- Email: [email protected]
Automatic liquid nitrogen pumping system for particle accelerators
NUCLEAR
INSTRUMENTS
AND
METHODS
24 (1963)
477--481; N O R T H - H O L L A N D
PUBLISHING
CO.
AUTOMATIC LIQUID NITROGEN PUMPING SYSTEM FOR PARTICLE ACCELERATORS* M. K. S A L O M A A
Laboratory/or Nuclear Science, Massachusetts Institute o/ Technology, Cambridge, Mass. R e c e i v e d 15 April 1963 A description is g i v e n of an a u t o m a t i c liquid n i t r o g e n p u m p i n g system. P u m p i n g of liquid nitrogen into cooling t r a p s u p to a pred e t e r m i n e d level is a c h i e v e d b y an a u t o m a t i c a l l y controlled c o m p r e s s e d air system. An i m p o r t a n t feature of t h e e q u i p m e n t
is t h e ability to switch a u t o m a t i c a l l y f r o m a n e m p t y liquid n i t r o g e n reservoir to a full one. T h i s allows for a m u c h m o r e flexible o p e r a t i o n t h a n in conventional s y s t e m s where the reservoir has to be replaced m a n u a l l y when e m p t y .
1. Introduction
automatically without a t t e n t i o n for a period of two to four days or more d e p e n d i n g on the size of the containers used a n d on the rate of liquid nitrogen consumption. Because of the a u t o m a t i c switching between two liquid nitrogen containers, an e m p t y container can be replaced at a n y time during the period when the o t h e r c o n t a i n e r is being used. This allows m u c h more flexible operation of the system.
A n u m b e r of systems h a v e been developed to control liquid nitrogen level in a cooling t r a p of a v a c u u m s y s t e m l ' 2 ' a , * ' 5 ) . The most c o m m o n l y used methods of control are p r o b a b l y those using resist or type sensing probes as described b y Schwartz a n d Wilson 1) a n d those which employ various types of float arrangements. In m a n y of t h e more m o d e r n units resistors have been replaced with t h e r m i s t o r s which are readily available, a n d also are more sensitive to t e m p e r a t u r e variations t h a n o r d i n a r y wire probes. However, these systems which h a v e been described require a t t e n t i o n w h e n e v e r the liquid nitrogen container empties. At the Rockefeller G e n e r a t o r installation at Massachusetts I n s t i t u t e of Technology the need arose a few years ago for a n a u t o m a t i c s y s t e m which could operate a u t o m a tically for several days without a t t e n t i o n a n d this requires t h a t the system includes provision for switching from a n e m p t y c o n t a i n e r to a full container. Similar needs h a v e arisen at other installations at M.I.T. a n d elsewhere. The system described below has been designed specifically for this purpose a n d can o p e r a t e fully
2. Description of the Circuitry There are two liquid nitrogen containers for each cooling trap. E a c h one of these is controlled b y a solenoid valve which allows nitrogen gas u n d e r pressure to flow from the reservoir cylinders a n d force liquid nitrogen into the trap. These solenoids are controlled b y the operation of RL1 a n d R L 2 (figs. 1 a n d 2) which in t u r n are a c t i v a t e d b y two t h e r m i s t o r s in the trap. W h e n the liquid nitrogen level falls below the level d e t e r m i n e d b y the lower t h e r m i s t o r RL2 operates one of the solenoid valves. The choice of valve is governed b y the can selector relay RL4 whic~ in t u r n is governed b y RL5 a n d RL7. RL5 a n d R L 7 are a c t i v a t e d b y thermistors in the liquid nitrogen containers. As the liquid nitrogen starts flowing into the t r a p RL2 opens almost immediately, b u t the solenoid valve remains open until the upper level relay RL1 has openend RL3. As soon as can no. 2 empties, the relay RL7 operates so as to p e r m i t operation of can no. I
* T h i s w o r k is s u p p o r t e d in p a r t t h r o u g h f u n d s p r o v i d e d b y t h e U.S. A t o m i c E n e r g y C o m m i s s i o n u n d e r c o n t r a c t AT(30 1)2098. 1) S. B. S c h w a r t z a n d A. E. Wilson, A d v a n c e s in Cryogenic E n g i n e e r i n g , Vol. 1 ( P l e n u m Press, Inc., N e w York, 1960). 2) j . E. Sherwood, Rev. Sci. Instr. 23 (1952) 446. a) R. D. Goodwin, Rev. Scl. I n s t r . 26 (1955) 1052. 4) G. F. Wells, Rev. Sci. Instr. 29 (1958) 893. 5) B. R i c h e l m a n n , Rev. Sci. Instr. 30 (1959) 598. 477
478
M. K. S A L O M A A
LIQUID N2 CAN SELECTOR RL 4
N2 GAS
N2 GAS
THERMISTOR
LIOUID N2 CAN I
LIQUID N2 CAN 2 ~
--
P
UPPER LEVEL THERMISTOR LOWER LEVEL THERMISTOR
Fig. 1. Block diagram of the system.
