GB2185560A - Liquid nitrogen distribution system - Google Patents

Abstract

Apparatus for providing a simulated space environment for the testing of articles under low temperature conditions comprising a liquid nitrogen head tank (D), liquid nitrogen subcooler pumps (H), a liquid nitrogen subcooler coil (G), a liquid nitrogen head tank makeup pumps (B), a low pressure liquid nitrogen storage tank (A), a high pressure liquid nitrogen storage tank (C), a liquid nitrogen transfer pump, a vacuum chamber (E) and thermal simulation heat exchanger shrouds (CC, F) contained within the vacuum chamber (E). These components are connected so that the apparatus can be operated both in a subcooled pressurized closed loop system and in a gravity convection system. While liquid nitrogen in this apparatus will produce a stable uniform temperature of -173 DEG C (-297 DEG F) within the shrouds (CC, F) containing the article to be tested, other liquids with similar low temperature characteristics may be used. <IMAGE>

Description

SPECIFICATION Liquid nitrogen distribution system This invention relates generally to the simulation of cold temperatures in facilities utilizing a liquid medium such as freon, liquid nitrogen and helium whose boiling point at one atmosphere provides the controlling temperature. Typical facilities in which this invention could be utilized are thermal vacuum test facilities, optical test facilities, wind tunnel test facilities, coating chambers and othertest and manufacturing facilities requiring the simulation of a stable thermal environment.

Thermal simulation has been accomplished infacilitiesto date by the application of one ofthefollow- ing systems: one, a closed loop, pressurized, forced flow, subcooled system; two, a gravity feed convec tion system; orthree, a boiling mode system. How- ever, there is no thermal simulation system known to the applicants which combines into one common operating system a closed loop, pressurized, forced flow subcooled system together with a gravity feed convection system.

Recently there has been an increasing concern as to the reliability and economic operation ofthermai simulation facilities due to the critical nature of the test specimens and increased length of test periods.

Thus there is an urgent need for a thermal simulation system which provides reliability, flexibility and economic operation over a wide range of requirements.

US Patent Nos 3566960,3851274and 4024903 may be pertinentto the present invention. None of the prior art patents teaches the utilization of liquid nitrogen, which, while never completely changing state, is allowed to change slightly into the gaseous form, and thus due to the natural convection thereof, tends to cool the device being tested in the vacuum chamber. Patent No.3851274 generally relates to use of liquid nitrogen in a cooling system for a lazerdevice. Patent No.3566960 relates to a cooling system for a process that is carried out in a vacuum chamber. Patent No.4024903 depicts a cooling system utilizing the natural circulation of cooling water.

In one aspect the invention provides apparatus for testing articles in a simulated space environment comprising: acirculation system meansforsimulating low temperatures of a space environment including: avacuum chamber; internal and external piping means cooperating therewith in both a subcooled pressurized closed loop forced feed system and in a gravity convection system; heat exchanger means in said vacuum chamber connected to said piping means and cooperating with both of said systems; and a head tank meansforsupplying liquid nitrogen (LN2) to both of said systems.

In a further aspect the invention provides a device for testing articles under lowtemperature conditions comprising: aheadtankforcontaining liquid having low temperature characteristics, subcooler pumps, a subcooler coil, head tank makeup pumps, a low pressure liquid storage tank, a high pressure liquid stor agetank,a liquid transfer pump, piping meansfor operatively connecting the aforesaid, and a vacuum chamber having thermal simulation shrouds therewithin; said thermal simulation shrouds within said vacuum chamber being comprised of several cylin drical zones with a top and bottom shroud providing an optically dense envelope surrounding an article to be tested.

The present invention provides a thermal simulation system in which the thermal shrouds can be maintained at a uniform controlled temperature with high heatloadswhichcould be equal to onesolar consta nt 1440 watts per square metre (130 watts per square foot). This same system can also provide a uniform thermal shroud temperature with high spot heat loads from the test environment. The thermal simulation system can also maintain thermal shrouds at a controlled temperature with heat loads such as 323 watts per square metre (30 watts per square foot). The above can be achieved through a gravityflowconvection system.The system has the advantages that the thermal shrouds can be maintained at a controlled temperature in the event of power failure thus protecting a test article, and that consumption of the circulating liquid medium can be minimised. The thermal simulation system as defined above can be operated in either a manual mode or an automatic mode. This invention provides a liquid nitrogen circulation system which results from the combination of the characteristics of a forced flow recirculation system utilizing mechanical pumps and the characteristics of a gravityconvec- tion system utilizing a head tank and open circuits through which fluid can circulate and behave according to natural convection.In combining these two fundamental systems, hardware modifications have been made which allow certain equipment to perform in either mode while still achieving the basic objective of heat transfer from a warm body (test specimen) to heat sink panels herein described as main shrouds or auxiliary shrouds.

