CONFIDENTIAL REPORT 2G-21 S49
	FORMER GERMAN SUBMARINE TYPE XXI
	COMPRESSED AIR PLANT
	SUMMARY
	The air systems installed in this type of vessel are not altered in major particulars from those on earlier vessels, although certain changes have been made to reduce the number of separate lines and the sizes of the manifolds.
	Compressors, manifolds, valves, piping and operating gear are all of the same general types as those on the 9C and 9D2 vessels.
	July, 1946
	PORTSMOUTH NAVAL SHIPYARD, PORTSMOUTH, N. H.
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	REPORT 2G-21 S49
	General
	1.  The air systems installation is greatly simplified in comparison with the systems on the XB vessels, and has many of the characteristics of the installations on IXC and IXD2 vessels.
	Compressed Air Plant and High Pressure Piping Arrangement
	1.  The compressed air plant consists of two diesel compressors in the engine room, one electric compressor in the pump room, 23 air flasks all located within the pressure hull, a manifold in the control room and interconnecting piping.
	2.  Each compressor is provided with a separate lead to the air manifold, and in each lead is a water separator and a filter.
	3.  The two leads from the diesel compressors are arranged to permit them to be connected together, and from this bilge piping section branches are taken off to the three after air banks, to the main engine and diesel compressor starting connections and to the bilge water tank (which in this type of vessel takes the place of the one main ballast tank on the 9C and 9D vessels which was equipped with flood valves.)  The branch to the starting air arrangement is supplied with a filter and with two 2920 to 427 psi reducing valves in parallel.  Further, the lead from the port diesel compressor has a branch which leads to air bank No. 4 in the magazine, while the corresponding starboard lead has a branch to air bank No. 5 in the control room.
	4.  The piping from the electric compressor has no branches between the unit and the high pressure air manifold.
	5.  The manifold is much smaller than that on earlier vessels.  No separate leads to air banks are provided, as all leads serve multiple purposes.  Connections provided are as follows:
	a)  From the starboard diesel compressor.
	b)  From the port diesel compressor.
	c)  From the electric compressor.
	d)  From the air transfer line.
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	REPORT 2G-21 S49
	e)  Two to the low pressure air manifolds.
	f)  To the hydraulic system.
	g)  To the negative tank (in design only, as the negative tank was later abandoned as a design element).
	h)  To the torpedo h.p. air manifold in the torpedo room forward.
	i)  To the mine ejection valve (this does not appear to be fitted to torpedo tubes in available vessels of this type).
	j)  To the high pressure blow manifold.
	6.  Branches on the piping connections listed above, arranged by the same subheads, are as follows:
	a)  To air bank No. 5.
	b)  To air bank No. 4 with a bridged connection to what was originally the lead to the negative tank, and to the antenna mast.
	c)  None.
	d)  To the Mae West filling connection.
	e)  None.
	f)  None.
	g)  See (b) above.
	h)  To air bank No. 7.
	i)  To air banks No. 6 and 8, and to the normal connections to the high pressure blow manifold.
	j)  To the regulating tank blow manifold.
	7. The arrangement of piping permits charging of air banks, operation of the antenna mast and operation of torpedo tubes, when surfaced or snorkeling, even though the high pressure air manifold is not in service.
	8.  The air volume in the flasks is sufficient to evacuate all main ballast tanks at a depth of 70 meters (230 feet) according to the General Information Book.  Total air content of the flasks is 7600 liters (268 cu.ft.).
	9.  The torpedo high pressure air manifold receives air from the high pressure air manifold either directly or via the line to the mine ejection valve, and has connections, to the windlass motor, to the torpedo charging connections, to the impulse flasks port and starboard, to the torpedo ventilating fittings and the torpedo test connections.
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	REPORT 2G-21 S49
	The one to the windlass operated by way of a regulating valve and relief valve, as the maximum operating pressure of the windlass motor is 85 psi.
	Low Pressure Air System
	1.  The low pressure air manifold is supplied by two lines from the high pressure air manifold, one of which has a 2920 to 171 psi reduction valve and the other of which has a regulating and a relief valve.
	2.  The manifold has direct connections as follows:
	a)  Port trim tanks.
	b)  Starboard trim tanks.
	c)  Pneumatic tool connections forward, torpedo flood and drain arrangements in the torpedo room, forward sanitary blow, and the sea chests forward.
	d)  The vent valve operating gear control valves.
	e)  RDF mast.
	f)  Gyro compass  air cooling piping.
	g)  Radar mast.
	h)  Pneumatic tool and sea chest blow connections aft, sanitary blow connection in the after compartments, and operating gear for the main engine and main clutches in the maneuvering room.
	i)  Periscope air fittings, snorkel mast motor, hovering gear (where fitted) and horn.