t h r o u g h the a p p r o p r i a t e solenoid valve. F u r t h e r , w h e n can no. 1 is empty, t h e n RL5 in t u r n operates a n d allows can no. 2 to r e t u r n to its function provided t h a t it has been filled in the m e a n t i m e . Relays R L 6 a n d RL8 are auxiliary relays which p r e v e n t t h e r m i s t o r relays from going back a n d forth between on a n d off positions. RL5 a n d RL7 can be opened b y pressing the reset switch after t h e liquid nitrogen can has been replaced with a full one. Two gas reservoir cylinders are employed to supply gas into all liquid nitrogen pumps. If cylinder no. 1 is in use, as soon as it empties, a Mercoid pressure switch signals the cylinder selector relay RL10 which in t u r n s h u t s off solenoid valve no. 1 a n d opens no. 2 thus p u t t i n g t a n k no. 2 in operation. As soon as cylinder no. 2 is empty, t h e reverse switching occurs p u t t i n g cylinder no. 1 b a c k into commission provided t h a t it has been replaced with a full one in the meantime. I n our
installation one gas cylinder usually lasts for a b o u t one week, I n case b o t h gas cylinders or b o t h liquid nitrogen containers become e m p t y RL11 a n d RL9 with t h e i r auxiliary relays t u r n on the a l a r m or shut off the v a c u u m system depending on the position of S W l . The relay RL12 operates t h e a l a r m or s h u t s off p u m p s in case of a power failure.
3. Mechanical Construction The liquid nitrogen p u m p used with this system is a modification of a p u m p widely used in various laboratories at Massachusetts I n s t i t u t e of Technology (fig. 3). I t is clamped on t h e c o n t a i n e r with a U-shaped steel clamp. The sensing t h e r m i s t o r is a Fenwal GD 23P2 t h e r m i s t o r , as are all the o t h e r t h e r m i s t o r s used in this system. I t is m o u n t e d on a piece of textolite so t h a t the tip of it is in t h e open. The lead from the t h e r m i s t o r is o r d i n a r y thermocouple wire.
!
: _~_~
~__~_+_,__~
,
+
+
+
+
j'`+'',, +
,A6 - ~ To Nt Tank 2 Solenoid valve
Mercoid
+Merco+d +
+ - + A5 - I~ TO Nz Tonk I Solenoid volve
-~-
',+ - ~ c '
J
® ® ~. P,,o,,l,+,,,,+
,i. ~,8
~
~
.
+
+
+
+
o
,
F i g . 2. C i r c u i t d i a g r a m .
BIO
~- ~
.
_
,
.
_
.
J I
RL3,6,8,9,
++.~-'--~,o To TO V~C Voc Syster Shut Off
st+, \
s~,
A,o+
.
