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Design and performance characteristics of sorption pumps
Design and performance characteristics of sorption pumps received 17 October 1970;accepted12January 1971 P Vijendran and C V G Nair, TechnicalPhysicsDivision,BhabhaAtomicResearchCentre, Trombay,Bombay-85,India
This paper describes the design and performance characteristics of two types of refrigerated Molecular Sieve Sorption pump that produces and maintains a pressure of the order of 10-3 torr in volume of 10-50 I. in case of evacuation of air from atmospheric pressure. The main feature of this method of pumping is its freedom from pumping fluids. Some of its applications along with its limitations are discussed. Applications to multistage sorption pumping and the use of sorption pumps in combination with mechanical dry pump are illustrated,
Introduction It has now been established that improvement of characteristics and life of many electro-vacuum devices requires the creation of not only a very high but also clean vacuum. "Clean" means absence of any hydrocarbon in the composition of the residual gases of the instruments, which enter mainly from the pumps used to evacuate the devices (for example, oil vapour pumps). The effective way of reducing the hydrocarbon content in vacuum devices is to use an oilless means of evacuation. This has partly been achieved by the development of Sputter-ion pumps in our laboratory. However, since Sputter-ion pumps do not require continuous backing and as they are clean and oilfree, it is often desirable to rough pump a system pumped by Sputter-ion pump also in an oil free manner. This has necessitated the development of Sorption pump which serves as an ideal roughing pump for Getter-ion pumps.
Principle of Sorption pump A Sorption pump is a static device and is basically a volume filled with porous material viz Molecular sieve and surrounded by a coolant. The principle of Sorption pump is well known. The pumping action produced by the phenomena of adsorption on a refrigerated surface was noted by Dewar in 1875 and he used it as a getter in the production of high vacua. Unlike other types of pumping, sorption pumping is a batch process and the pumping rate is dependent on the gas already sorbed and the ultimate pressure, for a given sorbent and a gas, is a function of the number of molecules already sorbed. Sorption pumping is highly selective. Various factors such as condensation points of gases, the attractive forces between gas and sorbent and the structure of the sorbent (its pore size etc) influence this selectivity.
A practical sorption pump: design and performance characteristics The condensation coefficient for ambient gas depends on surface temperature and the gas pressure and this is equal to unity at all pressures if the surface temperature is 20°K or lower. There is, therefore, a critical temperature of the surface and critical pressure for an ambient gas for condensation coefficient to be unity. Near the boiling point of a gas the condensation coefficient will be almost unity and greater than 0.1 even at
higher temperatures. This result is the basis of all cryopumps and also of traps. The net rate of pumping of molecules by 1 cm 2 of flat surface is given by: 1
d_~ = CvP-- a.._Molecules cm-~sec-t dt
r : N o of molecules sorbed per cm -~ ---the specific arrival rate C =condensation coefficient r ----the mean time of sojourn of molecules on the surface P =pressure in torr. The main problem of sorption pump design is to maintain the difference between the condensation rate (CvP) and the de-
sorption rate (-~-a) sufficiently large as to satisfy the pumping requirements. However, as C is a quantity which does not vary greatly, the condensation term is not subject to wide changes by design. It is, therefore, the desorption term which is mainly affected by the design. Further, in assessing the performance of Molecular sieve sorption pump, there are two factors to be considered: (1) pumping speed (2) and its capacity for adsorption of an appreciable amount of gas and the minimum pressure that can be achieved. In general, the pumping speed depends mainly on the design of the pump which determines how fast and efficiently the adsorbent can be brought to the liquid nitrogen (or coolant) temperature. This itself is a problem on account of the low thermal conductivity of Molecular sieve. In order to have high speed, the pump should be designed in such a way that the diameter of the tube and the filling of Molecular sieve do not unduly constrict the flow of gas into the pump. Thus, we have two opposing factors to be taken into account while designing the pump. Moreover, the sorbent should have a large area against the pump body wall and should not be too thick, so that cooling of the Molecular sieve is reasonably rapid ~. Gas to be pumped should have free access to a large area of the adsorbent. These factors have been taken into consideration while designing the two types of pumps which are shown diagrammatically in Figures l(a) and l(b). Pump I---(Model SP-10). The pump is of argon-arc welded stainless steel construction and consists of a stainless steel cylinder of 2 in. dia. and 9 in. length. Within it, along its axis, is a tube of ½ in. dia. extending up to the top and ending in a
Vacuum/volume 21/number 5. PergamonPress Ltd/Printedin GreatBritain
159
P Vijendran and C V G Nair: Design and performance characteristics of sorption pumps
SECTION
ON A'A'
To v a l v e
cork
scape
ret cylinder
~
casing
/Outer
Figure l(a). Cutaway view of Sorption pumps, Model: SP-10.
