S WHC-SA-3043- ~p

Heating, Ventilating, and Air Conditioning Deactivation Thermal Analysis of PUREX Plant

Prepared for the U.S. Department of Energy Assistant Secretary for Environmental Management

Hanford Company Richland, Washington

Management and Operations Contractor for the US. Deparlment of Energy under Contract DE-AC06-87RL10930

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i e a t i n g , V e n t i l a t i n g , A i r C o n d i t i o n i n g D e a c t i v a t i o n Thermal A n a l y s i s o f WREX P l a n t

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HEATING, VENTILATING, AND AIR CONDITIONING DEACTIVATION THERMAL ANALYSIS OF PUREX PLANT

W i l l i a m W. Chen and R o b e r t A. Gregon is West inghouse H a n f o r d Company

R i c h 1 and, Wash ing ton

ABSTRACT

1-hermal a n a l y s i s was per fo rmed f o r t h e proposed PUREX e x h a u s t sys tem a f t e r d e a c t i v a t i o n . The purpose o f t h e a n a l y s i s was t o d e t e r m i n e i f c o n d e n s a t i o n w i l l o c c u r i n a s u f f i c i e n t q u a n t i t y t o p l u g o r damage t h e f i l t r a t i o n c:omponents. A h e a t t r a n s f e r and f l u i d f l o w a n a l y s i s was p e r f o r m e d t o e v a l u a t e t h e t h e r m a l c h a r a c t e r i s t i c s o f t h e underground d u c t system, t h e deep bed g l a s s f i b e r (DBGF) f i l t e r No. 2, and t h e HEPA f i l t e r s i n t h e f o u r t h f i l t e r b u i l d i n g . The a n a l y s i s i s based on t h e ex t reme v a r i a t i o n s o f a i r t e m p e r a t u r e , r e l a t i v e h u m i d i t y , and dew p o i n t t e m p e r a t u r e u s i n g f i f t e e n y e a r s o f H a n f o r d w e a t h e r d a t a as a b a s i s . The r e s u l t s w i l l be used t o e v a l u a t e t h e need f o r e l e c t r i c h e a t e r s t h a t a r e p roposed f o r t h e PUREX canyon e x h a u s t t o p r e v e n t <.ondens a t i on.

R e s u l t s o f t h e a n a l y s i s i n d i c a t e t h a t a c o n d i t i o n may e x i s t i n t h e underground d u c t w o r k where t h e d u c t t e m p e r a t u r e can l e a d o r l a g t h e changes i n t h e ambien t a i r t e m p e r a t u r e . s u r f a c e s o f t h e underground e x h a u s t d u c t . was p e r f o r m e d assuming t h a t a l l o f t h e w a t e r i s removed f r o m t h e m o i s t a i r o v e r t h e i n s i d e s u r f a c e o f t h e c o n c r e t e d u c t a r e a i n t h e f u l l y d e v e l o p e d t u r b u l e n t boundary l a y e r w h i l e t h e m o i s t a i r i n t h e f r e e - s t r e a m w i l l n o t condense. The t o t a l m o i s t u r e accumula ted i n a 24 h o u r p e r i o d i s n e g l i g i b l y s m a l l . The r e s u l t s o f t h e a n a l y s e s a g r e e w i t h t h e p l a n t o p e r a t i n g e x p e r i e n c e s .

l h e f i l t e r s were des igned t o r e s i s t h i g h h u m i d i t y and d i r e c t w e t t i n g , f i l t e r p l u g g i n g due t o s l i g h t c o n d e n s a t i o n i n t h e u p s t r e a m d u c t i s n o t a concern .

T h i s c o n d i t i o n may c o n t r i b u t e t o c o n d e n s a t i o n on t h e i n s i d e A w o r s t case c o n s e r v a t i v e a n a l y s i s

Water p u d d l i n g wou ld n o t be expec ted .

I . INTRODUCTION

The o b j e c t i v e o f t h i s r e p o r t i s t o document t h e r e s u l t s o f a t h e r m a l a n a l y s i s t h a t was p e r f o r m e d f o r t h e proposed PUREX e x h a u s t sys tem a f t e r d e a c t i v a t i o n . The purpose o f t h e a n a l y s i s was t o d e t e r m i n e i f c o n d e n s a t i o n w i l l o c c u r i n a s u f f i c i e n t q u a n t i t y i n t h e c o n c r e t e d u c t t o r a i s e a c o n c e r n and t o p l u g o r damage t h e f i l t r a t i o n components. p e r f o r m e d t o e v a l u a t e t h e t e m p e r a t u r e c h a r a c t e r i s t i c s o f t h e underground d u c t system, t h e deep bed g l a s s f i b e r (DBGF) f i l t e r No. 2, and t h e HEPA f i l t e r s i n t h e f o u r t h f i l t e r b u i l d i n g shown i n F i g u r e 1. e x t r e m e v a r i a t i o n s o f a i r t e m p e r a t u r e , r e l a t i v e h u m i d i t y , and dew p o i n t t e m p e r a t u r e u s i n g s i x t e e n y e a r s o f H a n f o r d w e a t h e r d a t a as a b a s i s . The r e s u l t s w i l l be used t o e v a l u a t e t h e need f o r e l e c t r i c h e a t e r s t h a t a r e b e i n g c o n s i d e r e d f o r t h e PUREX canyon e x h a u s t t o p r e v e n t c o n d e n s a t i o n .

