Purging and Air Removal
Richard A. Beier
Mechanical Engineering Technology Department
Oklahoma State University
Stillwater, OK 74074
rick.beier@okstate.edu
Introduction
Ground source heat pump systems (GSHP) often use closed loops to couple the ground
with the heat pump as illustrated in Figure 1. Plastic pipe may be installed in vertical
boreholes, horizontal trenches, or bodies of water. The chosen configuration for a
particular application depends on the amount of available land area, underground
soil/rock structure and availability of surface water. In these systems water (or
water/antifreeze mixture) circulates through the plastic-pipe loops and exchanges heat
with the refrigerant through a heat exchanger at the heat pump. Then the water flows
through the loops to exchange heat with the ground (or surface water). In this way heat is
transferred between the heat pump and ground (or surface water).
Prior to operation, the closed loops must be filled with water (or water/antifreeze
mixture) and all the air in the loop must be removed. If a significant amount of air
remains in the loops, problems are likely to occur in the subsequent operation of the
GSHP. Air inside the loop may shorten the life of the circulating pump, which is cooled
by the circulating water. Usually multiple ground loops are connected together in a
header or manifold (Figure 2). The header is designed to distribute the water flow evenly
among the loops. Air pockets can lead to uneven water flow among the loops. In fact, an
air pocket can restrict water flow and even block the flow of water to one or more the
loops, causing them to be inactive. Uneven distribution of water flow rates among the
loops reduces the heat transfer capacity of the ground loops. Thus, removing air within a
loop system is very important.
Pressure Loss Created By Air Pockets
In a ground loop air pockets may form at locations of high elevation where bubbles
coalesce. The term air pocket refers to a larger volume of air that does not move with the
liquid flow but remains in one location. The term air bubble refers to a smaller volume of
air that moves with the liquid. Any air pocket creates a relatively large pressure drop
and can alter the liquid flow distribution through a multi-loop system.
Lubbers (2007) measured the additional pressure loss in water flow created due to an air
pocket. He studied the effects of an air pocket in a downward sloping pipe. In the
experiments both water and air are supplied upstream of the air pocket to maintain the
size of the air pocket. Although his geometry is different from a ground loop, the same
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mechanisms are expected to carry over to the ground loop geometry. Figure 3 shows
one flow pattern observed by Lubbers where a stationary air pocket is located in the
sloping pipe. At the lower end of the air pocket a hydraulic jump may exist between the
air pocket and the water-filled pipe. The mixing motion in the hydraulic jump entrains
air and forms smaller bubbles from the air pocket. These small bubbles move
downstream. In the purging process of a ground loop the objective is to eliminate such air
pockets. But in Lubbers’ experiment large pockets of air do not move. Instead, air
moves as smaller bubbles break off from the pocket and the bubbles are swept
downstream. This process of the air pocket shedding bubbles may take a substantial
amount of time to eliminate the air pocket.
Lubbers (2007) measured the additional pressure drop in the liquid flow caused by the
stationary air pocket. Figure 4 illustrates the total pressure drop (vertical axis) across the
downward sloping pipe. For these data the slope of the pipe is 10 degrees from the
horizontal plane. The water flow rate is on the horizontal axis (ft/sec). The pressure drop
for a liquid filled pipe (no air) is shown by the solid curve at the bottom of the graph.
The gray band represents the spread of data over a range of air flow rates. Note the large
increase in the pressure drop when a gas pocket is present, especially at low liquid flow
rates. The air pocket (Figure 3) is present for liquid flow rates between 0.7 and 4.3 ft/sec.
The pressure loss generally decreases as the average water rate increases. Also, the
pressure drop decreases as the air flow rate decreases. These trends correspond to larger
air pockets producing larger pressure drops.
Lubbers also studied the flow patterns to sweep air down a vertical pipe. The pressure
drop curve for a vertical pipe in Figure 5 shows the pressure drop decreases sharply with
increasing liquid velocity. This decrease is more abrupt than the decreases observed in
pipes with slope angles of 5 degrees to 30 degrees from the horizontal plane. Again, the
gray band represents the spread of data over a range of air flow rates. Lubber argues that
bubbles with diameters of 0.1 to 0.2 inches rise at velocity of about 0.7 to 0.8 ft/sec in a
still water column. The results in Figure 5 indicate a downward liquid velocity greater
than 0.8 ft/sec tends to sweep the bubbles downstream in a vertical pipe. Thus, air
bubbles are more readily swept down a vertical pipe than a sloping pipe.
In a pipe where the angle from the horizontal is less than 90 degrees, the bubbles have a
tendency to travel in the upper part of the cross section of the pipe with bubble-free liquid
flow in the bottom region. Then the bubbles are more likely to coalesce and form a
stationary gas pocket. A larger liquid flow rate is needed to carry the air downstream
compared with the case of a vertical pipe.