Copper vanes
Molecular $~eve5
SECTION
ON A ' A '
Experimental data and analysis
ii To v a l v e ton cork
,re mesh
li~ndpr mer ,linder
A'
3 ..... .~-Out
e r casing
Figure I(b). Cutaway view of Sorption pump. Model SP-50. 160
small tube of ~ in. dia. Nickel plated copper fins are silver brazed to the central tube and the bottom flange. These fins facilitate the heat transfer to the Molecular sieve which fills the space between the central tube and the outer cylinder. The upper part of the vessel is a tube ending in a ½ in. standard vacuum union. The exhaust fitting which is closed during operation with a viton stopper, is located below this union. This acts as a pressure release valve. This pump is designed for a charge of 180g of Molecular sieve. The Molecular sieve is regenerated by a cartridge heater which fits into the central tube (which is open at both ends). During operation, the heater is removed and the central tube also gets filled with liquid nitrogen, which helps in the fast cooling of the sorbent. Pump II--(Model SP-50). This pump holds 850 g of Molecular sieve. Its construction does not differ much from that of (I). The outer vessel is of 4½ in, dia. and 9 in. length and along its axis is a stainless steel wire-mesh cylinder of 2 in. dia., for holding the Molecular sieve and also to facilitate the passage of the gas to the Molecular sieve pellets. This pump utilizes a unique copper finned conductance system to provide fast and efficient heat transfer. Its upper part ends in a 1 in. standard vacuum union. Selection of sorbents. The performance of a Sorption pump is influenced by the choice of the sorbent. The materials so far used in the Sorption pump are either activated charcoal or synthetic zeolites. Each has its own advantages and disadvantages. In general, charcoal has the advantage of higher thermal conductivity, lower cost and low regeneration temperature. However, it has certain limitations. Its adsorption capacity is low. It is normally prepared by an uncontrolled process and it is therefore inconsistent in its properties. Oxygen, adsorbed on charcoal is held strongly and on heating, it comes off as carbon monoxide. It also gives rise to a lot of dusting. However, on account of its higher thermal conductivity, an initial fast pump down is achieved by using charcoal in the Sorption pump. Molecular sieve on the other hand, has the advantage of higher capacity, lower dusting and ease of regeneration and outgassing. Of the various types of Molecular sieve examined under our laboratory conditions where Relative Humidity varies from 50 to 80 per cent; the type--13X is found to be the most effective as it has a higher sorption capacity for water, compared to others and our further remarks relate to the results using this material in the pump.
Figure 2 shows schematically a simple experimental set up to study the various characteristics of the Sorption Pump. After loading into the pump, the Molecular sieve is activated by heating to 350°C either in atmosphere or in vacuum (with a Rotary Pump) using a cartridge heater, till it is fully outgassed. When the pump is submerged in liquid nitrogen, the Molecular sieve is cooled from inside and outside alike. Pressure measuremerits were made by using a Bourdon gauge* and a thermocouple gauger. Figure 3 illustrates the pump down curves achieved by the Sorption pumps SP-10 and SP-50. Maximum capacity of these pumps for pumping from atmospheric pressure to 10 3 torr has been found to be 10 and 50 I. respectively. * M/s Edwards High Vacuum Limited. t Technical Physics Division, BARC, Trombay, Bombay-85, India.
P Vijendran and C V G Nair:
Designand performancecharacteristicsof sorptionpumps
i
The;o%:ou0" __I-" t
~ T e s t
chamber
'~'-~_~--Bourdon
1 F-
[
gauge
I~Manifold V2
Sorptionpump-~.
VI~
f
1
Figure 2. Experimental set up. 10 2
In general,
!
IO
I
For S P - I 0
volume
V=f(P, T) where V = the a m o u n t of gas a d s o r b e d per g m of the a d s o r b e n t (in cc a t N T P ) P = t h e e q u i l i b r i u m pressure in torr, T = t h e cooling t e m p e r a t u r e in °K. A s is expected, the a m o u n t of gas a d s o r b e d increases with decrease of temperature. This is quite evident f r o m Figure 4 which shows the a d s o r p t i o n o f air o n M o l e c u l a r sieve at t e m p e r a t u r e s 77°K a n d 195°K.
2 l
11' F o r S P - 5 0 volume 20 l pre-chill time IOmin. (for both)
At -78*C
i
×
i0_! ~
ft.
I0 2
i
SorptionpumpSP-IO volume 2 L.
I0 21
I0
10-3
5o-"
I I I I r I i 2
3
4
5
6
7
I /
8
9
Time, rain
I
I
I0 II
j1At-196 °C
I I 1
12 13 14 15
Figure 3. Pump down time vs pressure of Sorption pumps SP-10 and SP-50. T h e basic characteristics are given below: Pump-Type
SP- 10
SP-50
Pressure range
760 to 10-2 torr
760 to 10-a torr
Evacuation time for max volume and lowest pressure starting from 760 torr
10 mira ( + 5 rain prechilling)
15 rain ( + 10 rain prechilling)
Initial ml Steady state ml/br
About 1250
About 5000
About 250
About 300
Figure 4. Pump down time vs pressure at different temperatures, Sorption pump SP-10.