	3.  The piping and valve connections to the trim tanks admit air to either half of the forward or after trim tanks, as desired, thereby displacing water to the related half of the trim tanks at the opposite end of the vessel.
	4.  The low pressure torpedo air arrangements are the same as those on earlier vessels except that stop valves are substituted for the cocks used in earlier vessels at the top of the WRT tanks.  The cock arrangement for selecting the flow of air to the torpedo tube or the WRT tank, with the interrelated venting of the opposite tank, is retained.
	5.  Angle cocks are employed on the sanitary tanks blow and vent line in lieu of valves.
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	REPORT 2G-21 S49
	6.  The sea chest blow arrangements are of the same type as those on earlier vessels.
	7.  The vent valve operating gear can all be operated simultaneously under the control of a single valve, or the tanks can be operated under the control of cocks as follows:
	a)  Bow buoyancy alone
	b)  MBT 5 alone
	c)  MBT 4 and 3 together
	d)  MBT 2 and 1 together
	8.  Although the low pressure air system diagram in the sketch book shows operation for the outboard engine air induction valves, port and starboard for the negative flood valves, no such power operations is provided for the air induction valves, and the negative flood valves have been sealed.  Power operation is provided however, for the snorkel quick closing valve, via a separate cock adjacent to the ballast tank vent valve operating cocks.
	9.  The piping to the RDF and radar mast has, in each case a regulating valve, a relief valve, and a cock which admits air either above or below the piston for lowering or raising the mast.
	10.  The gyro compass cooling line on this type of vessel has a 171 to 7 psi reducing valve in the line with a bypass in which is a regulating valve, and a relief valve.  The normal air blast cooling found on earlier types if vessels is retained.
	11.  The air-oil operation of the main clutch and main engine clutch operating gear found in earlier types is retained.
	12.  The snorkel hoist motor is a screw type air motor generally similar for the IMO pumps.  Admission of air to one or the other end of the pump determines the direction of rotation.  Raising or lowering the snorkel is accomplished by a gear on the motor shaft which engages a ratchet on the snorkel mast.  A four-way cock controls the admission of air and the venting of the motor.
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	REPORT 2G-21 S49
	High Pressure Blow System
	1.  This consists of a manifold which receives high pressure air via either of two lines from the high pressure air manifold or from air banks 6 and 8, if the high pressure air manifold is out of service.  A regulating valve is installed in each supply line to the manifold, with a pressure gauge and a relief valve. Lines lead from the manifold to the stern buoyancy tank, main ballast tank 1, 2, 3, and 4, and 5, and the bow buoyancy tank, with a stop check valve at the hull opening for each tank.  The lines to MBT 1, 3, 3 and 4 divide and supply air to both halves of each of these tanks.  Normally all tanks are blown simultaneously under control of the regulating valve, but each tank can be cut off individually at the manifold and at the hull valves.  The hull valves are normally locked open.
	Regulating Tank Blow System
	1.  This system receives air from a branch on one of the lines connecting the high pressure manifold and the high pressure blow manifold, by way of a regulating valve.  Four valves on the manifold connect to lines which serve each half of the regulating tank and the regulating bunker.  Transfer of water from one half to the other half of the same tank is accomplished by opening the appropriate valve or valves.
	Hydraulic System
	1.  The air line to the hydraulic system connects two air flasks by way of a regulating valve, oil separator and filter, and supplies make-up air to the air flasks which serves as volume tanks on the hydraulic system.
	Low Pressure (Exhaust Gas) Blow System
	This is the same as corresponding systems on earlier vessel, with piping from the main engine exhausts via a main blow valve to a manifold from which piping connections lead off to each of the five main ballast tanks.
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	REPORT 2G-21 S49
	Comment
	All evidence point to the fact that great efforts were made to conserve weight by reducing the size of the manifolds, and by employing multi-purpose piping leads whenever possible.
	The type of valve used is the same as that on earlier vessels except at the hull, where valves with packed stem glands are substituted for the earlier type.
	In general the air systems appear to serve their designed purposes satisfactorily.
	The exhaust gas blow system has the same advantages and disadvantages as that on earlier vessels.
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