This invention is applicable to facilities such as thermal vacuum space vehicle testfacilities, optical test facilities, wind tunnel test facilities, coating chambers and other test manufacturing facilities re quiringthesimulation of a stablethermal environment. The description herein is based on the application of a thermal vacuum test facility such as would be utilized for the testing of space vehicles. The thermal simulation system for such a facility utilizes liquid nitrogen as the circulating medium to simulate the temperatures of outer space in the test envelope.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:- Figure 1 is a schematic depiction of facility ofthe present invention showing the main component units; Figure2 shows the vacuum chamber from the system of Figure 1 and the components contained therewithin; Figure3 is a side elevational view of a cylindrical shroud zone; Figure 4is a cross-sectional view along line 4-4 of Figure 3.

Figure 1 shows a simplified schematic of the overall facility identifying the following main components: liquid nitrogen (LN2) head tank, LN2 subcooler pumps, LN2 subcooler, LN2 head tank makeup pumps, low pressure LNP storage tank, high pressurt LN2storagetank, LN2transfer pump and vacuum chamber containing the thermal simulation shroud.

Thevacuumchamberisshown enlarged in Figure 2. The thermal shroud (F) inside the chamber is comprised of several cylindrical zones, a top and bottom shroud providing a optically dense envelope surrounding the space vehicle. The thermal vacuum facility usually requires the application of auxiliary shroud panels placed at random locations inside the test envelope. One such auxiliary shroud zone (CC) is shown in Figure 2.

The Figure 1 view shows the overall circulation system means for assimulating low temperatures of a space environmentfortest purposes. Avacuum chamber E contains therewithin main thermal shrouds F and auxiliary shrouds CC. These shrouds, which are essentially heat exchangers, are shown in structural form inFigures 3 and 4 of the drawings.

Preferably, the main thermal shroud F comprises a plurality of cylindrical shroud zones. Figure 2 shows a cross-sectional view wherein a plurality ofthe cylindrical shrouds is depicted, with the top shroud and the bottom shroud of said grouping being lab elled. Along the vertical right side ofthis Figure, the liquid nitrogen return manifolds are shown. These are the manifolds indicated by reference numeral 30 in Figures 3 and 4 of the drawings. Along the vertical left side of Figure 2, the liquid nitrogen supply mani- folds are shown. These correspond to the manifolds 20 of Figures 3 and 4.

The liquid nitrogen (LN2) is circulated through passages in the thermal shroud panels, thereby providing a stable uniform temperature of -173"C (-297"F) of the shroud exposed to the test article. As previously mentioned,thethermal facility utilized for description ofthis invention utilizes liquid nitrogen as the circulating medium. This invention is applicable to other liquid as a circulating medium whose boiling points at one atmosphere will produce re- quired operating temperature. A requirementfor such liquids when used as a circulating medium with the apparatus herein is that the boiling point thereof at one atmosphere will produce the required test operating temperature.

Other components applicable to a thermal vacuum facility as depicted in Figure 1 are not shown herein, but are a part of this invention, asfollows: 1.vacuum pumping system to evacuatethe vacuum chamber; 2. gaseous nitrogen system to remove LN2from shroud; 3. gaseous nitrogen thermal system to maintain shrouds at + 1500C (+300 F) to - 157 C (-250 F).

The following description of the invention is based on the application of an LN2 thermal system as shown in Figure2thatwill maintainthethermal shrouds inside the vacuum chambershowntherein at or near LN2 temperatures. The criteria governing the design of this system are as follows: 1. thermal shroud uniform heat load 200 KW (3230 watts/square metre or 300 watts/ft2); 2. maximum shroud temperature minus 1760C (285 F); metres (58' - 6) in height.

3. thermal shroud size 9.14m (30 ft) x 17.83m (58 ft 6 in) in height.

The description set forth below is divided into three parts: the pressurized closed loop forced flow mode of operation; the gravity feed convection system mode of operation; and common system elements.

Part -Pressurizedclosedloop forced flow mode Liquid nitrogen (LN2) loss due to the heat load into the thermal shrouds (heat exchanger panel) F located in vacuum chamber E is supplied to the head tank D from the low pressure LN2 storage tank A. The LN2 head tank Dislocated at an elevation above vacu um chamber E. LN2 is withdrawn from the bottom of storage tank A and pumped to the head tank D through a 23 gpm cryogenic pump B. An alternate means of supplying makeup LN2tothe head is utilizing the high pressure LN2 storage tank Cto transfer LN2to the head tank D.The LN2 head tank D is operated ator nearly equal to atmospheric pressure, and hence contains liquid boiling at thermal equilibrium saturation condition of approximately one atmosphere, thus providing a cold liquid (LN2).

