is small (under the conditions of the "Tatneft'" PO, this initial period consists of about 240 days), after which the intensity increases and then remains constant up to 500 days of use. The intensities of failures of the unit modules of the deep-well centrifugal electric pumps (the submerged motors and the supply cables) which govern the reliability of the complete submerged assembly of the deep-well centrifugal electric pumps differ both in magnitude and in the manner of their variation: the intensity of failures of the submerged motors of the U9~TsN deep-well centrifugal electric pumps remains constant in the course of 500 days of operation, while the intensity of failures of the supply cables produced on the basis of KPBP and KPBK cable is smaller than that of the motors in the initial period of operation, and then increases. Consequently, in order to increase the reliability of the deep-well centrifugal electric pumps it is necessary to increase the reliability of the supply cables and the submerged motors, which requires the development of new cables and new submerged motors of improved reliability.

NEW COMPACT EVAPORATORS FOR CRYOGENIC LIQUID CONVERTERS O. K. Krasnikova, E. V. Onosovskii, S. N. Platonova, I. I. Gurevich, and V. V. Usanov

UDC 621.59.048:66.073.5:621.6.036

One of the uses of cryogenic technology is in fire fighting in coal mines using liquid nitrogen. Liquid nitrogen converters designed for this purpose require compact evaporators of moderate pressure with heat supply by natural atmospheric convection. The atmospheric evaporators used in the cryogenic technology cannot be used for mine converters since they generally use rolled-welded panels as heat-exchange surfaces [i]. These are large sized and their production technology is complex. Converter designs are also known [2] in which aluminum tubes are used as heat-exchange surface of the evaporators; these tubes are strengthened by longitudinal as well as cross ribs. Such evaporators could operate at moderate and high pressures but they are not compact. A new type of surface [3] in the form of a tube ribbed by wire and wire spiral (Fig. i) was adopted as the heat-exchange element. This tube has a high coefficient of ribbing (~8). The tube and the wire are made of aluminum alloy AMts which retains its strength at cyrogenic temperatures, increased pressure, and high atmospheric humidity. Studies of heat and mass exchange in the heat-exchange element of the proposed design were made* in an experimental model (Fig. 2) made of a 4.4-m-long ribbed tube. During the tests, nitrogen flow, nitrogen temperature (at seven points along the tube length), and atmospheric temperature T and humidity x were measured (in the experiments, T varied from 284 to 299 K and x from 5 to i0 g/kg). The experimental results were processed for the conditions at the beginning and end of the working period. The heat fluxes were calculated from the nitrogen temperature change in the tube sections and flow rate m as Q=m(ie

--fi),

where ii and ie are the nitrogen enthalpies at the inlet into the tube section and at its exit. The experimental heat-transfer coefficient per unit ribbed surface was determined for each section as Q KexP- Fr Atlg ' where Fr i s t h e r i b b e d t u b e s u r f a c e nitrogen and the atmosphere. *Engineers

and h t l g

t h e mean l o g a r i t h m i c

E. A. C h e r n y s h e v a a n d D. N. V a z i z o v a p a r t i c i p a t e d

temperature

head between

in the work.

Translated from Khimicheskoe i Neftyanoe Mashinostroenie, No. i, pp. 19-21, January, 1988. 32

0009-2355/88/0102-0032512.50

9 1988 Plenum Publishing Corporation

?

Fig. i.

Element of finned tube: i) spiral, 2) tube, and 3) wire. J

t --I 5 Fig. 2. Experimental model: 1) t a n k , 2) r i b b e d t u b e , 3) p o t e n t i o m e t e r s , 4) r o t a m e t e r , 5) psychrometer, and 6) thermocoupte. Later, from KexD, the experimental heat-exchange coefficient Sex D to the ribbed surface was determlned (wlthout allowance for the efflclency of _ibs exposed In the alr): aexp-ef =