A t h e r m a l and f l u i d f l o w a n a l y s i s was

The a n a l y s i s i s based on t h e

1

The pressure drop of moist air through the filter media due to Joule-Thomson effect is addressed. Calculation examples to demonstrate the effect are presented.

11. SYSTEM BACKGROUND

The ventilation system for the deactivated PUREX plant will consist cf a draw- through system as shown in Figure l. capacity of 40,000 ft3/min (18.88 m3/sec) will maintain negative pressure on the canyon which provides the motive force to bring outside air into the plant. The proposed design cascades ventilation air through various zones within the plant to the PUREX canyon where it exits via the air tunnel to the underground duct work. The exhaust air passes through the DBGF filter No. 2, the fourth filter building containing HEPA filters, the main stack, and finally to the atmosphere.

One of three canyon exhaust fans with a

111. OPERATING EXPERIENCES

Prior to 1982, air washers in the supply air inlet plenums were used throughout the year. Large quantities of moisture were added to the air stream providing a mechanism for condensation far greater than what will occur during surveillance and maintenance period. history, no filter plugging or large accumulations due to water condensation were recorded. Drains in the underground filter plenums eliminated condensate water that may have formed but no water was entrained into the air stream to plug the deep bed or HEPA Filters.

The east and west sample gallery hood exhaust system exhausts to underground HEPA filter housings. The cover blocks of the filter housings are exposed to the ambient temperature. During HEPA filter replacements in 1987 and 1992, no high water marks or water damage were observed on any of the filters or plenum walls.

On several occasions in 1992, contamination was found to be dripping from the sample gallery hood exhaust duct when the spray washers were turned on. dripping was attributed to a chemical compound in the ductwork that had an affinity for water. Since 1992, the spray washers have been left off from early spring until late fall which eliminated the contamination dripping from the duct work. history that duplicates the SGE (sample gallery exhaust) ventilation during deactivation. No dripping or wetting of the ductwork, piping, and walls has been detected or reported.

During the winter, the sample gallery reheat coil supplies 7OoF (21.11"C) air to the sample gallery. (12.78"C), due to the effect of the earth, have shown no condensation or dripping. This provides the confidence necessary t o predict that the sample gallery will not experience any condensation throughout the year.

Based on available operating

The

This also provided a three-year spring/summer operating

The cold walls that maintain a temperature of 55'F

IV. ANALYSIS PROCEDURES

2

The thermal and fluid flow characteristics were evaluated to determine the worst case variations at different locations along the flow path from the PUREX canyon to the 291-AE building (Figure 1). The basic mechanisms for condensation and frost formation were evaluated. cooling in the underground portions of the ductwork were also described and evaluated. The temperature variations between the duct and exhaust air are investigated to determine if the condensation phenomena and/or frost phenomena are likely under these conditions.

Condensation or Moisture Formation

The effects of heating and

Condensation Princiole

Condensation or moisture formation may occur from the humid air under certain flow and thermal conditions. When moist air is exposed to a surface that is colder than the dew point temperature of the air, a trace of moisture film will appear on the surface (McQuiston and Parker, 1988). This phase change phenomenon is called condensation. The dew point (DP) temperature on the wall surface is the controlling factor for condensation to occur. The following conditions prevent condensation from forming:

The unsaturated air temperature equals the duct surface temperature. The unsaturated air temperature is colder than the duct surface temperature. The air dew point temperature is colder than the duct surface temperature.

The thermodynamic process of atmospheric air flowing through the PUREX exhaust system is essentially a heating and cooling process without changing the humidity ratio; only sensible heat is added or removed. Therefore, the moist air that flows through the cold duct does not necessarily produce condensation.

Condensation Rate

Based on the Hanford weather data occurred during a 24 hour period on December 11, 1994, The data indicated the dry-bulb temperature in the range of 30 to 33"F, while the relative humidity was at a range of 95% to 99%. cumulative condensate was calculated based on persistent air temperature of 3 3 ° F (0.56"C) and relative humidity of 98%, for a period of 24 hours, while the duct wall temperature (32°F or O'C) was assumed to fall below the air dew point temperature of 32.5"F (0.28"C).

humidity ratio, a worse Aw = 0.000325 lb of H,O per lb of dry air was calculated for a 24 hour period.

(0.78 m3/kg), the mass flow rate was calculated as 3,200 lb/min (1,451.5 kg/min).

The

Since the condensation rate was calculated based on the variation of the

The average specific volume of the air was determined at 12.5 ft3/lb

3

Since the quantities of moist air condensation occur only for those contacting with the cool wall surface, most of the free-stream air simply f'lows through the duct without producing condensation. condensation occurs only for the moist air inside the laminar sublayer in a fully developed turbulent boundary layer. assumed that the moist air inside the fully developed turbulent boundary will be condensed.

The actual

However, it is conservatively

For fully developed boundary layer, the Reynolds number is calculated based on boundary layer length of 50 ft (15 .24 m) at 3,787,698. The boundary layer thickness 6 was calculated 11 in (27 .94 cm). condensation is 3,200 ft3 (90 .6 m 3 ) . The total condensation rate for all the air moisture in the 24 hour period is 608 lb (275.8 kg).