The LN2 head tank D contains an LN2 subcooler coil G that is an integral part of the pressurized closed loop forced feed system.

LN2 is supplied to the LN2subcoolerpumps (180 gpm) Hthrough conduits l,J and Kwhich raisethe system pressure to approximately 223 pa (150 psi).

The LN2 is transferred through conduit Lto the thermal shrouds F located in vacuum chamber E. The thermal shrouds F are exposed to the heat loads inside vacuum chamber E. As the fluid passesthrough the thermal shrouds F the pressure is sufficiently high thatthe heat gained by the flow liquidraisesthe pressure in thefluid stream but it remains as a liquid (does not change phase). The liquid isthentransferred through conduits M and N from the thermal shrouds F backtothe LN2subcoolercoil Gwhich is immersed in the LNP in the head tank D.Whenthe liquid passes through the coil, heat gained inside chamber E is exchanged to the colder liquid in the heat tank, causing liquid in the head tank to boil more vigorously. As the liquid in the headtankis boiled off, it is again made up from pumping from the low pressure LN2 tank A or by pressure transferfrom high pressure tank C, as previously described. After theliquidpassesthroughthesubcoolercoil G, itis nearlythe same temperature as the head tank liquid.

The liquid then flows to the pumps G through conduits l,J and Kand continues circulation.

LN2 for startup and system losses for the closed loop system is made up from the head tank D through conduit 0 to a suction venturi located near pump H suction.

Part2 - Gravity feed convection system Liquid nitrogen (LN2) loss due to the heat load into thermal shrouds is supplied to the head tank as pre viously described in Part 1. The primary difference in the gravity feed convection system is that the thermal shrouds F inside vacuum chamber E can be maintained at liquid nitrogen temperature without dependency on transfer pumps or electrical power.

LN2supplytothethermalshroudsFinsidevacuum chamber E is provided from the liquid inside the head tank D. Liquid flows through conduits P, J, Q and Rto the thermal shrouds. The inlettothethermal shroud zone connects to the lower portion of each shroud zone. The liquid then flows progressively upward through the shroud zone or passage.

As the liquid flows it picks up heatdueto heatfiux from inside chamber E, thereby decreasing its density slightly, creating a pressure differential between any point in the thermal shroud and a corresponding pointatthe same elevation inconduitJ.Astheliquid continues to flow upwardly in pipe M, sufficient heat has been gained that at some elevation at or below the head tank, the liquid becomes a two-phase mix turehaving less density than liquid in supply pipeJ at the same elevation, thereby providing the driving force for the liquid, overcoming system friction losses and causing the liquid to flow by natural con vection. The two phase mixture flows back to the headtankthrough conduits M and S.

As shown in Figures 3 and 4, the supply or input manifolds 20 are at the bottom of each heat exchan ger structure, while the outlet manifolds 30 are atthe top thereof. This, of course, corresponds with the gravity flow system effect, as described above. Of course, when the system is operated in the pressurized closed loop forced flow mode, the specific structure arrangement would not be as critical. This mode of forced flow as effected by an appropriate liquid nitrogen pump is described above. In addition to the heat exchanger structure of the thermal shrouds, it is also necessary that a liquid nitrogen head tank D be provided for supplying the liquid nitrogen in both the forced feed flow mode, as well as the gravity feed flow mode.

Part3 - Common system elements The liquid capacity of the LN2 head tank D is app roximately 7600 litres (2,000 gallons). The liquid capacity of the LN2tank is maintained at a level sufficient to maintain the LN2 subcooler coil G immersed in liquid. The LNP head tank incorporates a vapour space above the liquid that will allow phase separation of the two-phase flow returning to the head tank from the thermal shrouds through conduits M and S during the gravity feed convection system mode of operation (Part 2).

The LN2 head tank is provided with a low pressure vapourventing system T. The venting system includes demister U to prevent loss of LN2through venting wet vapour.

Thermal shrouds and IN2 distribution system The thermal shroud consists of several cylindrical shroud zones: a top shroud and a bottom shroud as shown in Figure 2. The LN2supply is connectedto the bottom of each zone; correspondingly, the LN2 outlet is located at the top of each zone. Figure 3 shows a typical detail of a portion of a cylindrical shroud zone. The LN2 thermal shroud is comprised of a number of thermal panels 12 each provided with a fluid passage 14. The spacing of the fluid passage 14 is dependent upon the heat load into the thermal shroud. Each fluid passage in a particularzone is connected to the inlet manifold 20 and an outlet manifold 30to provide parallel liquid distribution through the shroud zone (see Figure 4).This prov idesacontinuousupwardflowofliquid requiredfor the gravityfeed convection system modeofoper- ation.

