditions, especially in holes not over 3 inches x 3 inches. They also have held up well in much larger holes.
Every ship should have a large assortment of conical, square-ended and wedge-shaped wooden plugs at each repair station. The plugs should not be painted, for unpainted soft wood absorbs water and holds better. Plugs should be stowed in canvas bags secured to the overhead.
Combinations of conical, square-ended and wedge-shaped plugs may be used to get better conformation to the shape of the hole. It is best to wrap the plugs with lightweight cloth before inserting. The cloth will tend to keep the plugs in place, and also will fill some of the gaps between plugs. In most cases wooden plugs will not make a watertight fit, but by filling the remaining leaky area with rags, oakum and smaller wedges, the ingress of water can be greatly reduced. Square-ended plugs hold better than conical plugs in holes in plating one-fourth inch or less in thickness.
c. Welding a threaded pipe piece with pipe cap
A skilled welder can make an arrangement as shown in figure. This arrangement require a metal pipe piece with a threaded plug for blanking on one side; the other side should make a good surface contact with the end cover surface.
The diameter of the pipe should be considerably larger than the hole diameter. The pipe piece should be welded on the end cover while keeping the plug removed, it will need the water to flow through the pipe, once the welding is completed the pipe can be plugged. This method is relatively time consuming and require a skilled person. But the repair is highly reliable.
B. When not is operation
Cooler when not in operation can be drained completely and permanent repairs can be carried out. Cold repairs like applying DEVCON putty can also be considered which in this case will have sufficient time for curing.
a. A quick method is fitting a nut bolt with washer and Gasket RTV. Drain the cooler and remove the end cover. Clean the area with wire brush, and remove the weak rusted metal. Inserting a bolt in the hole diameter must be as close as possible. A washer with considerable larger diameter on both side of cover, applying Gasket RTV on the surfaces of washer. Tight the nut and left it for a curing period for few hours.
Now fit the cover back to the cooler and open the cooling water and check for any leakage.
b. Welding a patch over the damaged part. This require the surface preparation and heat treatment on the end cover. A patch of same material with a larger size than that of the hole is welded on the both side of hole. Time should be given for slow cooling. Check for any leakage, apply a coat of primer and then paint the cover. Now fit it back to the cooler, open the cooling water.
End covers of shell-tube type heat-exchanger are made up of casting or welding pieces. There are inlet & outlet connecting pipes, both of which are equipped with thermometers for cold medium and vent & drain plugs in inlet and outlet covers. For exchangers with the medium of S.W., the end covers must be equipped with zinc bars for the prevention from corrosion.
Various measures are being taken for repairing a marine diesel engine fresh cooler end cover which is badly corroded and holed. There are two general methods of repairing a hole: put something into it or put something over it. Choice of method depends upon the availability of repairing material and time to carryout the repair.
(A) While in operation,
When it is required to repair the heat exchanger end cover while it is in operation temporary cold repair is preferred. Cold repair methods such as using DEVCON require a curing period. But as the cooler is in operation the curing period is shall not be available. DEVCON emergency repair using underwater repair putty however is found to be an effective method to arrest the leakage and don't require any curing time.
Few other methods involve using a jubilee patch and clamp cannot be used because the shape of the end cover of cooler does not permit.
a. Hook bolt
It is a long bolt having the head end so shaped that the bolt can be hooked to plating through which it has been inserted. The common types are the T, the J and the L, so called because they resemble those letters. The long shanks are threaded and provided with nuts and washers. Steel or wooden strongbacks are used with them; generally the latter. A hookbolt has no regular head.
The head end of the bolt is inserted through a hole, and the bolt is rotated or adjusted until it cannot be pulled back through the hole. A pad or gasket, backed by a plank or a strong back, is then slid over the bolt and the patch is secured in place by taking up on the nut. It is generally necessary to use these bolts in pairs.
A variation of the hook bolt is the folding T. It resembles the T hook bolt, but it has a hinge where the shank joins the cross-piece, so that it is much like the tumble toggle bolt. This bolt may be folded and inserted through a small hole; when pulled back, the cross-piece catches on the hull plating.
b. Wooden plug:-
The simplest method of repairing a fairly small hole is to insert some kind of a plug. Plugs made of soft wood have been found rather effective under battle conditions, especially in holes not over 3 inches x 3 inches. They also have held up well in much larger holes.
Every ship should have a large assortment of conical, square-ended and wedge-shaped wooden plugs at each repair station. The plugs should not be painted, for unpainted soft wood absorbs water and holds better. Plugs should be stowed in canvas bags secured to the overhead.
Combinations of conical, square-ended and wedge-shaped plugs may be used to get better conformation to the shape of the hole. It is best to wrap the plugs with lightweight cloth before inserting. The cloth will tend to keep the plugs in place, and also will fill some of the gaps between plugs. In most cases wooden plugs will not make a watertight fit, but by filling the remaining leaky area with rags, oakum and smaller wedges, the ingress of water can be greatly reduced. Square-ended plugs hold better than conical plugs in holes in plating one-fourth inch or less in thickness.
c. Welding a threaded pipe piece with pipe cap
A skilled welder can make an arrangement as shown in figure. This arrangement require a metal pipe piece with a threaded plug for blanking on one side; the other side should make a good surface contact with the end cover surface.
The diameter of the pipe should be considerably larger than the hole diameter. The pipe piece should be welded on the end cover while keeping the plug removed, it will need the water to flow through the pipe, once the welding is completed the pipe can be plugged. This method is relatively time consuming and require a skilled person. But the repair is highly reliable.
B. When not is operation
Cooler when not in operation can be drained completely and permanent repairs can be carried out. Cold repairs like applying DEVCON putty can also be considered which in this case will have sufficient time for curing.
a. A quick method is fitting a nut bolt with washer and Gasket RTV. Drain the cooler and remove the end cover. Clean the area with wire brush, and remove the weak rusted metal. Inserting a bolt in the hole diameter must be as close as possible. A washer with considerable larger diameter on both side of cover, applying Gasket RTV on the surfaces of washer. Tight the nut and left it for a curing period for few hours.
Now fit the cover back to the cooler and open the cooling water and check for any leakage.
b. Welding a patch over the damaged part. This require the surface preparation and heat treatment on the end cover. A patch of same material with a larger size than that of the hole is welded on the both side of hole. Time should be given for slow cooling. Check for any leakage, apply a coat of primer and then paint the cover. Now fit it back to the cooler, open the cooling water.
The first system cleaning in place or chemical washing is decidedly simpler but does not always allow you to obtain the desired effect it may not address deeply embedded or inaccessible residues and contaminants, leading to incomplete cleaning outcomes. In such cases, during Plate Type Heat Exchanger Maintenance work, disassembly and complete regeneration is a must.
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