An experimental programme is presented for estimating cool-down time for foam insulated cryogenic transfer lines. The results show that there is good agreement between theoretical and experimental cool-down curves. The integral method is shown to be an accurate way to determine the position of cooling front as the cool-down proceeds. I t was also found that the mass f low rate does not significantly affect the cool-down of short transfer lines.

Cool-down of foam insulated cryogenic transfer lines

K. A. Durga Prasad, K. Srinivasan, and M. V. Krishna Murthy

Although many analytical and experimental investigations have been carried-out on uninsulated and vacuum-insulated transfer lines 1,2,3,4,s hardly any work has been reported on cool-down of externally insulated lines. Rigid foam insul- ations are becoming more popular for liquid nitrogen and liquid oxygen applications, in view of their low cost, com- pared to vacuum insulated lines, the considerable reduction of evaporation losses, and their manipulability to any desired shape. A recent analysis by Srinivasan et al 6 showed that the integral method can be conveniently used to estimate the minimum insulation thickness taking into account cool- down considerations. The present investigation proves that there is fair agreement between the analytical results ob- tained by Srinivasan et al and experimental results. Further- more it is shown that the mass flow rate does not influence the cool-down very significantly.

Experimental apparatus and procedure

The experiments were conducted on a copper tube of 13 mm outer diameter and 10.6 mm inner diameter. Polystyrene foam (commercially known as Thermocole) is used to insulate the line over a 1000 mm length. The density of foam used was 20 kg m -3 The outer diameter of the insulation was 120 mm which is slightly more than the minimum predicted in reference 6. The moisture barrier was produced by coating the insulation outer surface with a thin film of bitumen. The insulation, consists of two halves of a hollow cylinder, bonded on to the pipe wall using a glue. The test set-up is shown in Fig. 1. Thirty gauge copper constantau themocouples (Leeds and Northrop) were used for all temperature measurements, with liquid nitrogen used as a reference junction. The thermocouples were located at various radial positions to measure thermal layer penetration. The radial positions of the thermocouples are shown in Fig.2. The up-stream aa~d down-stream pipe wall temperatures were recorded on a Siemens Kompensograph III (2 channel line recorder) and thermal layer position was measured by recording temper- atures at different radial locations on a Hartman & Braun 12 channel point recorder.

The authors are with the Refrigeration and Airconditioning Laboratory, Department of Mechanical Engineering, Indian Institute of Technology, Madras 600036, India. Received 14 August 1974.

Thermoeouple Thermocouple ,3 \ ,4

Insulation Copper tube

IT Transfer system I I I I Pressure gauge d - - h

I I \ _P ess0 e regu,at,ng II v°'v" / ! i \

Mercury I I II II I i i I manometer Receiver i~ k.j i

Spring balance ~ t I - - -

L2 I I / . . . . I U cylinder r_--)

Liquid nitrogen dewar

Fig.1 Test section

The liquid transfer system comprises a 15 litre metal dewar connected to the test section through a transfer siphon. The ullage volume of the dewar is pressurized using dry nitrogen gas from a high pressure cylinder. The flow of the pressurant is controlled by a pressure regulator, and the supply dewar pressure is measured by a conventional U-tube mercury manometer. The receiving dewar is mount- ed on weigh-bridge. Transfer of liquid nitrogen is ac- hieved by opening the regulator on the high pressure cylin- der. The recorders are switched on, ullage volume pressure is brought to a desired value, and maintained at that value for a particular run. The cool-down is assumed to be complete when the down-stream end of the test section has reached a steady temperature approximately equal to liquid nitrogen temperature. At this stage the rate at which liquid nitrogen is collected in the receiving dewar is measured. This facilitates estimation of mass flow rate through the test section.

CRYOGENICS. NOVEMBER 1974 615

Resu l ts a n d d iscuss ion

The experimental data includes the time-temperature history of the down-stream end of the test section and the temperature recordings of the thermocouples located at different radial positions shown in Fig.2. Fig.3 shows the time-temperature history of the test section. The analy-

Fig .2 R a d i a l pos i t ions o f t h e r m o c o u p l e s

tical zero mass flow rate cool-down curve and the points on the experimental curve for different supply dewar pressures are plotted on Fig.3. It can be seen that the analytical zero mass flow rate curve very reasonably approximates the experimental cool-down curve for all mass flow rates. This fact proves that the supply dewar pressure, which in turn is proportional to the mass flow rate, does not influence the cool-down rate. The theoretical cool-down curve is obtained using the pool boiling correla- tions only for calculating the heat flux to the fluid. This suggests that the contribution of forced convection to heat flux inside the tube is negligibly small for short transfer sections. This result is in confirmation with the results obtained by Steward et al 7 and Sfinivasan et al.a

A typical temperature recording of a thennocouple located at a radius of 44 mm is shown in Fig.4. Similar curves were obtained for other radii shown in Fig.2.