350 W
W i t h a view to d e t e r m i n e t h e S o r p t i o n p u m p was set t o p u m p 2 I. capacity. T h e S o r p t i o n p u m p a n d cooled with liquid n i t r o g e n
Liquid Nitrogen consumption
Regeneration wattage (at 230 V) i
175 W
I°-2
I[ ~ I
2
3
4
5
6
7
i I I I t J l_J
8
Time,
9
I0
II
12 13 14 15
rain
of
o p t i m u m chilling time, the a n u n b a k e d test c h a m b e r of was valved into the c h a m b e r a n d the p u m p d o w n time-
161
P Vijendran and C V G Nair: D e s i g n and p e r f o r m a n c e characteristics pressure readings were taken. It was allowed to warm up to room temperature when the previously adsorbed gas was desorbed. The pump was then cooled with the isolation valve closed. The valve was opened after 5 rain and the readings were taken. The experiment was repeated for pre-chilling time of 10, 15, 20 and 45 rain. The data obtained has been presented graphically in Figure 5. It has been observed that 5-10 min pre~Tr
I
~rrr
I
of sorption p u m p s I0 z
IO
I
]"
No p r e - c h i l l
II
Pro-chill
5min,
]~
Pre-chill
IOmin.
Pro-chill
15 r a i n . , 2 0 , 4 5
== co n 10 -I
\ ,o3LI \\
I
3
1 I
4
b
1 J [
e
z
Time,
"-.
a
I
9
l
io
I
n
t
[
12 t$
l
I
14 t5
min
Figure 6. Successive pump down by Sorption pump SP-iO without
i0 "2 ~n>-- ..z~....°
~"-o
jo-~I II 21 3i 4I .5i 6] I 8l 9I I0I III 12I 13J 14I 15j 7
Time,
rain
Figure 5. Pump down time vs pressure for different chilling times of Sorption pump SP-IO. chilling of the Molecular sieve is sufficient for attaining the equilibrium temperature. It also brings out the fact that the pump design is more or less optimum as the equilibrium temperature is reached in an appreciably small time. The results given in Figure 6 show the influence on the ultimate vacuum achievable, of the quantity of gas adsorbed. The pump-down curves were obtained for a pro-chilled Sorption pump set to pump a test chamber of 2 1. capacity. The chamber was repeatedly let open to atmospheric pressure and repumped without allowing the Sorption pump to warm up to release the previously sorbed gas. The SP-10 pump could be used for not less than 5 cycles of operation at a time, without seriously affecting its performance with regard to its low pressure limit, on this set-up. For SP-50, the number of cycles of operation has been found to be at least 3 with a test chamber of 201. capacity. In an effort to find out the total quantity of gas that can be pumped the following procedure was adopted. With fresh reactivated Molecular sieve in the pump, a two litre flask at atmospheric pressure was connected to it through a valve. With the valve closed, the pump was pre-chilled for 15 min and then allowed to pump the flask. The pressure fall was monitored by the Bourdon and Thermocouple gauges. When the ultimate pressure was reached, the pump was isolated and the flask let to atmospheric pressure. This was again pumped to 162
I
z
any warm up to release previously sorbed gas. the ultimate by opening the valve. This process was repeated till the pump could not handle any more gas. The total quantity was computed by adding up the V(P~-P2) of all the runs. This worked out to be 135 torr 1./g. Then knowing the surface area to be 5,14 × 10"cmZ/g for this type of Molecular sieve, the quantity of gas for monomolecular coverage was evaluated and found to be ~ 14 torr l./g. This indicates quite a good design as the thermal beat transfer is good enough through the Molecular sieve to achieve capillary condensation there by enhancing the quantity of gas sorbed to be about 10 times the monomolecular coverage. Multi-stage pumping. A sorption pump, starting from atmospheric pressure and pumping only on its own, still has to adsorb a large quantity of gas. The ultimate pressure it can reach is, therefore, limited to a few millitorr. Further, single Sorption pump cannot handle a very large volume and the pressure attainable will, therefore, be limited to a few torr. Suppose, we pump the Sorption pump and the chamber (by means of another Sorption pump or mechanical pump) without refrigerating it, till the pressure falls to a fairly low value. The first roughing pump is then valved off and the pre-pumped Sorption pump is cooled. The number of molecules to be adsorbed now is 4-5 orders of magnitude less than when starting at atmospheric pressure and accordingly the ultimate pressure achievable is much lower. The results obtained are shown in Figure 7. Figure 7(a) compares pump-down by two Sorption pumps, working first in parallel and then in sequence to evacuate a test chamber. Sequential pumping has been found to be more effective as it makes more efficient use of the sorbent and yields faster pump down. The final pressure is however, limited by helium and neon for which the adsorption capacity of Molecular sieve cooled to liquid nitrogen temperature is very poor 3'~. Sorption pump in combination with a mechanical pump, is
P VUendran and C V G Nair: Design and performance characteristics of sorption pumps iO a
10 3 I st s t a g e
orption pump SP-IO) I0
I0
r
Pumps used in p a r a l l e l
TT
Pumps used in cascade, second pump isolated and p r e - c h i l l e d during f i r s t part of pumpdown.
