CONFIDENTIAL REPORT 2G-9C S49
	FORMER GERMAN SUBMARINE TYPE IX-C
	COMPRESSED AIR PLANT
	SUMMARY
	The compressed air installation consists of a high pressure (2920 psi) and a low pressure (171 psi) system, with individual operating units on each system operating at other pressures.  The low pressure system is used for a wide variety of purposes.
	Some of the arrangements provided are very flexible and others are quite rigid.  Cocks are used on the low pressure system to provide a means of quick opening and closing and for interlocking operations, but all other valves are screw operating.
	The construction of the normal type air valve is of interest and has been the subject of review.
	July, 1946
	PORTSMOUTH NAVAL SHIPYARD, PORTSMOUTH, N. H.
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	General
	1.  What would be the compressed air system on a U.S. submarine is, in the case of the German vessels, divided into two parts.
	a)  A combination of systems which are approximate counterparts of the high and intermediate pressure air systems in U.S. naval practice.
	b)  A portion of the main engine exhaust system which is the counterpart in U.S. naval practice of the low pressure blow system.
	Because these two parts are related in some measure, and because the Navy Filing Manual locates the low pressure blow system in this "S" group, both parts will be discussed herein.
	2.  On this type of vessel, the compressed air plant consists of an electric and a diesel air compressor, several banks of air flasks, connecting piping, valves and manifolds.
	3.  This basic plant is connected directly to the high pressure blow system with its related equipment, and to the torpedo charging equipment and torpedo impulse fittings in the torpedo rooms.
	4.  By way of pressure reducing valves and piping the high pressure air system is connected to the low pressure system, the starting air system, the variable tank and negative blow arrangements, the hydraulic system and the windlass motors.
	5.  The high pressure blow system manifolds, piping and valves supply air for blowing main and fuel ballast tanks, and the bow and stern buoyancy tanks.
	6.  The torpedo charging equipment consists of a manifold and piping in each torpedo room, for charging torpedo air flasks.
	7.  The low pressure air system manifolds and piping supply air to operate ballast tank vent valves, to blow, flood and drain torpedo tubes, to blow sea chests, to operate engine shaft couplings, to operate diving plane clutches, to raise and lower radio and radar antenna masts, to cool the gyro compass, to provide air impulse for marker buoys when released, to pneumatic tools and to the horn.
	8.  The starting air system supplies air for starting the main engines and the diesel air compressor.
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	9.  The variable (regulating) tank air arrangements provide air for list control by blowing variable water from one side of the vessel to the opposite side, and for blowing the regulating tanks.
	10.  The negative tank arrangements supply air for blowing the negative tank.
	11.  The air connection to the hydraulic system supplies air to the hydraulic accumulator.
	12.  The air arrangements to the windlass motors permit these to operate.
	13.  The exhaust gas blow systems, by means of piping, a manifold and valves in the superstructure, supplies exhaust gas in lieu of low pressure air to blow main and furl ballast tanks.  In other respects it is a part of the main engine outboard exhaust gas piping system.
	14.  Salvage air arrangements, while shown in sketch books and text material were not actually installed in surrendered vessels.
	Compressed Air Plant - Description
	15.  This consists of one four-stage reciprocating air compressor driven by an electric motor, one Junkers free piston engine operating a four-stage air compressor, and related flasks, piping, manifolds and valves.  Both compressors are located in the maneuvering room.  The manifold is located in the control room.
	16.  The motor weighs 1700 kg (3750 lb.) and is rated at .56 to 62 kw on a voltage range of 110 to 170 volts, with a related speed range of 535 to 600 rpm.
	17.  The direct-connected compressor weighs 1155 kg (2440 lb.) takes 76 to 84 PSe (75 to 83 H.P.) depending on revolutions and is rated at 14 to 15.4 liters per minute (.493 to .543) cu. ft. per minute).  The discharge pressure of the air is 205 kg/cm2 (2920 psi).
	18.  The Junkers compressor consists of a free piston diesel engine and compressor in a common housing.  The assembled weight is 560 kg (1234 lb.) excluding muffler, tail pipe and insulation.  It operates at 780 to 840 rpm, and is rated at approximately 50 horsepower.
