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 + 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.
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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 +
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;
L v2 For fully turbulent flow hf = f 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 is g i v e n i n f e e t o f a i r i n t h e Darcy- Weisbach e q u a t i o n ( 1 1 . his may b e c o n v e r t e d to i n c h e s o f water 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 head loss ( i n c h e s of w a t e r ) = -0147 x -03 x 5 x - 64 (4
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 t o v- head loss i n i n c h e s of water = 1.9 x 1 0 x - D
p e r f o o t of p i p e where D is 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" x v2 p e r f o o t .
For 3 i n c h p i p e , t h i s y i e l d s 7.6 x 10" x v2 p e r f o o t .
For 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 v p e r f o o t .
For 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 v per f o o t .
For t h e c a l c u l a t i o n o f p r e s s u r e d r o p , t h e e q u i v a l e n t p i p e l e n g t h shou ld be used . T h i s means t h a t 10 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 added f o r e a c h elbow and s i z e t r a n s i t i o n to b e used.
Exampl e : A radon sys tem must have a f low o f 6 5 CFM i n order to
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 of 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 elbows and f o r t y f e e t o f s t r a i g h t run . What w i l l b e t h e p r e s s u r e d r o p i n a f o u r i n c h p i p e ?
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 23.1 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 is . 0 9 sq f t , so t o c a r r y 65 cfm w i l l r e q u i r ~ ~ a v e l o c i t y of 722 f t . p e r minu te . 722 squared t i m e s 5.7 x 1 0 = -003 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 fan 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 l a r g e r p i p e
would reduce 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 produce so much r e s t r i c t i o n t h a t t h e sys tem might 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 fee t . The velocity would be 1324 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 are not d i f f i c u l t i f a sc ient i f ic calculator i s available. However, for use on the jobsite it i s f a r more convenient t o use a table look-up. The author has therefore compiled tables for use in calculating pressure drop for each of the commonly used pipe sizes. To use the tables, f irst determine the length of s t ra igh t pipe to be used. Then add to t h i s length ten times the diameter of each elbow or transition. Divide the pipe diameter by 1 2 first so the resul t w i l l be in feet . I f there are reducers, add 1 0 times the diameter ( i n f ee t ) of the downstream pipe from each reducer.
Round off the length t o the nearest 1 0 f ee t and use the nearest available flow ra te from the table. This w i l l not be the exact answer, but w i l l be close enough to determine whether the system w i l l have to be redesigned before i t is instal led. This i s a l o t easier and cheaper than ins ta l l ing a marginal system and finding out l a t e r that it does not work. The example above can be worked out using the tables. T h i s exercise is l e f t to the reader. I t can be seen tha t the resul ts from the tables i s about the same a s given by the calculations.
REFERENCES
Handbook of Engineering Fundamentals; Eshbach, 0. W.; John Wiley & Sons; New York, NY
A p p l i cation of Radon Reduction Methods ; Environmen t a l Protect ion Agency; EPA 625/5-88/024; 1988
HEAD LOSS USING 1 . 5 INCH PIPE (INCHES OF WATER)
FLOW VELOCITY CFM FEET/MIN 10
EQUIVALENT P I P E LENGTH I N FEET 20 30 4 0 50
HEAD LOSS USING
FLOW VELOCITY CFM FEET/MIN 70
1.5 INCH PIPE (INCHES OF WATER)
EQUIVALENT PIPE LENGTH IN FEET 80 90 100 110 12 0
HEAD LOSS USING 2 INCH P I P E (INCHES OF WATER)
FLOW VELOCITY CFM FEET/MIN
EQUIVALENT P I P E LENGTH I N FEET 2 0 3 0 4 0 5 0 60
HEAD LOSS USING 2 INCH PIPE (INCHES OF WATER)
FLOW VELOCITY EQUIVALENT PIPE LENGTH IN FEET CFM FEET/MIN 70 80 90 100 110 120
6.9 8.2 9 . 5 11 12 14 16 17 19 21 23 26 28 30 3 3 3 5 38 41 43 46 49 5 3 5 6 5 9 6 3 66 70 73 77 8 1 85 8 9 9 3 98
102 107 Ill 11 6 12 1
HEAD LOSS USING 3 INCH PIPE (INCHES OF WATER)
FLOW VELOCITY CFM FEET/MIK 10
EQUIVALENT PIPE LENGTH IN FEET 20 30 40 5 0 60
HEAD LOSS USING 3 INCH PIPE ( I N C H E S OF WATER)
FLOW VELOCITY CFM FEET/MIN 70
EQUIVALENT PIPE LENGTH IN FEET 80 90 100 110 12 0
HEAD LOSS USING 4 INCH PIPE (INCHES OF WATER)
FLOW VELOCITY CFM FEET/MIN 10
EQUIVALENT PIPE LENGTH IK FEET 20 30 40 5 0 60
HEAD LOSS USING 4 INCH PIPE (INCHES OF WATER)
FLOW CFM
VELOCITY FEET/MIN
EQUIVALENT PIPE LENGTH IN FEET 80 90 100 110 12 0
HEAD LOSS USING
FLOW VELOCITY CFM FEET/MIN 10
6 INCH PIPE (INCHES OF WATER)
EQUIVALENT PIPE LENGTH IN FEET 20 30 4 0 5 0 6 0
HEAD LOSS USING
FLOW VELOCITY CFM FEET/MIN 70
6 INCH PIPE (INCHES OF WATER)
EQUIVALENT PIPE LENGTH IN FEET 80 90 100 110 120
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