2 nd. stage /(Sorption pump SP-IO)
]Me/hanical I dry pump / I0
-2 nd. s t a g e ~ (Sorption pump|
f=0
, _ =o- ~ I -
~--2nd.
1
stage s t a r t s
i0-+ _
+o+/ jo.-3 _l
~×
\ ~×
I 0-21 --
I
I
2
f
3
I
4
I 5
Time4
I
6
I
7
t
8
I
9
I
,o--+I [ J
I0
I
2
I
I I
3 4
min
Figure 7(a). Two stage pumping of two Sorption pumps SP-10. generally used in pumping larger volumes. The objection to using a conventional type of rotary pump is the possibility of organic contamination due to the rotary pump oil. However, if a graphite bladed dry pump* is used, there is no risk of organic contamination and an extremely clean method of pumping is available. Figures 7(b) and (c) show the results of two stage pumping using the same vacuum system. In the first case, one Sorption pump was used to pre-pump the other Sorption pump and the system. In the second case, graphite bladed dry pump replaced the first stage Sorption pump. It can be seen that the combination of mechanical pump and S o r p t i o n pump is the more effective one. However, the ultimate vacuum attained was not improved much, as these Sorption pumps have not been designed for being effective at low pressures.
5
I 1 / I
6 7 Time,
\
8 9 I0 It rain
~
12 13 14 15
Figure 7Co). Two stage pumping of a given system with two combinations of pumps. o
Ist stage
iO l-~,~.1...Mechanical pump
I0 2 nd. stage ~ J ( S o r p t i o n pump S P - 5 0 )
~P m ~P k. O.
I0 +l I-- ×
Discussion
It is evident that the Molecular sieve Sorption pump is capable of adsorbing a limited quantity of gas, after which saturation is reached. The amount of gas adsorbed per gram of adsorbent is a function of pressure and temperature and it decreases with increase in temperature. It is, therefore, advisable to use the coolest refrigerant available to have the maximum efficiency of the pump. It is further observed that the amount of gas sorbed at any given temperature decreases rapidly with the decrease in pressure. It is this fact which seriously affects the lowest pressure attained by a Sorption pump using a limited quantity of M o l e c u l a r sieve in this type of design. The amount adsorbed also depends on the pre-treatment of Molecular sieve, which therefore, has to be activated before being loaded into the Sorption pump. This is achieved by heating the Molecular sieve to 350°C either in vacuum or in atmosphere. After a * Rotovae Pump from M/s Tawde Engineering, Bombay, India.
IO-Z~- x
tO -3
~
~o--+L I
4 8
1 t ! J
×-......×
I I I t I I I 1 I I
12 16 20 24 28 32 36 40 44 48 52 56 60 Time, rain
Figure 7(c). Two stage pumping of a system with combination of Sorption pump SP-50 and mechanical dry pump. number of cycles o f operation, the Molecular sieve becomes saturated with water vapour and periodic heating of the sieve becomes necessary. After the processing, the sieve must be sealed off from the atmosphere to prevent adsorption of water 163
P Vijendran and C V G Nair: Design and performance characteristics of sorption pumps vapour from atmosphere. The problem can be overcome to a great extent by using vapour traps in between Sorption pump and the system so that most of the water vapour in the system is taken care of by these traps. The pumping of air by a single stage Sorption pump is usually limited to 10 -3 torr. This is mainly due to the p o o r adsorption of neon and helium by Molecular sieve and the residual gas is enriched by these gases. The reduced capacity of Molecular sieve for neon and helium can be ascribed largely to the small molecular size of these gases (2.59 ,~ and 2.18/~ respectively) and their very low boiling p o i n t ? '4 (27°K and 4,1°K respectively). Multi-stage pumping can overcome this limitation and pressures in the range of 10-4-10 5 torr can be achieved 5. It has been found that in multistage pumped cascade systems, helium and neon can be well handled by partially trapping t h e m within the pump itself, if each p u m p in earlier stage is valved off while the gas flow is still in the viscous region. Flush pumping with dry nitrogen has also been successfully employed to achieve quicker p u m p down and lower pressures. However, these Sorption pumps are found to be inefficient at pressures below 10 -4 torr. This is mainly due to two reasons: (1) at pressures of 10 -4 torr and below, the thermal conduction
164
through the gas phase tends to become zero and it is very difficult to maintain the regions of the sorbent remote from the bath at the bath temperature and (2) sojourn times become longer at low pressures and coverages and the rate at which the gas penetrates the absorbent through the gas phase and by surface diffusion may become slow. These problems can be overcome to some extent by proper mechanical design, and efforts are being made towards this direction at present.
Acknowledgement The authors are grateful to Shri C A m b a s a n k a r a n for granting permission for the publication of this paper.
References 1 p A Redhead, J P Hobson and E V Kornelson, The Physical basis oJ Ultra High Vacuum. Chapman and Hall, London (1968). 2 B D Power, High Vacuum Pumping Equipment. Reinhold, New York (1966). 3 N W Robinson, The Physical Principles of Ultra High Vacuum Systems and Equipment. Chapman and Hall, London (1968). 4 L Holland, M J Falker and L Laurenson, Nuovo Cim Suppl, 5, 1967, 242-260. 5 F T Turner and M Feinleib, Trans Am Vacuum Soe Symp (196 I).