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	19.  The related compressor is rated at an effective air intake of 2 cu. maters per minute (70.6 cu. ft. p.m.) and a discharge of 10 liters per minute (.353 cu. ft. p.m.).
	20.  The air flasks have a total capacity of 6200 liters (219 cu. ft.) at 2920 p.s.i.  This is sufficient air to evacuate all main ballast, fuel ballast and bow and stern buoyancy tanks once at a depth of 180 feet, or 5.65 times the free air volume of these tanks.  Most of them are located in the superstructure, but six flasks which are part of four banks are located in the maneuvering room and engine room.
	21.  The maximum operating pressure of the system, as has been stated, is 2920 psi.
	22.  The electric compressor motor and controller are further described under the heading of electric auxiliaries.
	23.  Detail of materials, manufacturing clearances and adjustments is contained in the compressed air system instruction book (Beschreibung und Betriebsvorschrift für die Druckluftanlage auf U Boote Bauart IXC u IXD2)
	24.  The high pressure air manifold is a steel housing to which steel valve housings are welded.  Valve bonnets are a special bronze, spindles steel, discs steel, springs phosphor bronze and threaded seat insert is 18/8 chrome nickel steel.  Air piping is cooper plated steel.  Materials for valves are the same as those for the manifold.
	Compressed Air Plant - Operation
	25.  To start the system from empty tanks it is necessary either to operate the electric compressor, or to obtain air from shore of another ship via the transfer connection in the superstructure.
	26.  The electric compressor takes free air from the maneuvering room, compresses it and discharges it by way of a water separator and filter either to air bank No. 1 or to the manifold from which it can be distributed to other banks.
	27.  The diesel compressor requires compressed air for starting so cannot be started until sufficient pressure is available to set the starting pressures in the different compressor stages, and to start the diesel pistons.  This should be about 427 psi.  When started, the diesel compressor also drains air from the maneuvering room, compresses it and discharges it via the same water separator and air filter as the one used by the electric compressor.
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	28.  The manifold and air bank piping is so arranged that if it is necessary to shut down the manifold to effect repairs the vessel is not without air.  Air bank No. 1 has a direct connection to the L.P. torpedo air manifold in the after torpedo room, from which there is another connection to the main low pressure air manifold.  Air bank No. 6 can be connected to either the high pressure blow manifold, to the variable tank blowing and list control manifold or to both.  Air bank No. 8 has a direct connection to the low pressure torpedo air manifold in the forward torpedo room, which, like the corresponding manifold in the after torpedo room, has a connection to the main low pressure air manifold.
	High Pressure Blow System - Description
	29.  This consists of a main and stand-by supply line from the high pressure manifold, each with its own regulating valve, which are brought together, fitted with a pressure gauge and relief valve, beyond which the piping branches and connects to four manifolds, each with its own regulating valve and pressure gauge.  A valve on one manifold connects via piping and a stop-check hull valve to one tank.  In the amidships group, however, one valve leads to one side of one tank.  The manifolds are all located in the control room.
	30.  The tanks are grouped as follows on the manifolds:
	After Group - MBT 1     FBT 3 Amidship Group - FBT 4     MBT 5 Forward Group - FBT 6     FBT 7     MBT 8 Buoyancy Group - Stern Buoyancy     Bow Buoyancy
	The stop-check valves at the hull, the valves connecting the manifolds to the tanks, and the main manifold valve for the amidships group are normally locked open.
	High Pressure Blow System - Operation
	31.  Opening of the main blow valve or stand-by main blow valve, without other operation, will blow the amidships group alone.  By opening the valves to the after, forward and buoyancy manifolds as desired, these may be blown in any desired combination with the amidships group.
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	32.  Individual tanks may be blown separately, but to do so it is necessary to close all valves on the same manifold which do not connect to the tank to be blown, and if the tank is not in the amidship group, close the valve to the amidships group manifold as well.
	High Pressure Torpedo Charging Air - Description
	33.  This consists, in the forward torpedo room, of a main and stand-by supply line from the high pressure air manifold, to one of which is a direct connection to one air bank, and to the other of which is a direct connection to the low pressure air manifold.  These two lines are brought together and enter a manifold in the forward torpedo room which has the following connections:
	a)  A torpedo charging connection.
	b)  A connection, through a filter and reducing valve, to the anchor windlass motor.
	c)  A Mae West filling connection.
