PRESSURE DROP IN RADON CONTROL PIPES by:

William E. Belanger, P.E. U. S. Environmental Protection Agency Region I11 841 Chestnut Street Philadelphia, PA 19107 ABSTRACT

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Design of radon mitigation systems requires the designer to choose among the available fan sizes and to choose a fan with pressure-flow characteristics which match the application. The mitigator is also faced with a choice of pipe size and material, and must choose a routing through the structure which is acceptable to the customer and which will not disrupt system function with excessive pressure drop. This paper presents a tabulation of pressure drops for use in selecting pipe size and routing for radon control system pipes. The calculated pressure drop in the pipework can also be used to assist in fan selection. Disclaimer This paper does not necessarily reflect the policies of the Environmental Protection Agency. No endorsement of any named product should be implied.

PRESSURE DROP IN RADON CONTROL PIPES William E. Belanger, P.E. INTRODUCTION When a radon mitigation system is designed, the designer is faced with a number of decisions. In the ideal situation, there would be an adequate bed of crushed stone under the concrete floor of the basement. There would be a convenient place to run the suction pipe from the basement directly to the attic, and an electrical connection for the fan. There would be no need for elbows in the suction pipe, and the walls (or chase) would be big enough to accommodate whatever pipe size is chosen. Unfortunately, in a real mitigation these conditions are seldom found. Instead, the mitigator is forced to route the pipes through walls which are too small to accommodate 4 inch pipe, go around corners using many elbows, and generally install the pipework in a non-optimum fashion. The question that must be asked is whether the fan chosen can move the required volume of air through all this pipe. Will a bigger fan be needed, or will the pipe size and routing have to be changed? This paper presents a simple scheme to calculate the pressure drop in smooth-walled radon control pipe. Pipe sizes from 1.5 inch to 6 inch are treated. The calculations are for plastic pipe as is commonly used in radon control applications. Once the required flow has been determined and the pipe size and routing chosen, it is relatively easy to determine if the pressure drop will be excessive. If the required flow is not known, it can be determined experimentally. In general, the required flow will be sufficient to produce a -015 inch water column vacuum across the entire slab. While good radon reduction may be achieved with lesser flow under some conditions, "Application of Radon Reduction Methodsw (EPA/625/5-88-024) suggests that .015 inch of vacuum is needed to assure reliable system operation. CALCULATIONS Pressure drop in the pipe may be calculated from the equivalent length of straight pipe used in the system. This equivalent length may be calculated by adding the length of straight run to the equivalent length of the elbows and transitions. Each elbow and each reducer can be assumed to have an equivalent length of 10 times the diameter of the pipe. Thus an elbow in a 4 inch pipe would have an equivalent length of 10 times 1/3 foot or 3.3 feet. For a reducer, use either the outlet (downstream) size. A 3x4 reducer would have an equivalent length of either 3.3 feet of 4 inch pipe or 2.5 feet of 3 inch pipe.

DERIVATION OF PRESSURE DROP TABLES; For fully turbulent flow hf = f L v2

2g

This is the Darcy-Weisbach equation. ^

For laminar flow where :

hf is the head in feet of air (which must be transformed into inches of water). f is a friction factor which is a function of pipe roughness and other parameters.

L is the length of the pipe. V is the velocity of fluid flow. D id the pipe diameter.

Re is the Reynolds number. g is gravitational acceleration.

From the Moody diagram, f will be about -03 for smooth pipes at the flow rates which are most likely. f tends to decrease at large Reynolds numbers for very smooth pipes, but less so for slightly rough pipes. Because the internal roughness of the pipe is not closely controlled, Â is assumed to be -03 for all flow' rates. This will yield a somewhat higher pressure drop at very high flow rates, but will prevent underestimation of the pipe size required. The transition from laminar to turbulent flow in pipes occurs at Re between 2000 and 4000.

where:

v, the kinematic viscosity of air at sea level is 1.6 x 10- ft /sec.