This paper describes the design and performance characteristics of two types of refrigerated Molecular Sieve Sorption pump that produces and maintains a pressure of the order of 10-3 torr in volume of 10-50 I. in case of evacuation of air from atmospheric pressure. The main feature of this method of pumping is its freedom from pumping fluids. Some of its applications along with its limitations are discussed. Applications to multistage sorption pumping and the use of sorption pumps in combination with mechanical dry pump are illustrated,
Introduction It has now been established that improvement of characteristics and life of many electro-vacuum devices requires the creation of not only a very high but also clean vacuum. "Clean" means absence of any hydrocarbon in the composition of the residual gases of the instruments, which enter mainly from the pumps used to evacuate the devices (for example, oil vapour pumps). The effective way of reducing the hydrocarbon content in vacuum devices is to use an oilless means of evacuation. This has partly been achieved by the development of Sputter-ion pumps in our laboratory. However, since Sputter-ion pumps do not require continuous backing and as they are clean and oilfree, it is often desirable to rough pump a system pumped by Sputter-ion pump also in an oil free manner. This has necessitated the development of Sorption pump which serves as an ideal roughing pump for Getter-ion pumps.
Principle of Sorption pump A Sorption pump is a static device and is basically a volume filled with porous material viz Molecular sieve and surrounded by a coolant. The principle of Sorption pump is well known. The pumping action produced by the phenomena of adsorption on a refrigerated surface was noted by Dewar in 1875 and he used it as a getter in the production of high vacua. Unlike other types of pumping, sorption pumping is a batch process and the pumping rate is dependent on the gas already sorbed and the ultimate pressure, for a given sorbent and a gas, is a function of the number of molecules already sorbed. Sorption pumping is highly selective. Various factors such as condensation points of gases, the attractive forces between gas and sorbent and the structure of the sorbent (its pore size etc) influence this selectivity.
A practical sorption pump: design and performance characteristics The condensation coefficient for ambient gas depends on surface temperature and the gas pressure and this is equal to unity at all pressures if the surface temperature is 20°K or lower. There is, therefore, a critical temperature of the surface and critical pressure for an ambient gas for condensation coefficient to be unity. Near the boiling point of a gas the condensation coefficient will be almost unity and greater than 0.1 even at
higher temperatures. This result is the basis of all cryopumps and also of traps. The net rate of pumping of molecules by 1 cm 2 of flat surface is given by: 1
d_~ = CvP-- a.._Molecules cm-~sec-t dt
r : N o of molecules sorbed per cm -~ ---the specific arrival rate C =condensation coefficient r ----the mean time of sojourn of molecules on the surface P =pressure in torr. The main problem of sorption pump design is to maintain the difference between the condensation rate (CvP) and the de-
sorption rate (-~-a) sufficiently large as to satisfy the pumping requirements. However, as C is a quantity which does not vary greatly, the condensation term is not subject to wide changes by design. It is, therefore, the desorption term which is mainly affected by the design. Further, in assessing the performance of Molecular sieve sorption pump, there are two factors to be considered: (1) pumping speed (2) and its capacity for adsorption of an appreciable amount of gas and the minimum pressure that can be achieved. In general, the pumping speed depends mainly on the design of the pump which determines how fast and efficiently the adsorbent can be brought to the liquid nitrogen (or coolant) temperature. This itself is a problem on account of the low thermal conductivity of Molecular sieve. In order to have high speed, the pump should be designed in such a way that the diameter of the tube and the filling of Molecular sieve do not unduly constrict the flow of gas into the pump. Thus, we have two opposing factors to be taken into account while designing the pump. Moreover, the sorbent should have a large area against the pump body wall and should not be too thick, so that cooling of the Molecular sieve is reasonably rapid ~. Gas to be pumped should have free access to a large area of the adsorbent. These factors have been taken into consideration while designing the two types of pumps which are shown diagrammatically in Figures l(a) and l(b). Pump I---(Model SP-10). The pump is of argon-arc welded stainless steel construction and consists of a stainless steel cylinder of 2 in. dia. and 9 in. length. Within it, along its axis, is a tube of ½ in. dia. extending up to the top and ending in a
Vacuum/volume 21/number 5. PergamonPress Ltd/Printedin GreatBritain
159
P Vijendran and C V G Nair: Design and performance characteristics of sorption pumps
SECTION
ON A'A'
To v a l v e
cork
scape
ret cylinder
~
casing
/Outer
Figure l(a). Cutaway view of Sorption pumps, Model: SP-10.