	34.  In the after torpedo room the same dual service is provided, but there is no manifold.  Instead, there is merely a lead to the torpedo charging connection.
	35.  By opening the appropriate valve on the manifold on the forward torpedo room it is possible to supply high pressure air to torpedoes, to the windlass motor, or to inflate life jackets.  In the after torpedo room, opening of the valve supplies air for charging torpedoes.  In an emergency, it is possible, in either room, to use the alternate supply from the main high pressure manifold, or to obtain air from one air bank.
	Torpedo Impulse Air
	35.  This is obtained, in the forward torpedo room, from a branch on the line connecting the main high pressure manifold and the No. 7 air bank.  Valves are fitted in the line which permit the No. 7 air bank to be used as a separate supply if air via the manifold is not available.
	In the after torpedo room, a similar arrangement is provided.  In this case the branch comes off the line to the No. 2 air bank, which can be made to serve as an emergency supply if desired.
	As the torpedo air system beyond this point is a function of the torpedo tube operation, no further description is provided at this point.  The reader is referred to the section on torpedo tubes for further information.
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	Low Pressure Air System - Description
	36.  This system begins with a lead from the high pressure manifold which branches, each branch passing through a 205 to 12 atmosphere (2920 psi to 171 psi) reducing valve and is then brought to the low pressure manifold in the control room.  In addition to this source of supply there is a separate line connecting the low pressure manifold to the low pressure torpedo air manifolds in the forward and after torpedo room which, in turn, have connections to the high pressure air system.
	37.  The manifold has one valve for each of the following:
	a)  Vent valve operating.
	b)  Torpedo low pressure manifold forward (which also can serve as supply)
	c)  Torpedo low pressure manifold aft (which also serves as supply) from which branches leads to the engine clutch operating gear and two sea-chest blow connections.
	d)  Diving plane clutch operating gear.
	e)  RDF mast hoist.
	f)  Radar mast and radio antenna hoist.
	g)  Bilge pump sea-chest blow connection.
	h)  Gyro compass cooling.
	i)  Pneumatic tools.
	j)  Horn.
	All the valves on the manifold are normal stop valves except the one to the tool connections, which is a regulating valve.
	38.  The pressure reducing valves are regular Draeger valves.  They operate on a counterbalanced spring and diaphragm basis.  One spring on the high pressure side of the line pushes a gasketed valve piston onto a seat.  Another spring, separated from the low pressure chamber of the valve by a membrane, is provided with a bearing plate and a pin in the low pressure chamber, which last, bears on the center of the valve piston.  If there is no compression of the low pressure spring, the valve is closed by the high pressure spring pushing the piston onto its seat.  By compressing the low pressure spring, pressure offsetting that of the high pressure spring is placed on the valve piston, which is forced off its seat, thereby, admitting air to the low pressure chamber.  This increases the pressure in the low pressure chamber, thereby, deforming the diaphragm membrane and compressing the low pressure spring further, taking the outer pressure off the center of the valve piston which is reseated by pressure
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	from the high pressure spring, thereby, stopping the flow of air.  As the air pressure in the low pressure chamber falls, the low pressure spring takes charge and pushes the pin against the valve piston sufficiently to unseat it.  The valve is basically a commercial type unit and comes in a number of different sizes, with varying arrangements of springs, but the principle on which all operate is the same.  It relies for satisfactory operation on proper spring balance between a very large spring and a very small spring.  The large one, on the low pressure side, is readily accessible, but fatigue of the small spring has the effect of introducing increasingly higher pressure into the low pressure side of the valve, and the valve must be disassembled to do any work on the small spring.  The amount of maintenance probably accounts for the two reducing valves in parallel.
	39.  The line to the ballast tank vent operating gear leads via a main valve and another pressure gauge, to three four-way cocks in parallel, in the control room.  Each of these cocks connects, by way of two lines and a second four-way cock, to an air cylinder in which is the piston for opening and closing one group of vent valves.
	One of the three cocks first mentioned above control the air to separate operating gear for MBT 1 and FBT 3, one controls the air to a common unit actuating vents for FBT 4 and MBT 5, and one controls two units for vents on FBT 6 and 7, and MBT 8.