Horn
RL 4 and I0
~+,+AY
,:,,,,a,s
Potter and 8rumfiefd
~}
+
~+
' 0
i L' |
i
_-
+..+
~,
~
r i
,
I =~
=
II - 16 Potter and Brumfleld
'
t!
,.n
I
'L,
t
~-~+++ ~_
'
I I
I I
i
1
'I
'I -J
I
p I
I
+ I
l~
~L+,M,
/
+,+ ++ _,, ++_,,
+_+,,+,+ ~,;
+..,,:.++:u+
,
'
~+lt
+++,o,
'
/ ~L°,M,
IOK
B6
+
_
'
2+1/
J
B9
"ro,+,,N Con 2
~
+ B2 3
,OK
+ MAIN SYSTEM
,O~an,+q.N+,To,To.N+ ,'o'~. N, Con 2 Con I
•
/
TOp Shelf (4)
T
contror cabinet
o
i
• These numbe s refe
+
A4
l-++~od ~,~It
76v 3 0 0
~_~c~
+"°+~
•
,~_ I
~
~!:.~L~
'+
30K
A6.7
:i++~+,+;o+ .ro;+.+r rhe:'+:'°
'+
)OK
A2
A9
480
M. K. S A L O M A A
~
B y placing this thermistor at different heights in the liquid nitrogen container, one can adjust the point where the system switches over from one
3~
~._~-LIQUID NITROGEN P U M P
;: .::/
/ M E T A L F U N N E L WITH STYROFOAM SHELVES
/
AND LINING
I Ila" 0 D
TUBING
/ / WATERTIGHTSEALS ~
BRASS CUP - ~ ~
DR
/
/ TEXTOLITE PLATE
~THERMISTORS
NYLONSCREW
~--~1~
rE XTOLITE SHIELD
FIBERGLAS INSULATED COPPER TUBING
STAINLESS STEEL OUTER TUBING
LIQUIO NITROGEN
STEELWASHER RUBBER
VAGUUN Fig. 4. Cooling trap arrangement. (Shelves in the funnel prevent
moisture from being blown into the trap.) LIQUID NITROGEN CONTAINER
"/
~ /
!
/ /
/
\V
~
/ ,'[~I ,,:\
/
\
BRASS
~
EX OL E
\,
---- STAINLESSSTEEL INNER TUBING
Fig. 3. Liquid nitrogen pump.
container to another. If the tip of the thermistor is 14" from the bottom of the container (a 25 liter Superior Air Products container), the switching occurs i m m e d i a t e l y after the container has emptied. After the solenoid valve in the gas line has closed, there still remains some overpressure in the liquid nitrogen container causing pumping which m a y lead into overflowing at the trap. To bleed off the excess gas a hole is drilled with a no. 60 drill in the gas intake fitting. The sensing thermistors in the liquid nitrogen traps are mounted as shown in fig. 4. It is important that no ice forms on the thermistors because it will then act as an insulator. Fig. 5 shows the arrangement of the pressure switches and solenoid valves at the nitrogen gas cylinders. The solenoid valves must be of a variety
AUTOMATIC
LIQUID
NITROGEN
PUMPING
SYSTEM
481
Fig. 5. Compressed nitrogen s t stein.
equipped with a spring which closes the valve t i g h t when not energized. I m m e d i a t e l y before each valve there is a strainer.
4. Conclusion The s y s t e m described above has now been in operation at t h e Rockefeller Van de Graaff accelerator installation at Massachusetts I n s t i t u t e of Technology for more t h a n a year. I n principle it can be m a d e to operate on a n y
n u m b e r of liquid nitrogen containers b y replacing can selector relays RL4 with stepping relays. If pressurized liquid nitrogen containers are employed, the nitrogen gas cylinders and their control circuits can be eliminated b y using appropriate solenoid valves in the liquid nitrogen lines.