For a 4 inch pipe, this gives a transition velocity of 1 to feet per second. For 6-inch pipe the transition velocity is omewhat lower. In a 4-inch pipe, this corresponds to only 5 to 10 cfm, so most radon control systems will operate in the turbulent regime. In addition, the presence of elbows and other discontinuities will favor turbulent flow. For this reason, turbulent flow equations are used for pressure drop calculations in this paper. This will yield an estimate of the maximum expected pressure drop.

The p r e s s u r e d r o p h i s g i v e n i n f e e t o f a i r i n t h e Darcymay b e c o n v e r t e d t o i n c h e s o f water Weisbach e q u a t i o n ( 1 1 . b y m u l t i p l y i n g b y 0.0147. S o l v i n g f o r head l o s s i n i n c h e s of w a t e r f o r one f o o t o f p i p e y i e l d s t h e f o l l o w i n g : 1 v2 h e a d l o s s ( i n c h e s o f w a t e r ) = -0147 x - 0 3 x 5 x 64 (4

his

-

where:

V is v e l o c i t y i n f e e t p e r second or

where :

V is v e l o c i t y i n f e e t p e r minute.

^

Performing t h e a r i t h m e t i c , t h i s t r a n s l a t e s to

-v-

h e a d loss i n i n c h e s o f water = 1.9 x 1

0 x D

p e r f o o t o f p i p e where D i s i n f e e t . F o r 1.5 i n c h p i p e , t h i s y i e l d s 15.2 x 10" F o r 3 i n c h p i p e , t h i s y i e l d s 7.6 x 10"

x

F o r 4 i n c h p i p e , t h i s y i e l d s 5.7 x 1 0 " x F o r 6 i n c h p i p e , t h i s y i e l d s 3.8 x 1 0 " x

x

v2 v v

v2

per foot.

per foot. per foot.

per f o o t .

For t h e c a l c u l a t i o n of p r e s s u r e drop, t h e e q u i v a l e n t p i p e l e n g t h s h o u l d be u s e d . T h i s means t h a t 1 0 t i m e s t h e d i a m e t e r ( i n f e e t ) o f t h e p i p e s h o u l d be a d d e d f o r e a c h elbow a n d s i z e t r a n s i t i o n t o b e used. Exampl e : A radon s y s t e m must h a v e a f l o w o f 6 5 CFM i n order t o m a i n t a i n a r e q u i r e d vacuum o f .5 i n c h e s o f w a t e r . The p i p e r o u t i n g r e q u i r e s 7 e l b o w s a n d f o r t y f e e t o f s t r a i g h t r u n . What w i l l be the pressure drop i n a four inch pipe? Seven elbows i n f o u r i n c h p i p e a r e e q u i v a l e n t t o 3.3 x 7 o r 2 3 . 1 f e e t of s t r a i g h t p i p e . Adding t h e 40 f e e t o f s t r a i g h t r u n y i e l d s 63.1 f e e t e q u i v a l e n t l e n g t h . The cross s e c t i o n a l area of a 4 i n c h pipe i s . 0 9 sq f t , so t o c a r r y 6 5 cfm w i l l r e q u i r ~ ~v ea l o c i t y o f 7 2 2 f t . p e r m i n u t e . 722 s q u a r e d t i m e s 5.7 x 1 0 = - 0 0 3 i n c h e s o f water column p e r f o o t o r -19 i n c h e s o f w a t e r f o r t h e whole p i p e . The f a n must t h e r e f o r e o p e r a t e a t a b o u t .7 i n c h e s o f water ( - 5 +.19) a t 65 cfm t o make t h e system work. A larger pipe would r e d u c e t h e p r e s s u r e d r o p , w h i l e a smaller p i p e c o u l d p r o d u c e so much r e s t r i c t i o n t h a t t h e s y s t e m m i g h t f a i l t o work.