	40.  The two torpedo low pressure manifolds are each provided with a connection for pneumatic tools. a line to the marker buoy stowage, and a line to the group of valves which control the flooding, blowing and draining of the torpedo tubes.  This last group of valves, for each tube, consists of a stop valve, a relief valve and pressure gauge, a four-way cock which admits air to the torpedo tube on the WRT tank while venting the other one of the pair, and a three-way cock to permit selective venting of either or both ends of the torpedo tube.  There is also an individual cock in the vent line of each WRT tank and a common three-way cock which permits selective use of either WRT tank by any torpedo tube.
	41.  The sea-chest blow arrangements merely provide a connection to the sea side of certain sea valves for the purpose of clearing clogged strainers.  They are all operated from the adjacent compartment.
	42.  The connection to the engine clutch operating gear leads, by way of a stop valve and a pressure gauge, to a four-way cock at each engine.  The cock has one line to each of two
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	air-oil cylinders, and can introduce air pressure on one of then while venting the other, threby forcing oil into one end or the other of a cylinder in which is a piston which operated the main engine shaft clutch.
	43.  The line to the bow and stern diving plane clutches branches at the diving stations, with a separate valve for each set of planes, and runs fore and aft to the torpedo rooms where it connects to the clutch operating gear.
	44.  The lines to the masts connect to air cylinders for raising and lowering the masts.
	45.  The line to the horn leads, by way of a lever-operated back-up valve and a stop valve at the hull, to the horn.
	Low Pressure Air System - Operation
	46.  It is possible to operate the ballast tank vent valves in either of two ways:
	a)  by setting the three cocks for the operation desired and then admitting air from the adjacent stop valve, or -
	b)  opening the stop valve and then operating the cocks serially to obtain the desired operation.  It is also possible, if the secondary cock adjacent to the air cylinder for the vent valve is rotated to vent both ends of the cylinder, to operate the vent valves directly by means of a lever.  Note, however, that there is a minimum of selectivity, for the three primary cocks in the control room are permanently piped to specific air cylinders, and the air cylinders control specific tanks in fixed combinations.
	47.  The only tributaries of the torpedo low pressure air manifolds which call for description are the combination of valves related to the torpedo tube filling and draining.  On this combination, operation of one four-way cock admits air under pressure either to the WRT tank or to the torpedo tube with which the cock is associated.  Related operation of a cock on the tube drain line then permits flow of water from the WRT tank to the tube, or vice versa, as desired.  Further, the operation of an additional three-way cock in the air line permits blowing or venting one or both ends of the tube.  A further three-way cock in the sir line to the WRT tank, when operated together with a three-way cock on the common drain line to the WRT tanks, permits a choice of draining to or flooding from either the port or starboard WRT tank.
	48.  There is no special operating indicated for the sea-chest blow connections.
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	49.  The main engine clutch operating gear is all located in the engine room.  The description covers the operation and need not be amplified.
	50.  The lines to the plane clutch operating gear terminate at a cylinder with a single-acting piston in each of the two torpedo rooms.  Admission of air through the valve at the after diving plane station causes the piston at the stern plane mechanism to be displaced, thereby disconnecting the electric motor and connecting the hand drive shafting which leads forward to the control room.  The operation for the bow planes is similar.  In the absence of air, either clutch is operable by hand, and restoration of power operation must be accomplished by hand for the affected planes.
	51.  In the case of the radio, RDF and radar mast, opening of the valve in the air line adjacent to the mast to be operated admits air below an air piston in a cylinder connected to the base of the mast.  The mast is raised by the air pressure acting on the lower face of the piston.  When the mast is fully raised, it is mechanically secured in position, and the air pressure is released.  To lower the mast, air pressure is again applied, the mechanical securing is released, and the cylinder is then vented, the weight of the mast being sufficient to push the air from the cylinder and restore the mast to its stowed position.
	52.  The line to the horn has a spring-loaded, lever-operated piston valve in the conning tower, which is operable by an extension rod from the bridge.  When the hull valve has been opened, operation of the piston valve admits air to the horn.