Acknowledgements The a u t h o r is grateful to Prof. S. C. Collins for the discussions on this system.
INSTRUMENTS
AND
METHODS
24 (1963)
477--481; N O R T H - H O L L A N D
PUBLISHING
CO.
AUTOMATIC LIQUID NITROGEN PUMPING SYSTEM FOR PARTICLE ACCELERATORS* M. K. S A L O M A A
Laboratory/or Nuclear Science, Massachusetts Institute o/ Technology, Cambridge, Mass. R e c e i v e d 15 April 1963 A description is g i v e n of an a u t o m a t i c liquid n i t r o g e n p u m p i n g system. P u m p i n g of liquid nitrogen into cooling t r a p s u p to a pred e t e r m i n e d level is a c h i e v e d b y an a u t o m a t i c a l l y controlled c o m p r e s s e d air system. An i m p o r t a n t feature of t h e e q u i p m e n t
is t h e ability to switch a u t o m a t i c a l l y f r o m a n e m p t y liquid n i t r o g e n reservoir to a full one. T h i s allows for a m u c h m o r e flexible o p e r a t i o n t h a n in conventional s y s t e m s where the reservoir has to be replaced m a n u a l l y when e m p t y .
1. Introduction
automatically without a t t e n t i o n for a period of two to four days or more d e p e n d i n g on the size of the containers used a n d on the rate of liquid nitrogen consumption. Because of the a u t o m a t i c switching between two liquid nitrogen containers, an e m p t y container can be replaced at a n y time during the period when the o t h e r c o n t a i n e r is being used. This allows m u c h more flexible operation of the system.
A n u m b e r of systems h a v e been developed to control liquid nitrogen level in a cooling t r a p of a v a c u u m s y s t e m l ' 2 ' a , * ' 5 ) . The most c o m m o n l y used methods of control are p r o b a b l y those using resist or type sensing probes as described b y Schwartz a n d Wilson 1) a n d those which employ various types of float arrangements. In m a n y of t h e more m o d e r n units resistors have been replaced with t h e r m i s t o r s which are readily available, a n d also are more sensitive to t e m p e r a t u r e variations t h a n o r d i n a r y wire probes. However, these systems which h a v e been described require a t t e n t i o n w h e n e v e r the liquid nitrogen container empties. At the Rockefeller G e n e r a t o r installation at Massachusetts I n s t i t u t e of Technology the need arose a few years ago for a n a u t o m a t i c s y s t e m which could operate a u t o m a tically for several days without a t t e n t i o n a n d this requires t h a t the system includes provision for switching from a n e m p t y c o n t a i n e r to a full container. Similar needs h a v e arisen at other installations at M.I.T. a n d elsewhere. The system described below has been designed specifically for this purpose a n d can o p e r a t e fully
2. Description of the Circuitry There are two liquid nitrogen containers for each cooling trap. E a c h one of these is controlled b y a solenoid valve which allows nitrogen gas u n d e r pressure to flow from the reservoir cylinders a n d force liquid nitrogen into the trap. These solenoids are controlled b y the operation of RL1 a n d R L 2 (figs. 1 a n d 2) which in t u r n are a c t i v a t e d b y two t h e r m i s t o r s in the trap. W h e n the liquid nitrogen level falls below the level d e t e r m i n e d b y the lower t h e r m i s t o r RL2 operates one of the solenoid valves. The choice of valve is governed b y the can selector relay RL4 whic~ in t u r n is governed b y RL5 a n d RL7. RL5 a n d R L 7 are a c t i v a t e d b y thermistors in the liquid nitrogen containers. As the liquid nitrogen starts flowing into the t r a p RL2 opens almost immediately, b u t the solenoid valve remains open until the upper level relay RL1 has openend RL3. As soon as can no. 2 empties, the relay RL7 operates so as to p e r m i t operation of can no. I
* T h i s w o r k is s u p p o r t e d in p a r t t h r o u g h f u n d s p r o v i d e d b y t h e U.S. A t o m i c E n e r g y C o m m i s s i o n u n d e r c o n t r a c t AT(30 1)2098. 1) S. B. S c h w a r t z a n d A. E. Wilson, A d v a n c e s in Cryogenic E n g i n e e r i n g , Vol. 1 ( P l e n u m Press, Inc., N e w York, 1960). 2) j . E. Sherwood, Rev. Sci. Instr. 23 (1952) 446. a) R. D. Goodwin, Rev. Scl. I n s t r . 26 (1955) 1052. 4) G. F. Wells, Rev. Sci. Instr. 29 (1958) 893. 5) B. R i c h e l m a n n , Rev. Sci. Instr. 30 (1959) 598. 477
478
M. K. S A L O M A A
LIQUID N2 CAN SELECTOR RL 4
N2 GAS
N2 GAS
THERMISTOR
LIOUID N2 CAN I
LIQUID N2 CAN 2 ~
--
P
UPPER LEVEL THERMISTOR LOWER LEVEL THERMISTOR
Fig. 1. Block diagram of the system.