2 1 hour period is 1,600 ft2 (148.6 m2). Therefore the condensate rate over unit duct surface area is 0.016 lb of H20/hr-ft2 (0 .078 kg of H20/hr-m2) or a total of the moisture at 0.38 lb H20/ft (1 .85 kg H20/m2) per day on the duct wall surface.

The remaining non-condensed moist air outside the boundary layer will flow through the duct. because the accumulatd quantity is small, dripping or puddling will not be expected.

interior is not in a saturated state, the sorption characteristics, capillary suction, and moisture transfer capabilities of the concrete may be able to absorb a small amount of moisture (Jonasson, 1985) .

cannot produce dropwise condensation. concrete surface with a thin layer of film with no dripping or puddling.

The total volume for

The total surface area for this amount of condensate to spread over in a

The condensate on the wall surface causes no concern

If the amercoat on the concrete has been deteriorated or concrete

For the concrete surface in the air duct, this rate of condensation The mositure will be spread over the

As a result of the assumed condensation, the humidity ratio or moisture contents in the flowing air will be reduced when the air flows through the D8GF filter No. 2.

- Foq and Frost Formation

Fog is a disperse system consisting of liquid drops suspended in a gas. The conversion of vapor to fog is a discontinuous process occurring under critical conditions when vapor reaches the supersaturation state (Amelin, 1967). The air flow in this study has not experienced any isentropic expansion into the wet region in the enthalpylentropy diagram (Obert, 1960) and the supersaturation condition has never been attained. Thus, fog formation inside the air duct is not possible.

If the fog or gaseous suspensions are flowing from the environment into the plant with the moist air, the warming effect inside the building and air duct will evaporate the moist fog. The fog can be sustained only if the plant has a source to continuously supply the mositure and the temperature in the duct is much colder than the outside air. As the moist air flows through a complicated and winding path inside the plant, the moist drops may impact a

surface and deposit along the pathway. lasted only few hours, a limited amount of moisture was deposited in the air duct.

Frost forms when the moist air is exposed to a surface that is colder than the dew point temperature of the air which is far below 32'F (0°C). Under this condition, the moisture content of the air passes directly from the gaseous state to the soiid state forming a porous layer of frost on the cooled surface. The rates of increase of the frost thickness require a continuous cooler temperature and high humidity environment. Otherwise, the solid frost begins to melt and evaporates into the air stream. When the air/frost interface temperature temporarily warms back to 32°F (O"C), even in a high humidity environment, the frost layer will occasionally undergo cycles of me1 ting and refreezing (Trammell, 1968). Considering the high affinity the air has for moisture, the frost melts and is evaporated by the dry air.

Air and Duct Temperature

7he worst temperature variations contained in the Hanford weather data for the past sixteen years were reviewed. The two wettest years, and the two coldest years, and the 1994 winter data were used as the basis for performing the heat transfer analyses. Heat transfer calculations and numerical examples for the conditions described are presented be low.

Because the fog conditions in Hanford

Transient Heat Conduction

Based on the coldest and wettest Hanford weather data, transient heat conduction calculaticns were performed to determine the worse environmental temperature conditions and the depth of the freezing layer penetration into the soil resulting from the worst weather scenario (Schneider, 1955).

Two examples are used to demonstrate the depth of frozen layer penetration into the underground soil under extreme weather conditions.

ExamDle 1: the surface temperature of the plant environment drops down to 18'F (-7.8'C). These cold conditions continue for 14 days. The temperature of the ground before the weather change is assumed to be 35°F (1.7 "C). After this period, the temperatures at different distances from the surface are calculated at (Schneider, 1955):

On a cold winter day, after a sudden change in weather conditions,

x = 4 ft (1.220 m), T = 32.1"F (0.08"C). x = 5 ft (1.524 m), T = 33.6-F (0.89"C). x = 6 ft (1.829 m), T = 34.3"F (1.28"C).

Examole 2: temperature to 0°F (-17.8"C). 40°F (4.4"C) before the onset of the cold wave, the freezing temperature penetrations were calculated to different depth after various hours,

During the winter, a sudden cold wave reduces the ambient air If the earth was at a uniform temperature of

6 hours, 12 hours,

x = 0.9 x 241.8 x 6 = 5.9 in (14.98 cm). x = 0.9 x 2J1.8 x 12 = 8.4 in (21.34 cm).

5

24 hours ,

T h i s i n d i c a t e s t h a t t h e f r o z e n l a y e r p e n e t r a t e s o n l y l e s s t h a n 1 ft

x = 0 .9 x 241.8 x 24 = 11.8 i n (29.97 cm).

(0 .3048 m) i n 24 hours .