Copper vanes
Molecular $~eve5
SECTION
ON A ' A '
Experimental data and analysis
ii To v a l v e ton cork
,re mesh
li~ndpr mer ,linder
A'
3 ..... .~-Out
e r casing
Figure I(b). Cutaway view of Sorption pump. Model SP-50. 160
small tube of ~ in. dia. Nickel plated copper fins are silver brazed to the central tube and the bottom flange. These fins facilitate the heat transfer to the Molecular sieve which fills the space between the central tube and the outer cylinder. The upper part of the vessel is a tube ending in a ½ in. standard vacuum union. The exhaust fitting which is closed during operation with a viton stopper, is located below this union. This acts as a pressure release valve. This pump is designed for a charge of 180g of Molecular sieve. The Molecular sieve is regenerated by a cartridge heater which fits into the central tube (which is open at both ends). During operation, the heater is removed and the central tube also gets filled with liquid nitrogen, which helps in the fast cooling of the sorbent. Pump II--(Model SP-50). This pump holds 850 g of Molecular sieve. Its construction does not differ much from that of (I). The outer vessel is of 4½ in, dia. and 9 in. length and along its axis is a stainless steel wire-mesh cylinder of 2 in. dia., for holding the Molecular sieve and also to facilitate the passage of the gas to the Molecular sieve pellets. This pump utilizes a unique copper finned conductance system to provide fast and efficient heat transfer. Its upper part ends in a 1 in. standard vacuum union. Selection of sorbents. The performance of a Sorption pump is influenced by the choice of the sorbent. The materials so far used in the Sorption pump are either activated charcoal or synthetic zeolites. Each has its own advantages and disadvantages. In general, charcoal has the advantage of higher thermal conductivity, lower cost and low regeneration temperature. However, it has certain limitations. Its adsorption capacity is low. It is normally prepared by an uncontrolled process and it is therefore inconsistent in its properties. Oxygen, adsorbed on charcoal is held strongly and on heating, it comes off as carbon monoxide. It also gives rise to a lot of dusting. However, on account of its higher thermal conductivity, an initial fast pump down is achieved by using charcoal in the Sorption pump. Molecular sieve on the other hand, has the advantage of higher capacity, lower dusting and ease of regeneration and outgassing. Of the various types of Molecular sieve examined under our laboratory conditions where Relative Humidity varies from 50 to 80 per cent; the type--13X is found to be the most effective as it has a higher sorption capacity for water, compared to others and our further remarks relate to the results using this material in the pump.
Figure 2 shows schematically a simple experimental set up to study the various characteristics of the Sorption Pump. After loading into the pump, the Molecular sieve is activated by heating to 350°C either in atmosphere or in vacuum (with a Rotary Pump) using a cartridge heater, till it is fully outgassed. When the pump is submerged in liquid nitrogen, the Molecular sieve is cooled from inside and outside alike. Pressure measuremerits were made by using a Bourdon gauge* and a thermocouple gauger. Figure 3 illustrates the pump down curves achieved by the Sorption pumps SP-10 and SP-50. Maximum capacity of these pumps for pumping from atmospheric pressure to 10 3 torr has been found to be 10 and 50 I. respectively. * M/s Edwards High Vacuum Limited. t Technical Physics Division, BARC, Trombay, Bombay-85, India.
P Vijendran and C V G Nair:
Designand performancecharacteristicsof sorptionpumps
i
The;o%:ou0" __I-" t
~ T e s t
chamber
'~'-~_~--Bourdon
1 F-
[
gauge
I~Manifold V2
Sorptionpump-~.
VI~
f
1
Figure 2. Experimental set up. 10 2
In general,
!
IO
I
For S P - I 0
volume
V=f(P, T) where V = the a m o u n t of gas a d s o r b e d per g m of the a d s o r b e n t (in cc a t N T P ) P = t h e e q u i l i b r i u m pressure in torr, T = t h e cooling t e m p e r a t u r e in °K. A s is expected, the a m o u n t of gas a d s o r b e d increases with decrease of temperature. This is quite evident f r o m Figure 4 which shows the a d s o r p t i o n o f air o n M o l e c u l a r sieve at t e m p e r a t u r e s 77°K a n d 195°K.
2 l
11' F o r S P - 5 0 volume 20 l pre-chill time IOmin. (for both)
At -78*C
i
×
i0_! ~
ft.
I0 2
i
SorptionpumpSP-IO volume 2 L.
I0 21
I0
10-3
5o-"
I I I I r I i 2
3
4
5
6
7
I /
8
9
Time, rain
I
I
I0 II
j1At-196 °C
I I 1
12 13 14 15
Figure 3. Pump down time vs pressure of Sorption pumps SP-10 and SP-50. T h e basic characteristics are given below: Pump-Type
SP- 10
SP-50
Pressure range
760 to 10-2 torr
760 to 10-a torr
Evacuation time for max volume and lowest pressure starting from 760 torr
10 mira ( + 5 rain prechilling)
15 rain ( + 10 rain prechilling)
Initial ml Steady state ml/br
About 1250
About 5000
About 250
About 300
Figure 4. Pump down time vs pressure at different temperatures, Sorption pump SP-10.