t h r o u g h the a p p r o p r i a t e solenoid valve. F u r t h e r , w h e n can no. 1 is empty, t h e n RL5 in t u r n operates a n d allows can no. 2 to r e t u r n to its function provided t h a t it has been filled in the m e a n t i m e . Relays R L 6 a n d RL8 are auxiliary relays which p r e v e n t t h e r m i s t o r relays from going back a n d forth between on a n d off positions. RL5 a n d RL7 can be opened b y pressing the reset switch after t h e liquid nitrogen can has been replaced with a full one. Two gas reservoir cylinders are employed to supply gas into all liquid nitrogen pumps. If cylinder no. 1 is in use, as soon as it empties, a Mercoid pressure switch signals the cylinder selector relay RL10 which in t u r n s h u t s off solenoid valve no. 1 a n d opens no. 2 thus p u t t i n g t a n k no. 2 in operation. As soon as cylinder no. 2 is empty, t h e reverse switching occurs p u t t i n g cylinder no. 1 b a c k into commission provided t h a t it has been replaced with a full one in the meantime. I n our
installation one gas cylinder usually lasts for a b o u t one week, I n case b o t h gas cylinders or b o t h liquid nitrogen containers become e m p t y RL11 a n d RL9 with t h e i r auxiliary relays t u r n on the a l a r m or shut off the v a c u u m system depending on the position of S W l . The relay RL12 operates t h e a l a r m or s h u t s off p u m p s in case of a power failure.
3. Mechanical Construction The liquid nitrogen p u m p used with this system is a modification of a p u m p widely used in various laboratories at Massachusetts I n s t i t u t e of Technology (fig. 3). I t is clamped on t h e c o n t a i n e r with a U-shaped steel clamp. The sensing t h e r m i s t o r is a Fenwal GD 23P2 t h e r m i s t o r , as are all the o t h e r t h e r m i s t o r s used in this system. I t is m o u n t e d on a piece of textolite so t h a t the tip of it is in t h e open. The lead from the t h e r m i s t o r is o r d i n a r y thermocouple wire.
!
: _~_~
~__~_+_,__~
,
+
+
+
+
j'`+'',, +
,A6 - ~ To Nt Tank 2 Solenoid valve
Mercoid
+Merco+d +
+ - + A5 - I~ TO Nz Tonk I Solenoid volve
-~-
',+ - ~ c '
J
® ® ~. P,,o,,l,+,,,,+
,i. ~,8
~
~
.
+
+
+
+
o
,
F i g . 2. C i r c u i t d i a g r a m .
BIO
~- ~
.
_
,
.
_
.
J I
RL3,6,8,9,
++.~-'--~,o To TO V~C Voc Syster Shut Off
st+, \
s~,
A,o+
.