F l o w i n q A i r Temoerature

The w i n t e r a tmospher i c a i r f l o w s f rom t h e PUREX canyon b u i l d i n g and e n t e r s t h e i n c l i n e d a i r d u c t system. The w a l l s u r f a c e t e m p e r a t u r e o f t h e a i r d u c t was assumed t o be a t 55°F (12.8-C) a t 30 f t (9.144 m) below g r a d e and c o o l e d down t o n e a r t h e env i ronmen t tempera tu re a t t h e upper s e c t i o n c l o s e t o t h e g rade l e v e l . t h e f l o w i n g a i r t e m p e r a t u r e a t t h e t u r b u l e n t boundary l a y e r due t o t h e warmer w a l l . T h i s s e c t i o n c a l c u l a t e s t h e m o i s t a i r t e m p e r a t u r e change due t o t h e e f f e c t o f warm c o n c r e t e d u c t w a l l . The underground c o n c r e t e d u c t c r o s s - s e c t i o n a r e a v a r i e s f r o m t h e l o w e r p o r t i o n o f 1 1 ' x 5 ' - 9 " (3 .35 m x 1.75 m) t o t h e upper p o r t i o n o f 8 ' x 8 ' (2 .44 m x 2.44 m). The upper s e c t i o n i s n e a r t h e g rade l e v e l , i t s t e m p e r a t u r e i s c l o s e r t o t h e e n v i r o m e n t a l t e m p e r a t u r e .

Heat t r a n s f e r c a l c u l a t i o n s were pe r fo rmed t o d e t e r m i n e t h e i n c r e a s e o f

C o n s i d e r 40,000 c fm (1,132 m3) f l o w r a t e , t h e f l o w v e l o c i t y t h r o u g h 11' x 5 ' - 9 " (3 .35 m x 1 .75 m) d u c t i s 10.54 f t / s e c ( 3 . 2 1 m/sec). h y d r a u l i c d i a m e t e r i s 90.6 i n (2.3 rn). f a l l s w i t h i n t h e t u r b u l e n t f l o w r e g i o n . Assume P r a n d t l number i s 0.7 f o r a i r . The h e a t t r a n s f e r c o e f f i c i e n t was c a l c u l a t e d a t 0.0196 B t u / h r - i n 2 - " F (16.02 W/m2-K) u s i n g t h e c o r r e l a t i o n (Lance t , 1959) Nu = 0.042 (Re)'.' (P r ) "3 .

The e q u i v a l e n t The Reynolds number i s 578,529 w h i c h

Assume t h a t t h e m o i s t a i r a t 32°F (0°C) f l o w s t h r o u g h t h e c o n c r e t e d u c t s e c t i o n a t a t e m p e r a t u r e o f 55'F (12 .8 "C) , t h e h e a t b a l a n c e e q u a t i o n may be a p p l i e d t o c a l c u l a t e t h e f l o w i n g a i r t empera tu re a t 34.4 " F ( 1 . 3 3 " C ) . T h i s demons t ra tes t h a t t h e a i r t empera tu re i n t h e boundary l a y e r w i l l be warmed up t o 34.4 " F (1 .33"C) , w h i l e t h e a i r t empera tu re o u t s i d e o f t h e t h i n boundary l a y e r rema ins a t 32'F (O'C).

Duc t Wa l l Temperature

A n o t h e r c a l c u l a t i o n shows t h e c o o l i n g e f f e c t on t h e w a l l s u r f a c e i f c o o l a i r i s f l o w i n g c o n t i n u o u s l y t h r o u g h t h e d u c t . w a l l r e q u i r e s t e m p e r a t u r e g r a d i e n t i n t h e c o n c r e t e w a l l t o i n i t i a t e t h e c o n d u c t i o n h e a t f l o w . Hand c a l c u l a t i o n s appeared t o be f a i r l y i n v o l v e d . t r a n s i e n t h e a t t r a n s f e r a n a l y s i s u s i n g a s i m p l i f i e d f i n i t e - e l e m e n t model (ANSYS, 1993) was pe r fo rmed t o de te rm ine t h e t e m p e r a t u r e d i s t r i b u t i o n s t h r o u g h o u t a c o n c r e t e s e c t i o n .

The underg round warm d u c t w a l l s u r f a c e t e m p e r a t u r e may be c o o l e d i f a i r a t 32°F c o n t i n u o u s l y f l o w i n g t h r o u g h . s u r f a c e , t h e d u c t s u r r o u n d i n g remains a t 55°F (12.8"C).

The f i n i t e - e l e m e n t mesh d e n o t i n g t h e node and e lemen t number ing i s shown i n F i g u r e 2. The 3-D t h e r m a l s o l i d e lements ( s o l i d 7 0 ) , has e i g h t nodes w i t h a s i n g l e deg ree o f freedom, tempera tu re , a t each node was used. C o n v e c t i v e

The c o n v e c t i v e c o o l i n g f r o m t h e

A

However, a t a l a r g e d i s t a n c e away f r o m t h e

6

boundary c o n d i t i o n , w i t h h e a t t r a n s f e r c o e f f i c i e n t o f 0 .0196 B t u / i n 2 - h r - " F (16.02 W/m'-K), was a p p l i e d a t t h e i n s i d e w a l l s u r f a c e . C o n c r e t e m a t e r i a l p r o p e r t i e s were used.

A s m a l l i n i t i a l t e m p e r a t u r e g r a d i e n t was a p p l i e d t o s t a r t t h e t r a n s i e n t s o l u t i o n . R e s u l t s i n d i c a t e t h a t t h e tempera tu re d i s t r i b u t i o n t h r o u g h o u t t h e s e c t i o n o f t h e w a l l reached a s teady s t a t e t e m p e r a t u r e o f a p p r o x i m a t e l y 42'F (5.56"C) a f t e r 4 hours.