The analytically estimated and experimentally determined relation between thermal layer position and time is plotted in Fig.5. There is a very close agreement between analytical and experimental results. However, the analysis predicts a slightly pessimistic rate of penetration of the thermal layer. The reasons may be that the values used in the theoretical analysis may not be exactly the same as the properties of the insulation used in the experiments. The other probable cause may be that the thermocouple is measuring the temperature in the foam cell. Thus, delay might have been induced in the measurement of exact temperature due to low conductivity of the blowing gas in the cell.

320

28C

240

20O

v

160

I-

120

8 0

40

Analytical curve Experimental points

. Y , g

I I I I I I I I I I I0 2 0 3 0 4 0 5 0 60 70 8 0 9 0 I 0 0 I10 o

Fig.3 Cool-down c u r v e f o r p o l y s t y r e n e f o a m insu la ted t rans fe r l ine T i m e , s

616 CRYOGENICS. NOVEMBER 1974

310,

v 3 0 0

2

E

I =

~- 290

2 8 0 0 4 2 0

0 0

• 8 =

Supply dewar • Symbol pressure, cm H9

x 10.(3 o I1.0 • 13.0 " 11.2 o 15.0 • 7.0 = 10.2 • 12 .0

I I I I l I I I I I I I I

6 0 120 180 2 4 0 3 0 0 360

Time, s

Fig.4 Variation of temperature with time at a radius of 4.4 cm

During the experimental investigation it was observed that the foam produces a crashing noise towards the end of cool-down. This can be attributed to high thermal con- traction of the foain, which is prevented by the metal tube of lower thermal contraction coefficient. This fact suggests that it is advisable to provide a sufficient gap between the pipe and the insulation, so that at the end of cool-down the insulation just holds tim pipe tight. If differential contrac- tion is not provided the insulation may get degraded due to destruction of cell structure. 9

C o n c l u s i o n s

An experimenral investigation was carried out to establish the validity of earlier ana_lytical results on cool-down of foam insulated cryogenic transfer lines. There is good agreement between the analytically estimated and experi- mentally determined cool-down rates of the test section. The integral method used in the analysis gives sufficiently accurate results. The thernral layer position in the insulation during cool-down can be predicted with reasonable accuracy using the integral method. It was further shown that the transfer line cool-down in the case of short transfer sections is independent of supply dewar pressure or mass flow rate. If small gaps are provided between insulation and pipe wall degradation of insulation due to differential contraction may be avoided. Since the thermal layer has not penetrated

I 0 0

90

80

70

6 0

I- 50

4O

30

20

IO

0

I Theoret ica l curve Experimental points

Supply dewar Symbol pressure, cm Hg

x IQO o II.O • 13.0 " I I .2 o ;/ • 7 . 0 = 10.2 , • 12.0

I 2 3 4 5

Radius, cm

Fig.5 Thermal layer position as a function of time

to the outer surface of insulation even at the end of cool- down, the insulation thickness used in the experiments is slightly more than the mininmm predicted by the analysis. Thus, the value predicted by the analysis for the insulation thickness is pessimistic.

R e f e r e n c e s

1 Burke, J. C., Byrnes, W. R., Post, A. H., Rucc~a, F. E. Advances in Cryogenic Engineering, Vol 4 (1960) 378

2 Drake, E. M., Ruccia, F. E., Ruder, J. M. Advances in Cryogenic Engineering, Vol 6 (1961) 323

3 Bronson, J. C. et al Advances in Cryogenic Engineering, Wol 7 (1961) 198

4 Chi, J. W. H. Advances in Cryogenic I'nginecring, Vol 10 (1965) 330

5 Leonhard, K. E., Getty, R. C., Franks, D. E. ~dvances in Cryogenic Engineering, Vol 12 ( 19671 331

6 Srinivasan, K., Seshagiri Rao, V., Krishna Muithy, M. V. Paper S-5. ICEC5 (Kyoto, 1974)

7 Stewards, W. G., Smith, R. V., Brennan, J. A. Advances in Cryogenic Engineering, Vol 15 (1970) 354

8 Srinivasan, K., Seshagiri Rao, V., Krishna Murthy, M. V. Cryogenics 14 (1974) 489

9 Durgaprasad, K. A. M Tech Dissertation, Indian Institute o f Technology, Madras (1974)

C R Y O G E N I C S . N O V E M B E R 1974 617