I Kexp

! Sin

Fr

6t

Fr

Fin

~

Fin

'

where Sin is the heat-exchange coefficient from the tube wall to nitrogen, F r the surface per m ribbed tube length, Fin the inner surface per m tube length, 6t the tube wall thickness, and %m the thermal conductivity coefficient of the tube metal. The following data were obtained during the tests. Under natural (dry) convection for a frost-free surface, Sco n = 7 W/(ma'K). In the frost formation zone, the heat exchange coefficient initially (~0.5 h) along the tube length varied from 12 to 20 W/(m2"K); this value initially rose but fell later. At the end of the working period, the heat-exchange coefficient along the tube length in the frost formation zone rose from 6 to i0 W/(m2"K). The increase of the heat-exchange coefficient in the early frost formation period was also reported by other investigators [4, 5]. Thus, it was pointed out [4] that this coefficient doubled in 0.5 to 2 h. This is explained by the increased surface roughness following frosting, presence of the heat of phase transition, and the rise of the heat flux component due to the increasing darkening of the frost surface compared with that of the metal surface. Wlth the increasing duration of evaporator operation, the surface frost layer thickens and the heat-exchange coefficient falls because of the increasing thermal resistance of the frost. The following frost distribution features were detected in the surface studied: the frost layer increases predominantly on the outer spiral surface, its inner surface remaining practically free. This made for a fairly long life of the evaporator without any significant increase of the under-recovery of heat. Let us evaluate the heat-exchange coefficient ~ef theoretically from the available data [4, 6, 7] for conditions close to the test conditions. When calculating the heat exchange coefficient, the total heat flux to the surface should be considered:

33

9I

IO

I

i

& J

0

~

;00

T~

- Tsp, K

Fig. 3. Change of frost thickness 6 relative to the temperature difference of the atmosphere T~ and the spiral surface Tsp under different conditions: 9 T = 297 K, x = 6.5 g/kg, and T = 400 min; 4) T = 297 K, x = 9.5 g/kg, and Y = 300 min; A) T~ = 297.4 K, x = 10.5 g/kg, and ~ = 300 min; 9) T~ = 297.2 K, x = 8 . 7 g/kg, and T = 450 min; and .) T~ = 291 K, x = 7 g/kg, and 9 = 405 min.

Fig. 4.

Distribution of frost freezing on the spiral surfaces.

q = qcon+ % + qr, w h e r e qcon i s t h e c o n v e c t i v e h e a t f l u x , qp t h e h e a t o f p h a s e t r a n s i t i o n of water vapor s o l i d o r l i q u i d p h a s e , and q r t h e c o m p o n e n t o f t h e r m a l r a d i a t i o n energy. To d e t e r m i n e a e f , i t i s n e c e s s a r y t o know t h e t e m p e r a t u r e surface, i.e., the spiral or frost surface temperature. In the t h e m a i n w o r k i n g s u r f a c e i s t h e s p i r a l r i b whose t e m p e r a t u r e i s for the rib efficiency; the latter d e p e n d s on t h e r i b m a t e r i a l , c o n t a c t w i t h t h e t u b e , and t h e h e a t - e x c h a n g e c o e f f i c i e n t in the conditions, the rib efficiency was N95% [ 6 ] . The f r o s t

surface

temperature

was d e t e r m i n e d

into

of the cold heat-exchanger proposed evaporator design, determined with allowance its geometric sizes, rib atmosphere. For the test

by t h e m e t h o d o f

[7]

6=0,t2 [v(Tfr--~p)] ~ where 6 is the frost layer thickness, ~ the frost growth duration, f a c e temperatures of the frost and spiral, respectively.

and T f r and Tsp t h e s u r -

Figure 3 shows the frost thickness change relative to the temperature difference between the atmosphere T~ and the spiral surface for different working regimes. The calculated effective heat-exchange coefficients determined from the evaporator working conditions in the initial period were 12 to 17 W/(m2.K) and the experimental values for these conditions were 12 to 20 W/(m2"K),thereby showing good agreement between the experimental and calculated values. At the end of the evaporator working period, the heat-exchange coefficients varied from 14 to 17 W/(m2"K) and the experimental values from 6 to i0 W/(m2-K). Such a divergence could be explained by the inadequate accuracy of the frost surface temperature determination because of its uneven thickness. The heat-exchange coefficient [8] 34

coefficients obtained could be used to calculate the heat-transfer

Kt =

i

6t

!