F i l t e r System Condensat ion

P o s s i b l e c o n d e n s a t i o n and m o i s t u r e e f f e c t s on t h e f i l t e r systems a r e d e s c r i b e d i n t h i s s e c t i o n .

P r e f i l t e r (Deep Bed F i l t e r No. 2 2

I f c o n d e n s a t i o n was assumed t o have o c c u r r e d a t t h e c o l d w a l l s u r f a c e s i n t h e upper p o r t i o n o f t h e i n c l i n e d a i r d u c t system, t h e m o i s t u r e c o n t e n t o f t h e a i r wou ld be reduced and f u r t h e r e f f e c t o f c o n d e n s a t i o n on t h e deep bed f i l t e r wou ld be dec reased because t h e a i r e n t e r i n g t h e f i l t e r i s d r y e r .

G e n e r a l l y , a s u f f i c i e n t q u a n t i t y o f m o i s t u r e i n d r o p l e t f o r m i s hazardous t o a f i l t e r system. l o n g as p l u g g i n g does n o t occu r . F i l t r a t i o n i s ach ieved by v a r i o u s mechanisms depend ing on t h e p a r t i c l e s i z e and f l o w v e l o c i t y (Dav ies , 1973) . The p e n e t r a t i o n mechanism i s c o n t r o l l e d by e i t h e r m o l e c u l a r d i f f u s i o n , i n t e r c e p t i o n , Brownian mo t ion , o r i n e r t i a l i m p a c t i n g depend ing on t h e a i r f l o w v e l o c i t y .

I n good q u a l i t y f i l t e r s , o f l o w r e s i s t a n c e and h i g h e f f i c i e n c y a g a i n s t submic ron p a r t i c l e s , t h e a i r f l o w obeys D a r c y ' s l a w wh ich i s u s u a l l y c o n s i d e r e d t o be v a l i d f o r c r e e p i n g f l o w w i t h l o w Reynolds number. The l o w Reynolds number f l o w i s dominated by v i s c o u s f o r c e . between t h e a i r p a r t i c l e s and t h e f i l t e r f i b e r s may keep t h e f i l t e r s u r f a c e t e m p e r a t u r e f r o m d e c r e a s i n g be low t h e a i r t e m p e r a t u r e . f i l t e r s u r f a c e tempera tu re i s g e n e r a l l y c l o s e t o t h e a i r f l o w t e m p e r a t u r e .

However, t h e f i l t e r s a r e des igned t o have r e s i s t m o i s t u r e as

The v i s c o u s e f f e c t

As a r e s u l t , t h e

H i q h E f f i c i e n c y P a r t i c u l a t e A i r F i l t e r

I f c o n d e n s a t i o n o c c u r s i n t h e underground d u c t work t h e a i r f l o w i n g t h r o u g h t h e HEPA f i l t e r s i n t h e 291-AE b u i l d i n g w i l l be d r y e r .

B e f o r e t h e a i r reaches t h e HEPA f i l t e r s , i t has t o f l o w t h r o u g h t h e c o n c r e t e d u c t wh ich has a w a l l s u r f a c e t e m p e r a t u r e c l o s e t o t h a t o f t h e i n l e t p lenum w a l l l o c a t e d a t t h e e n t r a n c e t o t h e HEPA f i l t e r . s h o u l d be t h e same o r c o l d e r t h a n t h a t o f t h e HEPA f i l t e r . t e m p e r a t u r e i s a t o r c o l d e r t h a n t h e dew p o i n t o f t h e f l o w i n g a i r , c o n d e n s a t i o n w i l l o c c u r a t t h e plenum.

Condensa t ion w i l l n o t t a k e p l a c e i n t h e f i l t e r s whe the r o r n o t c o n d e n s a t i o n o c c u r s a t t h e plenum. The a i r w i l l s i m p l y f l o w t h r o u g h t h e f i l t e r s . The HEPA f i l t e r media i s t r e a t e d w i t h a w a t e r - r e s i s t a n t b i n d e r and w i l l t o l e r a t e b o t h

The p lenum w a l l t e m p e r a t u r e I f t h e p lenum w a l l

7

h i g h h u m i d i t y and d i r e c t w e t t i n g ( F l a n d e r s , 1988) . S e v e r a l t e s t s ( F i r s t and L e i t h , 1976) i n d i c a t e d t h a t no d e g r a d a t i o n i n f i l t e r i n g a b i l i t y o f t h e f i l t e r s a f t e r exposure t o a s t e a m - d r o p l e t atmosphere.

Joule-Thomson E f f e c t s

The Joule-Thomson exper iment (Jones and Hawkins, 1960; J. K e s t i n , 1979) may b e a r t h e resembiance o f t h e m o i s t a i r f l o w i n g t h r o u g h f i l t e r media and produce c o n d e n s a t i o n . t o c o o l i n g i s t h a t t h e i n i t i a l t e m p e r a t u r e o f t h e gas must be be low t h e maximum i n v e r s i o n a i r t e m p e r a t u r e w h i c h i s 628°F (331°C) . The t e m p e r a t u r e o f t h e f l o w i n g a i r i s below t h e maximum i n v e r s i o n t e m p e r a t u r e .