	Starting Air System - Description and Operation
	53.  The starting air system consists of a line from the high pressure air manifold which leads, by way of a stop valve, a filter, a 205 to 75 atmosphere (2920 to 1066 psi) reducing valve and a relief valve to a starting air flask with a pressure gauge.  The 1066 pound line to the air bottle also has a branch to each engine, which reaches the engine by way of a pressure gauge, a stop valve, the main engine starting valve, a 75 to 30 atmosphere (1066 to 427 psi) reducing valve, a second gauge and the air starting and reversing arrangements on the engine.  The reducing valves are similar in principle to those described under the low pressure air system.
	54.  The starting air flask is normally kept charged to 1066 psi, acts as a volume tank and supplies air which is admitted to the desired engine by opening the related main starting valve.  It also serves as an emergency source of air in case air from the high pressure manifold is not available.
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	55.  Starting air for the diesel compressor is supplied when a starting air flask for the main engines is installed, from a branch on the discharge line from the compressor to the high pressure air manifold, which in this case acts as a supply line to the diesel compressor, and is fitted with a stop valve, filter, a 205 to 30 atmosphere (2920 to 427 psi) reducing valve and a relief valve.  Opening of the stop valve admits air to the air starting mechanism of the diesel compressor.
	56.  In lieu of the arrangements described in the foregoing three paragraphs, some vessel of this type have no starting air flask for the main engines, but instead have a line from the high pressure air manifold which leads, via a reducing valve, a filter and a parallel arrangement of two 205 to 30 atmosphere reducing valves (2920 to 427 psi) with stop valves on the high and low side of each reducer, to a common line serving the main engines and the diesel compressor.  In this case one reducer acts as a standby for the other, and, if the system is in operation, the opening of the starting valve to an engine or the compressor admits air to the desired unit for starting.
	Regulator Tank Piping - Description and Operation
	57.  The piping leads from the high pressure air manifold, by way of a regulating valve, a gauge and a relief valve, to a manifold in the control room.  The manifold has four connections, one to each half of the regulating tank and one to each half of the regulating bunker.  Piping from each lead runs to port and starboard and connects to the vent line of the related tank.  By opening one or more of the manifold valves, which are all stop valves, and opening the regulating valve, air is admitted to either half of either tank.  If, in addition, valves on the seawater piping to the tanks are opened, a differential pressure between one half and the other of one tank, or between half of one tank and half of the other tank, will cause a flow of water which provided compensation for a list, trim compensation, or both simultaneously.  Further it is physically possible to blow to sea via the regulating tank flood valve.
	Negative Blow Piping
	58.  This piping from the high pressure air manifold connects, by way of a regulating valve, pressure gauge and relief valve to the negative tank inboard vent line.  By opening the port or starboard hull valve in the vent line, it is possible to put pressure on either half of the negative tank and, if the flood valve is then opened, to blow the negative tank to sea.
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	Hydraulic Air Piping
	59.  This piping leads from the high pressure air manifold, by way of a 205 to 48 atmosphere reducing valve (2920 to 682 psi), a filter, and water separator and a stop valve, to the top of the air-oil flasks which store oil under pressure for the hydraulic system.  The air is used to provide an initial pressure for the hydraulic pumps to operate against, and as a source of make-up air for the flasks if and as necessary.  Further description is provided in the section on hydraulic systems.
	Windlass Air Piping
	60.  This piping leads from the torpedo high pressure air manifold in the forward torpedo room, by way of a filter, a parallel arrangement of a 205 to 4 atmosphere (2920 to 57 psi) reducing valve, a gauge and a relief valve, to an air motor which operates on a multiple screw basis similar to that of an IMO pump.  Opening of the supply valve, either in the forward torpedo room or air deck, starts the motor.  Further description is provided in the section on mooring arrangements.
	Low Pressure (Exhaust Gas) Blow System - Description and Operation
	61.  The exhaust gas blow system consists of a pipe extending forward from a point between the inboard and outboard main engine exhaust valves port and starboard, which is joined together and led forward to a manifold in the superstructure over the control room.  There is a stop valve in the pipe to the manifold which serves as a main blow valve.
	62.  From the manifold, individual piping runs extend to MBT 1, FBT 3, FBT 4, each half of MBT 5, to FBT 6, FBT 7 and MBT 8.  The eight associated valves at the manifold are normally open, although they are operable from within the control room.