+

l

'

(2)

+

~m

ainFin

Ea n

ae~(F +riP +n~ Fsp ) i o ' ~q w

where F o is the outer surface per m tube length, F w and Fsp the respective surfaces of wire and rib spiral per m tube length, and qw and qr the efficiency of wire and rib spiral~ It should be pointed out that for dry convection, in the early working period, the heattransfer coefficient per m of ribbed tube length is calculated without allowance for the thermal resistance of the frost layer. The heat-transfer coefficient is determined using Eq. 2, also for sections covered by a frost layer of thickness <6 mm. The structural features of the ribbed tube is such that frost (at thickness <6 mm) is disposed unevenly along the spiral, its thickness more on the spiral outer surface, and its inner surface covered only by a thin frost layer (Fig. 4). This feature does not permit determining accurately the effective heat-transfer area of the frost-covered spiral surface. At frost layer thickness ~6 mm, the ribbed surface is smoothed and heat exchange from the atmosphere to the heat-exchange surface could be regarded as heat exchange to the cylinder. In this case, the calculated heat-transfer coefficient per m tube length allowing for the thermal resistance of the frost layer is found from K=

l

I

~

I

__dfr +

t

'

(3)

where dfr = dor + 26; dor the outer diameter of the ribbed tube and %fr the thermal conductivity coefficient of the frost layer (according to [4], %fr = 7"10-4 Pfr)- The frost density Pfr determined during the tests on the spiral surface at ~150 and N220 K was =70 and 250 kg/m s, respectively. The test results obtained in the experimental evaporator and the calculated values showed that fairly intense heat and mass transfer occurred on this heat-exchange surface. Prototypes of compact evaporators of coil and cassette types were therefore designed and produced. Studies were made with the new evaporators in a wide range of temperature and humidity variations over different durations. The results pointed to efficient working throughout the evaporator operation in a fairly wide range of atmospheric temperature (253 to 290 K) and humidity (0.7 to 10.4 g/kg) variations. The high under-recovery of heat (At u = 28 K) in one regime is explained by the peculiar heat and mass transfer conditions at T = 274 K and relative humidity r = i00%. Since the frozen surface tends toward the triple point of water vapors (i.e. 273 K) with the increasing frost layer thickness above 273 K, the temperature head between the heat-exchanger surface and air decreases sharply; the mass transport process, whose moving force is the gradient of water vapor concentration around the cold surface, weakens and heat transfer is impaired. An analysis of the test results of evaporators points out that the least under-recovery of heat (AT = 2 to 3 K) for nitrogen was in regimes of minimum atmospheric temperature (253 to 263 K) and low absolute moisture (0.7 to 1.5 g/kg). At nitrogen specific consumption not exceeding 3.9 kg/(m2-h) and T : 253 to 290 K, the evaporators ensure AT u ~ I0 K for not less than 8 h of operation. Based on the test results, a new cassette-type compact evaporator was designed for nitrogen generation in mines. It successfully completed the tests under mine conditions and has been adopted for commercial production [9]. The heat-transfer surface designed could be recommended for converters using other cryogenic liquids. LITERATURE CITED i.

Cryogenic Equipment. Catalog of Central Research Institute of Scientific Information and Technical-Economic Investigations on Chemical and Petroleum Engineering, Moscow (1980).

35

2. 3. 4. 5. 6. 7. 8. 9.

36

Foreign Cryogenic Equipment at the "Cryogenic Technology 80" Exhibition, Ekspressinformatsiya, Series KhM-6 (1981), No. i. Inventor's Certificate No. 718692 USSR, MKI F28 F 1/36. G. N. Napalkov, Heat and Mass Transfer under Frost Forming Conditions [in Russian], Mashinostroenie, Moscow (1983). V. S. Ivanova, "Heat exchange of ribbed air coolers during frost formation," Kho!od. Tekh., No. ii, 57-61 (1978). V. A. Martynov and O. M. Popov, "Resolving the problem of the efficiency of wire rib wound on a tube," Tr. NPO "Kriogenmash," 146-151 (1983). K. D. Kremers, "Frost formation on vertical cylinders under convection conditions," Teploperedacha, No. i, 1-7 (1982). M. A. Mikheev and I. M. Mikheeva, Principles of Heat Transfer [in Russian], Energiya, Moscow (1977). D. M. Anchugov, I. V. Zavorotnov, O. K. Krasnikova, et al., "Gasification unit PGKhKA 1.0-03/1.6," Khim. Neft. Mashinostr., No. ii, 36-37 (1987).