A system for remotely controlled filling of small Dewars with liquid nitrogen from a central storage Dewar vessel is described, consisting of a plunger type pump with an electromechanical driver and electromechanical ball type valves for distribution of liquid nitrogen. The preset nitrogen level in the small Dewars is kept constant by automatic refilling. The delivery is adjustable in steps by frequency change from 2.5 to 25 cm3/s, and delivery height up to 2 meters is assured

Kříž, J.

Němec, V.

Vognar, M.

Šimáně, Č.

Acta Polytechnica

English

A system for remotely controlled filling of small Dewars with liquid nitrogen from a central storage Dewar vessel is described, consisting of a plunger type pump with an electromechanical driver and electromechanical ball type valves for distribution of liquid nitrogen. The preset nitrogen level in the small Dewars is kept constant by automatic refilling. The delivery is adjustable in steps by frequency change from 2.5 to 25 cm3/s, and delivery height up to 2 meters is assured.</p

Č. Šimáně

M. Vognar

J. Kříž

V. Němec

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A Small Transfer and Distribution System for Liquid Nitrogen

CTU Open Journal Systems (Czech Technical University, Prague / České vysoké učení technické v Praze)

1 IntroductionThe system was proposed formultiple, remotely controlledselective filling of one of three smallDewars with liquid nitrogen (LN)from a storage Dewar vessel. Theliquid nitrogen level in the selectedDewar is to be kept at a presetheight by automatic refilling.The liquid nitrogen is transferredby a specially constructed electro-mechanical pump from a standardnon-pressurized storage Dewar. Weavoided the solution of LN transferfrom the storage Dewar by pressurenitrogen gas provided either by anelectric heater situated in the De-war, by self-pressurization or by drynitrogen supply from outside. Thepump itself, consisting of cylinder,plunger and valves, is immersed inLN above the bottom of the storageDewar vessel, the electromagneticdriving system being situated out-side. The travel of the core in thesolenoid of the driving system istransferred to the plunger by a rod.In such a way the Joule heat fromthe driver, the input power of whichis about 10 W, does not contributeto evaporation of liquid nitrogen.The distribution of LN between thethree small Dewars is accomplishedby a three-way valve with electro-mechanical actuators.2 ElectromechanicalLN pumpAll parts of the pump (Fig. 1)in contact with LN are made from stainless steel. The deliveryvalve is of a diaphragm type, and the intake valve is of a balltype. The diaphragm of the delivery valve is held in the closedposition by a spring, the ball of the intake valve by gravity andhydrostatic pressure of the liquid nitrogen column above it.The intake valve operates successfully even when pressure inthe cylinder volume is reduced during suction. Due to theshort delivery period, the plunger need not to be providedwith a special sealing against the cylinder wall to preventleakage.The electromagnetic solenoid type driver of the pump,sketched in Fig. 1, has an axially sliding iron core. The num-ber of windings of the coil is 1400, resistance 15  andinductance 0.5 Hy (when the core is in central position).Without current, the core is held in the extreme upper po-sition by a spring. During the current pulse, the core movesto the central position delivering pumping work. The staticcharacteristic of the system, representing the resultant ofmagnetic force at coil current value 1.7 A and the resultantof the elastic force of the spring, acting on the core, isrepresented in Fig. 2 as a function of the plunger position.This force compensates the inertial forces of the mechanicalparts of the system and of the column of the transferred LNin the exhaust tubing during the delivery period and thehydrostatic pressure. The full stroke is maintained up to10 Hz working frequency of the pump.The solenoid of the pump driver is excited by rectangularcurrent pulses, peak current 1.7 A, duty cycle adjustable from0.12 to 0.25 and repetition rate 1 to 10 Hz. The necessaryvoltage for peak current 1.7 A is 25.5 V, maximum ampe-re-turns equal to 2380 A. Integrated circuit 555 is used in thepulse current generator, directly switching a Darlington typetransistor. The delivery period of the pump corresponds tothe peak current, the intake period to zero current in the coil.The return of the plunger during the intake period is assured©  Czech Technical University