	63.  Normal operation involves starting the main engines as soon as the vessel surfaces, and by throttling the exhaust gas line diverting all or part of the exhaust gas via the main blow valve and the manifold to the tanks listed above, which are blown simultaneously when the main blow valve is opened.  Pressure gauges in the engine room and at the main blow valve assist in controlling the exhaust gas pressure, which is not supposed to exceed 10 psi.
	Comments:
	The compressed air storage capacity is smaller than that on U.S. submarines.  The diesel compressor, while it was
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	installed as an auxiliary to the electric, in later types of vessel became the principal compressor with the electric compressor as a standby unit.  The diesel compressor is a compact, efficient unit which the Germans found very reliable, although there has been some informal report of maintenance difficulty by American crews which may be in large part caused by unfamiliarity and lack of adequate instruction material in english.  The extensive use of cast iron on the electric compressor is unfortunate.  In other respects the compressor is like the Hardie-Tynes type currently used by the Navy.
	The high pressure air system is designed to provide alternate paths for some essential services in case of a need to secure the high pressure manifold.  Air for blowing tanks, however, is not available unless the manifold is in operation.
	The valves on the manifold are of considerable interest, for they are of the non-packed stem type with a lapped fit between the collar on the stem and the related shoulder inside the valve bonnet, and with conical discs and seats.  One of these valves has been cycled 30000 times at the Naval Shipyard, Portsmouth without developing a leak either through the stem or seat.  It was easily operable at all times, and required no forcing to seat properly at any time.  Informal information from members of U.S. Naval crews on German vessels does not bear out the test described, for they report considerable trouble with leaky manifold valves.  It is possible that, from the standpoint of the effect of depth charges, the ease of operation encountered could be a disadvantage if there were any tendency for the valve to back off the seat under shock conditions.
	The arrangement and operation of the high pressure blowing arrangements is not entirely satisfactory.  Screw type valves are used throughout, and a change in set-up cannot be made instantaneously, as it requires operation of two or more valves to effect the change and start blowing.
	The vent valve operating arrangements are also relatively inflexible, although the mechanics of the individual vent valves and their operating gear are extremely simple.  The extensive use of cocks is of interest.
	The torpedo tube draining and filling system is also simple but effective.  Here too, cocks play a major role.
	The one-way plane clutch operating gear is understood to be accounted for by the fact that corrective work is necessary at the location of the plane operating gear in any event, so a return line to reverse the clutching operation is unnecessary.  This does not appear satisfactory, for the reestablishment of power operation with the existing
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	arrangements requires a certain amount of interlocking operation between the torpedo room and the control room.  Without such interlock, the planesman may find himself with a dead handwheel.
	The exhaust gas blow system has both good and bad points.
	Use of exhaust gas for blowing tanks on the surface eliminates the need to install a low pressure blower with its related piping and fittings.  The location of the piping and manifold in the superstructure saves space within the pressure hull, and eliminates the need for high pressure fittings.
	At the same time, the system as engineered permits water and/or air to siphon from one tank to another, as the only valves which separate one tank from another are the ones at the manifold, which are normally left open.
	The Germans have been at great pains to specify that blowing with the low pressure system should be done only when there is no angle on the boat.  In addition to the instructions contained in the instruction books on the exhaust gas piping system, part VI of the special war experiences book - machinery section - is devoted to discussion of the hazards.  Mention is made of running under, caused by lack of teamwork between the control and engine rooms, difficulty of maintaining tight valves with the small handwheels provided, and uncontrollable masses of water in the blow piping caused by lack of valve tightness and the effect of this water on trim and weight.  Further remarks on the influence of heating and cooling on the tightness of the valves is covered on page 106 of the same publication.  Still further notice is given on page 112 of the publication with reference to inadvertent blowing of ballast tanks when snorkeling, caused by lack of tightness in the exhaust gas blow system valves, and the resultant difficulty in controlling depth.
	With reference to the need to retain an "0" angle mentioned above, this is related to the method employed for blowing in which all tanks are blown simultaneously.  The effect of any pronounced angle is to introduce an appreciable difference in pressure between one tank and another at the point where the blow line enters the tank.  The result of blowing with a pronounced trim angle would be that the exhaust gases, taking the path of least resistance, would blow the highest tank first and, in a dynamic state where the trim angle was changing, could permit tanks previously blown to flood again by way of the open blow lines.
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