The individual liquid passages 14 are connected to the LN2 inlet manifold 20 through a supply jumper tubeSJTandtotheLN2outletmanifoldthroughan outletjumpertube OJT (see Figure 4). The outlet jumpertubeswill be provided with an orifice coupling OC. The sizing of the orifice will insure fluid distribution throughout each shroud zone enabling a uniform thermal shroud temperature to be maintained.

The entire LN2 distribution system including external conduits in Figure 1 will be sized to minimize pressure drop for the gravity feed convection system modeofoperation and insurethata balanced liquid flow distribution can be achieved for the operation of the combined systems.

Auxiliary shrouds and external LN2 usage The auxiliary LN2 shrouds CC shown in Figure 1 may be spaced at random inside vacuum chamber E.

External equipment DD requiring LN2 supply are cryopumps and vacuum roughing linetraps.The nature and location of these items may not be applicable to LN2 supply from the gravity feed convection system (Part2). However, supply piping is provided through conduitsW,X,Yand Zto allow liquid supply to these elements. The fluid then returns to the head tank Dthrough conduitAA.

When operating in the pressurized closed loop forced flow mode (Part 1), liquid is supplied to the auxiliary shrouds CC and external equipment DD through conduits V, X, Y and Z. Liquid is then returned to the subcooler coil G in the head tankthroug h conduits AAand BB.

When the main thermal shrouds F located in vacuum chamber E are in the gravity feed mode of operation (Part 2), a reduced floworyogenic pump on LN2 subcooler skid H can be utilized to supply liquid to the auxiliary shrouds CC and external equipment DD. Liquid is supplied to the main thermal shrouds F and the LN2 subcooler pump skid H through a common supply header conduit J. Liquid is then supplied to the auxiliary shrouds CC and external equipment DD through conduits V, X, Y and Z. Liquid is then returned to the head tank D from the above equipment through conduitAA.

Claims (13)

1. Apparatus for testing articles in a simulated space environment comprising: a circulation system means for simulating low temperatures of a space environment including: a vacuum chamber; internal and external piping means cooperating therewith in both a subcooled pressurized closed loop forced feed system and in a gravity convection system; heat exchanger means in said vacuum chamber connected to said piping means and cooperating with both of said systems; and a head tank meansforsupplying liquid nitrogen (LN2) to both of said systems.

2. Apparatus according to Claim 1, wherein said heat exchanger means are properly sized for ac commodating the flow requirements of both the sub- cooled pressurized closed loop forced feed system and the gravity convection system.

3. Apparatus accordingto Claim 1 or2,wherein said head tank means includes a head tankstructure which is physically located above said vacuum chamber so that sufficient liquid head is provided for fluid circulation through the system when the phys icai principlesoftwo-phasefloware utilized on a re turn leg of piping from said heat exchanger means, and said head tank having sufficientvolumethere withintoallowforvapourspaceintheupperarea thereof together with a main vent for demisting re turn vapourfrom said headtanksothatsamewil be dry.

4. Apparatus according to Claim 3, together with a demister, a low pressure reliefvalve, and outlet piping.

5. Apparatus according to any preceding claim, together with external liquid nitrogen equipment.

6. Apparatus according to any preceding claim, togetherwith subcooler means provided in associa tion with the liquid nitrogen head tank,

7. Apparatus according to any preceding claim, together with liquid pumps for transferring fluid under pressure through the system.

8. A device for testing articles under lowtem perature conditions including: a vacuum chamber having thermal simulation shrouds therewithin; said thermal simulation shrouds within said vacuum chamber being comprised of several cylindrical zones with a top and bottom shroud providing an op tically dense envelope surrounding an article to be tested.

9. A device according to Claim 8, together with auxiliaryshrouds placed at random locations inside the test envelope.

10. A device according to Claim 8 or 9, including meansforcirculating liquid nitrogen (LN2)through passages in said thermal shrouds whereby a stable uniform temperature of - 1 730C (- 287"F) is provided for the test a rticle.

11. A device according to Claim 8,9 or 10, wherein said thermal shrouds comprise a plurality of panels, an inlet manifold connected to each of said panels, and an outlet manifold connected likewise thereto.

12. A device according to any of Claims 8 to 11, together with an auxiliary shroud connected be tween said top and bottom shrouds.

13. Adevice fortesting articles under lowtem perature conditions substantially as hereinbefore de scribed, with reference to and as illustrated in the ac companying drawings.