I f a i r i s compressed t o a p r e s s u r e o f 200 atm (2,940 p s i o r 20,265 KPa) and a t e m p e r a t u r e o f 126°F (52"C) , a f t e r t h r o t t l i n g t o a p r e s s u r e o f 1 atm ( 1 4 . 7 p s i o r 101.325 KPa), i t w i l l be c o o l e d t o 73°F (23°C) . F o r a s m a l l p r e s s u r e d r o p o f f e w i n c h e s o r c e n t i m e t e r s , such as f l o w t h r o u g h p o r o u s med ia , t h e t e m p e r a t u r e d r o p and t h e a i r c o o l i n g e f f e c t due t o t h e Joule-Thomson phenomena i s n e g l i g i b l e .

- F r o s t F o r m a t i o n on HEPA F i l t e r s

The p o s s i b l e f r o s t c o n d i t i o n s i n t h e HEPA f i l t e r b u i l d i n g a r e e v a l u a t e d u n d e r t h e assumed f r o z e n t e m p e r a t u r e i n t h e env i ronment . Two c o n d i t i o n s may s u b j e c t a c o o l s u r f a c e t o f r o s t f o r m a t i o n : n o c t u r n a l r a d i a t i o n , wh ich u s u a l l y o c c u r s a t n i g h t ; and when a s u r f a c e t h a t has c o o l e d b e l o w 32°F (0°C) c o n t a c t s t h e m o i s t a i r w i t h a dew p o i n t t e m p e r a t u r e g r e a t e r t h a n t h e s u r f a c e t e m p e r a t u r e .

The f i r s t c o n d i t i o n i s u n l i k e l y t o o c c u r because t h e HEPA f i l t e r s a r e housed i n a b u i l d i n g and n o t exposed t o n o c t u r n a l r a d i a t i o n . T h e r e f o r e , t h e c o o l i n g w i l l n o t be p r e s e n t i n s i d e t h e b u i l d i n g .

The second c o n d i t i o n becomes p o s s i b l e when a s u r f a c e c o o l s t o a s u b z e r o t e m p e r a t u r e w h i c h i s be low t h e dew p o i n t o f t h e m o i s t a i r . I f t h e w a l l s u r f a c e t e m p e r a t u r e o f t h e HEPA f i l t e r b u i l d i n g i s c o o l e d down w e l l b e l o w t h e f r e e z i n g p o i n t , t h e c o n c r e t e d u c t and i n l e t p lenum s u r f a c e t e m p e r a t u r e w i l l be n e a r or a t t h e f r e e z i n g p o i n t t e m p e r a t u r e . Under t h i s c o n d i t i o n , t h e w a t e r va.por l e a v i n g t h e a i r and a t t a c h i n g t o t h e s u r f a c e may pass d i r e c t l y f r o m ga.seous t o s o l i d s t a t e f o r m i n g a porous l a y e r o f f r o s t .

T e s t s ( B r i a n e t c , 1970) i n d i c a t e t h a t c o n t i n u o u s f r o s t f o r m a t i o n r e q u i r e s v e r y l o w sub-zero t e m p e r a t u r e and h i g h s p e c i f i c h u m i d i t y e n v i r o n m e n t . f r o s t f o r m a t i o n , f r o s t g r o w t h r e q u i r e s a s u s t a i n e d c o o l i n g mechanism, a c o n t i n u o u s t e m p e r a t u r e decrease o f t h e c o o l e d s u r f a c e and an i n c r e a s e o f t h e s p e c i f i c h u m i d i t y o f t h e a i r s t r e a m . I n t h e absence o f t h i s p e r s i s t e n t c o o l i n g phenomena, f r o s t b u i l d u p i s n o t p o s s i b l e (Trammel l , 1968) . t i m e t h e s u r f a c e t e m p e r a t u r e reaches 32'F ( O " C ) , m e l t i n g w i l l o c c u r (Jones and P a r k e r , 1975) . W i t h t e m p e r a t u r e f l u c t u a t i o n s f r o m t h e H a n f o r d w e a t h e r d a t a , f r o s t f o r m a t i o n on c o l d s u r f a c e s may have o c c u r r e d as a t r a n s i e n t c o n d i t i o n .

The b a s i c c o n d i t i o n f o r t h e Joule-Thomson e f f e c t t o g i v e r i s e

when t h e s u r f a c e has been exposed t o

A f t e r t h e

I f a t any

8

If condensation has already occurred as the cold air flows through the concrete duct or inlet plenum, the same amount o f air that flows into the HEPA filters would have reduced the moisture content or humidity ratio. reduced humidity ratio due to the loss of moisture content in the duct, the effect on the filters is not a concern.

With a

V. RESULTS AND DISCUSSIONS

lhe results of a transient heat conduction analysis show that the depth of frozen soil, resulting from a subfreezing surface temperature of 18°F (-7.78"C) penetrate 6 ft (1.83 m) below grade after 14 days. These results agree with the publications from the U. S. Weather Bureau map that show the average and maximum depths of frost penetration as 12 inches (30.48 cm) and 36 inches (91.44 cm), respectively, in the southeastern part of Washington state (Strock, 1965). This indicates that only a small upper section of the air duct perimeter near the grade level may come close to the freezing temperature temporarily under the possible worst environmental condition.