For example, a 3 inch pipe of the same length and same number of elbows would have an equivalent length of 57.5 f e e t . The velocity would be 1 3 2 4 fpm and the pressure drop would be -76 inches of water. A small fan would not adequately run t h i s sys tem. PRESSURE DROP TABLES

The calculations presented above a r e not d i f f i c u l t i f a s c i e n t i f i c calculator i s available. However, f o r use on the jobsite it i s f a r more convenient t o use a t a b l e look-up. The author has therefore compiled tables f o r use i n c a l c u l a t i n g pressure drop f o r each of the commonly used pipe s i z e s . To use the tables, f i r s t determine the length of s t r a i g h t pipe t o be used. Then add t o t h i s length ten times the diameter of each elbow or transition. Divide the pipe diameter by 1 2 f i r s t so the r e s u l t w i l l be i n f e e t . I f there are reducers, add 1 0 times the diameter ( i n f e e t ) of the downstream pipe from each reducer. Round off the length t o the nearest 1 0 f e e t and use the nearest available flow r a t e from the table. This w i l l not be the exact answer, but w i l l be c l o s e enough t o determine whether the system w i l l have t o be redesigned before i t i s i n s t a l l e d . This i s a l o t e a s i e r and cheaper than i n s t a l l i n g a marginal system and finding out l a t e r t h a t i t does not work. The example above can be worked out using the tables. T h i s exercise i s l e f t t o the reader. I t can be seen t h a t the r e s u l t s from the t a b l e s i s about the same a s given by the calculations. REFERENCES

Handbook of Engineering Fundamentals; Eshbach, 0. W.; & Sons; New York, NY

John Wiley

A p p l i cation of Radon Reduction Methods ; Environmen t a l Protect ion Agency; EPA 625/5-88/024; 1 9 8 8

HEAD LOSS USING FLOW CFM

VELOCITY FEET/MIN

10

1 . 5 INCH P I P E

20

(INCHES OF WATER)

EQUIVALENT P I P E LENGTH I N FEET 30 40 50

HEAD LOSS USING

FLOW CFM

VELOCITY FEET/MIN

70

1.5 INCH PIPE

80

(INCHES

OF

WATER)

EQUIVALENT PIPE LENGTH IN FEET 90 100 110

120

HEAD LOSS USING FLOW CFM

VELOCITY FEET/MIN

2 INCH P I P E

20

(INCHES OF WATER)

EQUIVALENT P I P E LENGTH I N FEET 30 40 50

60

HEAD LOSS USING FLOW CFM

2 INCH PIPE

VELOCITY FEET/MIN

70

80

6.9 8.2 9.5

11

12 14 16

17 19 21 23 26 28 30 33 35

38 41 43 46 49 53 56 59 63 66 70 73 77 81 85 89 93 98 102 107

Ill 11 6 121

(INCHES OF WATER)

EQUIVALENT PIPE LENGTH IN FEET 90 100 110

120

HEAD LOSS USING FLOW CFM

VELOCITY FEET/MIK

10

3 INCH PIPE

20

(INCHES OF WATER)

EQUIVALENT PIPE LENGTH IN FEET 30 40 50

60

HEAD LOSS USING FLOW CFM

VELOCITY FEET/MIN

70

3 INCH P I P E

80

( I N C H E S OF WATER)

EQUIVALENT PIPE LENGTH IN FEET 90 100 110

12 0

HEAD LOSS USING FLOW CFM

VELOCITY FEET/MIN

10

4 INCH PIPE

20

(INCHES

OF WATER)

EQUIVALENT PIPE LENGTH IK FEET 30 40 50

60

HEAD LOSS USING

FLOW CFM

4 INCH PIPE

VELOCITY FEET/MIN

80

(INCHES OF WATER)

EQUIVALENT PIPE LENGTH IN FEET 90 100 110

12 0

HEAD LOSS USING

FLOW CFM

VELOCITY FEET/MIN

10

6 INCH PIPE

20

(INCHES OF WATER)

EQUIVALENT PIPE LENGTH IN FEET 30 40 50

60

HEAD LOSS USING FLOW CFM

VELOCITY FEET/MIN

70

6 INCH PIPE

80

(INCHES OF WATER)

EQUIVALENT PIPE LENGTH IN FEET 90 100 110

120