Publishing House http://ctn.cvut.cz/ap/ 29Acta Polytechnica Vol. 41  No.1/2001A Small Transfer and DistributionSystem for Liquid NitrogenČ. Šimáně, M. Vognar, J. Kříž, V. NěmecA system for remotely controlled filling of small Dewars with liquid nitrogen from a central storage Dewar vessel is described, consisting of aplunger type pump with an electromechanical driver and electromechanical ball type valves for distribution of liquid nitrogen. The presetnitrogen level in the small Dewars is kept constant by automatic refilling. The delivery is adjustable in steps by frequency change from 2.5 to25 cm3/s, and delivery height up to 2 meters is assured.Keywords: liquid nitrogen, transfer system, solenoid pump, solenoid valve, automatic level control.Fig. 1: Pump for liquidnitrogenFig. 2: Force P of the driver unit as function of the plunger position xby the spring in the driver. At 1 cm plunger stroke the suctionvolume is 2.5 cm3. The delivery is adjustable in steps by fre-quency change from 2.5 to 25 cm3/s, and delivery height up to2 meters is assured. The use of pulse excitation of the electro-magnet with an adjustable duty cycle enables the energy to beconcentrated on a short delivery period, thus reducing themean heat dissipation in the electromagnet coil.3 Three-way LN valveThe distribution of LN to three Dewars is enabled bya three-way solenoid valve (Fig. 3) consisting of individualball valves with a common body. Each ball valve is actuated bya solenoid through a rod in contact with the ball of the valve,which is held in the closed position by a spring. Withoutcurrent in the solenoid coil, its core is held in the extremeupper position by a spring. When the magnet coil is excited,the core shifts towards the central position and only afterhaving reached a certain amount of kinetic energy does itstroke on the rod in contact with the ball. In such a way theball is set free even if it happens to have been frozen to theseal. The input power of each of the electromagnets is 3.6 W,z = 2500, coil resistance 105 , i = 0.185 A. The switchingcircuitry ensures that, when any one of the valves is open, theother two remain closed. As long the valve is open, the LNpump is in action unless the LN level in the correspondingDewar is already at the preset height.The levels in the small Dewar flasks are controlled bythermocouple sensors placed at preset heights. When the LNcomes into contact with the thermocouple sensor, the currentin the pump is interrupted and is renewed when the LNdescends. The unwanted cooling of the thermocouple sensorby cold LN vapors during the filling period is compensated byheat supplied to the active part of the sensor by conductionfrom the warmer parts of the apparatus, keeping the sensortemperature above that of LN.The common body of the valve is made from teflon tominimize evaporation of liquid nitrogen and at the same timeto insulate the valves thermally from the solenoids. For reli-able action of the system the air moisture must be preventedfrom entering the inner parts of the system which are incontact with LN. This is accomplished by rubber gaskets,both in the pumping system and in the distribution valve,situated at places remaining at all times at or near to theroom temperature.The transfer of LN to one of the three Dewars is started bypushing the button switch of the corresponding valve. Thestarting of the pump is bound to the opening of the valve.The filling and maintenance of the preset LN level in theDewar is fully automatic.4 ConclusionNo effects of atmospheric moisture on the function of thedistribution valves were observed. In particular, the systemdid not manifest any tendency to freezing of the nitrogen andclogging the transfer lines or the distribution valves.References.[1] Vognar, M., Šimáně, Č. : Laboratory apparatus for prepara-tion of 123I from 124Xe by photonuclear reaction.Acta Polytechnica, 5(1996), pp. 49–56Prof. Ing. Čestmír Šimáně, DrSc.Ing. Miroslav VognarJiří KřížVladimír NěmecDept. of Dosimetry and Appl. of Ionizing Rad.phone: +420 2 2323657, +420 2 2315212fax: +420 2 2320861e-mail: vognar@br.fjfi.cvut.czCzech Technical University in PragueFaculty of Nucleus Sciences and Physical EngineeringBřehová 7, 115 19 Praha 1, Czech Republic30 ©  Czech Technical University Publishing House http://ctn.cvut.cz/ap/Acta Polytechnica Vol. 41  No.1/2001Fig. 3: Three-way valve

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A Small Transfer and Distribution System for Liquid Nitrogen

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