P,n extreme weather condition which may never occur in Hanford was analyzed. A plant environmental surface temperature of -27°F (-32.8'C) lasting for 60 days was assumed. Results indicate that the frozen layer may penetrate into the soil at a depth of 7 ft (2.134 m).

The earth has been shown to maintain a constant temperature of 5 5 ° F (12.78"C) throughout the year at the lower section of air duct. Cool moist air at 32°F (0°C) flowing through the 5 5 ° F (12.78"C) air duct can warm up only 2 ° F (1.11OC) near the boundary layer while a large quantity of air at the free 5tream remains ?t 32°F (0°C). P. cnntinuous flow of cold air at 32°F (0°C) may lower the wall temperature to 42°F (5.56-C) when reaching a steady state condition in four hours. The 42°F (5.56"C) temperature is above the dew point temperature of the air.

If the air flows at a velocity of 10.54 ft/sec (3.213 m/sec), the time for air to travel from the duct entrance to the deep bed filter no. 2 takes approximately 25 seconds. lower than the steady state temperature. Condensation in the lower section of the air duct during the winter is not possible.

A condition may exist in the underground ductwork where the duct temperature can lead or lag the changes in the ambient air temperature due to the thermal storage characteristics of the underground duct. contribute to condensation on the inside surfaces of the underground exhaust duct.

The duct wall temperature cannot be cooled much

This condition may

The coldest and wettest weather data for the past sixteen years have shown that only short periods in the early spring and winter o f 1994 could contribute to some slight condensation which is barely enough to dampen the duct wall surfaces.

However, condensation rate was conservatively calculated based on a worst recorded data during the 1994 winter. the range of 30 to 3 3 ° F (-1.1"C to 0.56"C), while the relative humidity was in

The dry-bulb temperature occurred in

9

the range of 95 to 99% over a period of 24 hours, assuming that the moist air flowing through the boundary layer in the concrete duct is condensed in 24 hour period. be accumulated on a one square foot area (0.093 m2) in 24 hours. amount of flowing air in the free stream does not candense.

This small amount of moistura may be evaporated under the condition of temperature variations without producing significant weting. operating histories, no significant condensation has been observed, even when the water spray system was in operation.

If condensation occurs on the concrete duct wall surfaces, the humidity ratio for the air downstream is reduced. As a result, the filter housing temperature is above the dew point temperature of the air; therefore, condensation does not take place. The potential for condensation on the HEPA filter housing surfaces will be less of a concern.

Results indicated that only 0.38 lb (0.172 kg) of moisture may A large

From the plant

1-he pressure drop through the filter system is insignificant to cause any temperature drop and condensation. designed to resist high humidity and direct wetting (Burchsted, 1978; F-landers, 1988), so a small amount of moisture will be of no concern.

On the other hand, the filters are

Frost formation on the HEPA filters is not possible because frost forms only on much colder surfaces, such as the duct wall or inlet plenum, before the (:old air flows into the filters. No enduring mechanisms, such as continuous c.ooling and high humidity environment, exist to build up such effects on the HEPA filters.

VI. CONCLUSION

Condensation heat transfer and fluid flow analyses were completed to evaluate the effects of condensate in underground duct and filters in the PUREX plant. lhe results of the analysis indicate that the ductwork upstream of the DBGF filters bed No. 2 is the most likely place for condensation to form.

A worst case conservative analysis was performed assuming that all of the water is removed from the moist air over the inside surface of the concrete duct area in the fully developed turbulent boundary layer while the moist air in the free stream will not condense. The total moisture accumulated in a 24 hour period is 0.38 lb H,0/ft2 (1.85 kg H20/m2) which is barely enough to dampen the concrete wall surfaces. Water dripping and puddling will not be expected.

Turbulent heat transfer analysis has shown that 32'F (0°C) air flowing through the duct will be warmed to 34.4'F (1.33"C) inside the thin laminar sublayer while the air in the free stream remains at 32°F (0°C). Further analysis has indicated that the surface of the duct wall may be cooled down to 42°F (5.56"C) after reaching a steady state condition in four hours.

Based on an air flow velocity through the concrete duct at 10.54 ft/sec (3.213 m/s), the air has a resident time of approximately 25 seconds traveling from

10

t h e underground d u c t e n t r a n c e t o t h e DBGF No. 2 f i l t e r . l e a d / l a g t e m p e r a t u r e c h a r a c t e r i s t i c s o f t h e underground d u c t a r e o f s h o r t d u r a t i o n a l l o w i n g t h e d u c t t o f o l l o w changes i n a i r t e m p e r a t u r e r a t h e r c l o s e l y w h i c h reduces t h e c o n d i t i o n f o r p o s s i b l e c o n d e n s a t i o n . c o n d e n s a t i o n o c c u r r i n g i n t h e d u c t w o r k ups t ream o f t h e DBGF No. 2 f i l t e r w i l l p r o v i d e d r y e r a i r i n t h e downstream d u c t and f i l t e r s .