350 W
W i t h a view to d e t e r m i n e t h e S o r p t i o n p u m p was set t o p u m p 2 I. capacity. T h e S o r p t i o n p u m p a n d cooled with liquid n i t r o g e n
Liquid Nitrogen consumption
Regeneration wattage (at 230 V) i
175 W
I°-2
I[ ~ I
2
3
4
5
6
7
i I I I t J l_J
8
Time,
9
I0
II
12 13 14 15
rain
of
o p t i m u m chilling time, the a n u n b a k e d test c h a m b e r of was valved into the c h a m b e r a n d the p u m p d o w n time-
161
P Vijendran and C V G Nair: D e s i g n and p e r f o r m a n c e characteristics pressure readings were taken. It was allowed to warm up to room temperature when the previously adsorbed gas was desorbed. The pump was then cooled with the isolation valve closed. The valve was opened after 5 rain and the readings were taken. The experiment was repeated for pre-chilling time of 10, 15, 20 and 45 rain. The data obtained has been presented graphically in Figure 5. It has been observed that 5-10 min pre~Tr
I
~rrr
I
of sorption p u m p s I0 z
IO
I
]"
No p r e - c h i l l
II
Pro-chill
5min,
]~
Pre-chill
IOmin.
Pro-chill
15 r a i n . , 2 0 , 4 5
== co n 10 -I
\ ,o3LI \\
I
3
1 I
4
b
1 J [
e
z
Time,
"-.
a
I
9
l
io
I
n
t
[
12 t$
l
I
14 t5
min
Figure 6. Successive pump down by Sorption pump SP-iO without
i0 "2 ~n>-- ..z~....°
~"-o
jo-~I II 21 3i 4I .5i 6] I 8l 9I I0I III 12I 13J 14I 15j 7
Time,
rain
Figure 5. Pump down time vs pressure for different chilling times of Sorption pump SP-IO. chilling of the Molecular sieve is sufficient for attaining the equilibrium temperature. It also brings out the fact that the pump design is more or less optimum as the equilibrium temperature is reached in an appreciably small time. The results given in Figure 6 show the influence on the ultimate vacuum achievable, of the quantity of gas adsorbed. The pump-down curves were obtained for a pro-chilled Sorption pump set to pump a test chamber of 2 1. capacity. The chamber was repeatedly let open to atmospheric pressure and repumped without allowing the Sorption pump to warm up to release the previously sorbed gas. The SP-10 pump could be used for not less than 5 cycles of operation at a time, without seriously affecting its performance with regard to its low pressure limit, on this set-up. For SP-50, the number of cycles of operation has been found to be at least 3 with a test chamber of 201. capacity. In an effort to find out the total quantity of gas that can be pumped the following procedure was adopted. With fresh reactivated Molecular sieve in the pump, a two litre flask at atmospheric pressure was connected to it through a valve. With the valve closed, the pump was pre-chilled for 15 min and then allowed to pump the flask. The pressure fall was monitored by the Bourdon and Thermocouple gauges. When the ultimate pressure was reached, the pump was isolated and the flask let to atmospheric pressure. This was again pumped to 162
I
z
any warm up to release previously sorbed gas. the ultimate by opening the valve. This process was repeated till the pump could not handle any more gas. The total quantity was computed by adding up the V(P~-P2) of all the runs. This worked out to be 135 torr 1./g. Then knowing the surface area to be 5,14 × 10"cmZ/g for this type of Molecular sieve, the quantity of gas for monomolecular coverage was evaluated and found to be ~ 14 torr l./g. This indicates quite a good design as the thermal beat transfer is good enough through the Molecular sieve to achieve capillary condensation there by enhancing the quantity of gas sorbed to be about 10 times the monomolecular coverage. Multi-stage pumping. A sorption pump, starting from atmospheric pressure and pumping only on its own, still has to adsorb a large quantity of gas. The ultimate pressure it can reach is, therefore, limited to a few millitorr. Further, single Sorption pump cannot handle a very large volume and the pressure attainable will, therefore, be limited to a few torr. Suppose, we pump the Sorption pump and the chamber (by means of another Sorption pump or mechanical pump) without refrigerating it, till the pressure falls to a fairly low value. The first roughing pump is then valved off and the pre-pumped Sorption pump is cooled. The number of molecules to be adsorbed now is 4-5 orders of magnitude less than when starting at atmospheric pressure and accordingly the ultimate pressure achievable is much lower. The results obtained are shown in Figure 7. Figure 7(a) compares pump-down by two Sorption pumps, working first in parallel and then in sequence to evacuate a test chamber. Sequential pumping has been found to be more effective as it makes more efficient use of the sorbent and yields faster pump down. The final pressure is however, limited by helium and neon for which the adsorption capacity of Molecular sieve cooled to liquid nitrogen temperature is very poor 3'~. Sorption pump in combination with a mechanical pump, is
P VUendran and C V G Nair: Design and performance characteristics of sorption pumps iO a
10 3 I st s t a g e
orption pump SP-IO) I0
I0
r
Pumps used in p a r a l l e l
TT
Pumps used in cascade, second pump isolated and p r e - c h i l l e d during f i r s t part of pumpdown.
2 nd. stage /(Sorption pump SP-IO)
]Me/hanical I dry pump / I0
-2 nd. s t a g e ~ (Sorption pump|
f=0
, _ =o- ~ I -
~--2nd.