Horn
RL 4 and I0
~+,+AY
,:,,,,a,s
Potter and 8rumfiefd
~}
+
~+
' 0
i L' |
i
_-
+..+
~,
~
r i
,
I =~
=
II - 16 Potter and Brumfleld
'
t!
,.n
I
'L,
t
~-~+++ ~_
'
I I
I I
i
1
'I
'I -J
I
p I
I
+ I
l~
~L+,M,
/
+,+ ++ _,, ++_,,
+_+,,+,+ ~,;
+..,,:.++:u+
,
'
~+lt
+++,o,
'
/ ~L°,M,
IOK
B6
+
_
'
2+1/
J
B9
"ro,+,,N Con 2
~
+ B2 3
,OK
+ MAIN SYSTEM
,O~an,+q.N+,To,To.N+ ,'o'~. N, Con 2 Con I
•
/
TOp Shelf (4)
T
contror cabinet
o
i
• These numbe s refe
+
A4
l-++~od ~,~It
76v 3 0 0
~_~c~
+"°+~
•
,~_ I
~
~!:.~L~
'+
30K
A6.7
:i++~+,+;o+ .ro;+.+r rhe:'+:'°
'+
)OK
A2
A9
480
M. K. S A L O M A A
~
B y placing this thermistor at different heights in the liquid nitrogen container, one can adjust the point where the system switches over from one
3~
~._~-LIQUID NITROGEN P U M P
;: .::/
/ M E T A L F U N N E L WITH STYROFOAM SHELVES
/
AND LINING
I Ila" 0 D
TUBING
/ / WATERTIGHTSEALS ~
BRASS CUP - ~ ~
DR
/
/ TEXTOLITE PLATE
~THERMISTORS
NYLONSCREW
~--~1~
rE XTOLITE SHIELD
FIBERGLAS INSULATED COPPER TUBING
STAINLESS STEEL OUTER TUBING
LIQUIO NITROGEN
STEELWASHER RUBBER
VAGUUN Fig. 4. Cooling trap arrangement. (Shelves in the funnel prevent
moisture from being blown into the trap.) LIQUID NITROGEN CONTAINER
"/
~ /
!
/ /
/
\V
~
/ ,'[~I ,,:\
/
\
BRASS
~
EX OL E
\,
---- STAINLESSSTEEL INNER TUBING
Fig. 3. Liquid nitrogen pump.
container to another. If the tip of the thermistor is 14" from the bottom of the container (a 25 liter Superior Air Products container), the switching occurs i m m e d i a t e l y after the container has emptied. After the solenoid valve in the gas line has closed, there still remains some overpressure in the liquid nitrogen container causing pumping which m a y lead into overflowing at the trap. To bleed off the excess gas a hole is drilled with a no. 60 drill in the gas intake fitting. The sensing thermistors in the liquid nitrogen traps are mounted as shown in fig. 4. It is important that no ice forms on the thermistors because it will then act as an insulator. Fig. 5 shows the arrangement of the pressure switches and solenoid valves at the nitrogen gas cylinders. The solenoid valves must be of a variety
AUTOMATIC
LIQUID
NITROGEN
PUMPING
SYSTEM
481
Fig. 5. Compressed nitrogen s t stein.
equipped with a spring which closes the valve t i g h t when not energized. I m m e d i a t e l y before each valve there is a strainer.
4. Conclusion The s y s t e m described above has now been in operation at t h e Rockefeller Van de Graaff accelerator installation at Massachusetts I n s t i t u t e of Technology for more t h a n a year. I n principle it can be m a d e to operate on a n y
n u m b e r of liquid nitrogen containers b y replacing can selector relays RL4 with stepping relays. If pressurized liquid nitrogen containers are employed, the nitrogen gas cylinders and their control circuits can be eliminated b y using appropriate solenoid valves in the liquid nitrogen lines.
Acknowledgements The a u t h o r is grateful to Prof. S. C. Collins for the discussions on this system.