If no c o n d e n s a t i o n o c c u r s b e f o r e t h e deep bed f i l t e r s , t h e n t h e same m o i s t a i r i s f l o w i n g t h r o u g h t h e c o n c r e t e d u c t , t h e HEPA f i l t e r b u i l d i n g , and t h e e x h a u s t sys tem w i t h o u t f u r t h e r c o n d e n s a t i o n . f i l t e r sys tem i s i n s i g n i f i c a n t t o cause any t e m p e r a t u r e d r o p t h a t w i l l i n i t i a t e c o n d e n s a t i o n . h i g h h u m i d i t y and d i r e c t w e t t i n g ( B u r c h s t e d , 1978; F l a n d e r s , 1988) , f i l t e r p l u g g i n g due t o c o n d e n s a t i o n i s n o t a concern . T e s t s ( F i r s t and L e i t h , 1976) i n d i c a t e d no d e g r a d a t i o n i n f i l t e r i n g a b i l i t y a f t e r exposure t o a steam- d r o p l e t atmosphere. e x p e r i e n c e s . t h e r e f o r e , i t i s conc luded t h a t no e l e c t r i c a l h e a t i n g sys tem i s r e q u i r e d f o r t h e PUREX canyon e x h a u s t t o p r e v e n t c o n d e n s a t i o n .

V I I . REFERENCES

T h e r e f o r e , t h e

On t h e o t h e r hand,

The p r e s s u r e d r o p t h r o u g h t h e

On t h e o t h e r hand, t h e f i l t e r s were d e s i g n e d t o r e s i s t

The r e s u l t s o f t h e a n a l y s e s a l s o a g r e e w i t h t h e o p e r a t i n g

Amel in , A. G . , 1967, Theory o f f o g Format ion, 2nd ed., I s r a e l Program f o r S c i e n t i f i c T r a n s l a t i o n L t d . , Jerusa lem, I s r a e l .

ANSYS, 1993, "Eng ineer ing A n a l y s i s Computer Program," V e r s i o n 5.OA, Swanson A n a l y s i s Systems, I n c . , Houston, P e n n s y l v a n i a .

B r i a n , P. L . T., and R . C . R e i d and Y . T. Shah, 1970, F r o s t D e p o s i t i o n on Cold Sur faces , I n d . Eng. Chem. Fundamental , V o l . 9, No. 3, pp. 375-380.

B u r c h s t e d , C . A. and J. E. Kahn and A. B . F u l l e r , 1978, Nuc lea r Air C lean ing Book, ERDA 76-21, Oak Ridge N a t i o n a l L a b o r a t o r y , Oak R idge, Tennessee.

D a v i e s , C . N., 1973, Air F i l t r a t i o n , Academic Press , New York , N . Y .

F i r s t , M. W . and D . L e i t h , 1976, Entra inment Separa to r Per formance, Paper No 101, Proceed ings o f t h e 1 4 t h ERDA A i r C l e a n i n g Conference, pp. 694-710.

F l a n d e r s , 1988, Muclear Grade HEPA F i l t e r s , B u l l e t i n No. 812E, F l a n d e r s F i l t e r s I n c . , Washington, NC.

Jonasson, J. E . , 1985, Mois tu re F i x a t i o n and M o i s t u r e T r a n s f e r i n c o n c r e t e , Paper No. H5/11, 8 t h I n t e r n a t i o n a l Conference on S t r u c t u r a l Mechan ics i n R e a c t o r Techno logy , August 19-23, 1985, B r u s s e l s , Be lg ium.

Jones, J. B., and G. A. Hawkins, 1960, Eng ineer ing Thermodynamics, John W i l e y & sons, I n c . , New York, N.Y.

Jones, 8 . W. and J. D. Parker , 1975, F r o s t Format ion W i t h V a r y i n g Envi ronmenta l Parameters, J. o f Heat T r a n s f e r , May, 1975, pp. 255-259.

Kays, W . M . and M. E. Crawford , 1993, Convect ive Heat and Mass T r a n s f e r ,

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3rd ed., McGraw Hill Book Company, New York, N . Y .

Kestin, J., 1979, A Course i n Thermodynamics, Revised Edition, Volume One, Hemisphere Publishing Corporation, New York, N.Y.

Lancet, R. T., 1959, The E f f e c t o f Sur face Roughness on t h e Convec t ion Heat- T r a n s f e r C o e f f i c i e n t f o r f u l l y Developed T u r b u l e n t Flow i n Duc ts W i t h U n i f o r m Heat f l u x , J. o f Heat Transfer, May, 1959, pp. 168-174.

Mcpuiston, F. C. and J. D. Parker, 1988, Heat ing , V e n t i l a t i n g , and Air C o n d i t i o n i n g , A n a l y s i s and Des ign , 3rd ed., John Wiley & Sons, Inc.

Obert, E. F., 1960, Concepts o f Thermodynamics, McGraw Hill Book Company, New York, N.Y.

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Strock, C and R. L. Koral, 1965, Handbook o f Air C o n d i t i o n i n g H e a t i n g and V e n t i l a t i n g , 2nd ed., Industrial Press Inc., New York, N. Y .

r . I ' Trammell, G. J., and D. C. Little and E. M. Killgore, 1968, A Study o f F r o s t

Formed on A F l a t P l a t e Held a t Sub-Zero Temperatures, ASHRAE Journal, J u l y

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1968, pp. 42-47.

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