1
stage s t a r t s
i0-+ _
+o+/ jo.-3 _l
~×
\ ~×
I 0-21 --
I
I
2
f
3
I
4
I 5
Time4
I
6
I
7
t
8
I
9
I
,o--+I [ J
I0
I
2
I
I I
3 4
min
Figure 7(a). Two stage pumping of two Sorption pumps SP-10. generally used in pumping larger volumes. The objection to using a conventional type of rotary pump is the possibility of organic contamination due to the rotary pump oil. However, if a graphite bladed dry pump* is used, there is no risk of organic contamination and an extremely clean method of pumping is available. Figures 7(b) and (c) show the results of two stage pumping using the same vacuum system. In the first case, one Sorption pump was used to pre-pump the other Sorption pump and the system. In the second case, graphite bladed dry pump replaced the first stage Sorption pump. It can be seen that the combination of mechanical pump and S o r p t i o n pump is the more effective one. However, the ultimate vacuum attained was not improved much, as these Sorption pumps have not been designed for being effective at low pressures.
5
I 1 / I
6 7 Time,
\
8 9 I0 It rain
~
12 13 14 15
Figure 7Co). Two stage pumping of a given system with two combinations of pumps. o
Ist stage
iO l-~,~.1...Mechanical pump
I0 2 nd. stage ~ J ( S o r p t i o n pump S P - 5 0 )
~P m ~P k. O.
I0 +l I-- ×
Discussion
It is evident that the Molecular sieve Sorption pump is capable of adsorbing a limited quantity of gas, after which saturation is reached. The amount of gas adsorbed per gram of adsorbent is a function of pressure and temperature and it decreases with increase in temperature. It is, therefore, advisable to use the coolest refrigerant available to have the maximum efficiency of the pump. It is further observed that the amount of gas sorbed at any given temperature decreases rapidly with the decrease in pressure. It is this fact which seriously affects the lowest pressure attained by a Sorption pump using a limited quantity of M o l e c u l a r sieve in this type of design. The amount adsorbed also depends on the pre-treatment of Molecular sieve, which therefore, has to be activated before being loaded into the Sorption pump. This is achieved by heating the Molecular sieve to 350°C either in vacuum or in atmosphere. After a * Rotovae Pump from M/s Tawde Engineering, Bombay, India.
IO-Z~- x
tO -3
~
~o--+L I
4 8
1 t ! J
×-......×
I I I t I I I 1 I I
12 16 20 24 28 32 36 40 44 48 52 56 60 Time, rain
Figure 7(c). Two stage pumping of a system with combination of Sorption pump SP-50 and mechanical dry pump. number of cycles o f operation, the Molecular sieve becomes saturated with water vapour and periodic heating of the sieve becomes necessary. After the processing, the sieve must be sealed off from the atmosphere to prevent adsorption of water 163
P Vijendran and C V G Nair: Design and performance characteristics of sorption pumps vapour from atmosphere. The problem can be overcome to a great extent by using vapour traps in between Sorption pump and the system so that most of the water vapour in the system is taken care of by these traps. The pumping of air by a single stage Sorption pump is usually limited to 10 -3 torr. This is mainly due to the p o o r adsorption of neon and helium by Molecular sieve and the residual gas is enriched by these gases. The reduced capacity of Molecular sieve for neon and helium can be ascribed largely to the small molecular size of these gases (2.59 ,~ and 2.18/~ respectively) and their very low boiling p o i n t ? '4 (27°K and 4,1°K respectively). Multi-stage pumping can overcome this limitation and pressures in the range of 10-4-10 5 torr can be achieved 5. It has been found that in multistage pumped cascade systems, helium and neon can be well handled by partially trapping t h e m within the pump itself, if each p u m p in earlier stage is valved off while the gas flow is still in the viscous region. Flush pumping with dry nitrogen has also been successfully employed to achieve quicker p u m p down and lower pressures. However, these Sorption pumps are found to be inefficient at pressures below 10 -4 torr. This is mainly due to two reasons: (1) at pressures of 10 -4 torr and below, the thermal conduction
164
through the gas phase tends to become zero and it is very difficult to maintain the regions of the sorbent remote from the bath at the bath temperature and (2) sojourn times become longer at low pressures and coverages and the rate at which the gas penetrates the absorbent through the gas phase and by surface diffusion may become slow. These problems can be overcome to some extent by proper mechanical design, and efforts are being made towards this direction at present.
Acknowledgement The authors are grateful to Shri C A m b a s a n k a r a n for granting permission for the publication of this paper.
References 1 p A Redhead, J P Hobson and E V Kornelson, The Physical basis oJ Ultra High Vacuum. Chapman and Hall, London (1968). 2 B D Power, High Vacuum Pumping Equipment. Reinhold, New York (1966). 3 N W Robinson, The Physical Principles of Ultra High Vacuum Systems and Equipment. Chapman and Hall, London (1968). 4 L Holland, M J Falker and L Laurenson, Nuovo Cim Suppl, 5, 1967, 242-260. 5 F T Turner and M Feinleib, Trans Am Vacuum Soe Symp (196 I).