At moderate liquid flow velocities Lubbers’ experiments demonstrate the transport of air
occurs by sweeping small bubbles downstream. Larger air pockets must be broken down
into smaller bubbles in order to eliminate the gas pockets. The breakdown process takes a
significant amount of time to complete.
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Size of Air Bubbles
The transport of air in water pipelines has been studied in applications of civil
engineering. Escarameia (2005) and Lubbers (2007) have recently reviewed the technical
information on the movement of air in pipes and pipelines. Drag and buoyancy forces are
exerted on an air bubble in flowing water. The drag force is proportional to the projected
cross-sectional area of the bubble. The buoyancy force is proportional to the volume of
the bubble. A convenient ratio is the drag force on a bubble over the buoyancy force on
the bubble. The ratio of forces is related to the bubble diameter by
drag force
d2
∝ 3
buoyancy force d
(1)
As the diameter decreases, the drag force pushing the bubble downstream increases
relative to the buoyancy force. Thus, smaller bubbles are more easily swept downstream.
Researchers (Levich, 1962; Hinze, 1955) have argued that there is a maximum stable
bubble diameter in turbulent liquid flow through a horizontal pipe. In their analysis the
air fraction is small enough so that bubbles do not coalesce. Pressure fluctuations in
turbulent flow tend to change the shape and breakup a bubbles. On the other hand,
surface tension forces tend to resist any deformation of the bubble. Based on these
principles Hesketh et al. (1987) have developed an equation to estimate the largest stable
bubble size for air being carried by in turbulent flow.
Removal Velocity for Air Pocket Movement
Of particular interest is the minimum water velocity to move air through a pipe system.
Researchers have carried out experiments to determine the minimum velocity needed to
move air bubbles downward through a sloping pipe. Usually the experimental results are
written in terms of a Froude number (Ervine, 1998)
Fr =
Uc
(2)
gD
where
Uc = minimum or critical water velocity for air pocket movement
g = gravitational constant
D = pipe diameter
The dimensionless Froude number represents the ratio of inertia forces to gravitational
forces.
Experiments suggest the value of the critical Froude number for air movement depends
on the downward slope of the pipe, which is represented by the angle, θ, with respect to a
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horizontal line. Ervine (1998) reviewed the experimental work by others and plotted the
data on a single graph. A summary of data from Gardenberger (1957), Kent (1952),
Kalinske and Robertson (1943) and Martin (1976) is illustrated in Figure 6. The results
suggest the removal velocity (through the Froude number) increases with increasing pipe
slope until the angle becomes 50 to 60 degrees. The data from Gardenberger (1957)
indicate the required removal velocity decreases at steeper angles.
The graph suggests a Froude number as large as one is needed to move air pockets
through pipes. For a one-inch nominal SDR-11 HDPE pipe with an inside diameter of
1.077 inches the critical velocity is
U c = Fr g D = (1.0)
(32.2ft / sec 2 )(1.077 in )
12 in / ft
(3)
U c = 2 ft / sec
Note the result is written with only one significant digit, because the scatter among
available data sets suggests significant uncertainty. The results for other pipe sizes are
listed in Table 1 for pipes with slopes of 50 degrees and 0 degrees (horizontal pipes).
The removal velocity increases to 3 ft/sec for a 4-inch pipe with a 50-degree slope.
These removal velocities correspond to the peak Froude number in Figure 6. If the pipe
is nearly horizontal, the removal velocity according to Figure 6 corresponds to a Froude
number of about 0.6.
The dependence of the removal velocity on the slope of the pipe, θ, has been studied by
several researchers. Escarameia (2005) recommends the equation
Uc
= b + 0.56 (sin θ) 0.5
gD
(4)
The equation may be valid for angles up to 40 degrees. The value of b ranges from 0.45
to 0.61 based on the volume of the air pocket relative to the diameter of the pipe. In
practice one does not know the size of the air pockets to be removed. Thus, a value of
0.6 for a can be used.
Wisner et al. (1975) write their expression for the critical velocity as
Uc
= 0.825 + 0.25 (sin θ) 0.5
gD
(5)
The relation between removal velocity and slope are the same in Equations 4 and 5. Both
expressions suggest a Froude number of about 1 is needed to move air through a pipe
with a slope of 40 degrees. More information about removal velocity is given in reviews
by Lubbers (2007) and Escarameia (2007).
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Velocity in Ground-Loop Systems
The design of the piping layout for a set of boreholes should take into account the need
for purging the system of air. Shut-off valves and purging connections need to be located
to allow one to purge individual sections of the system. From the discussion above,
sufficient water velocity is needed in the purging process to remove air from the system.
For discussion purposes water flow through a header is analyzed here.
A header (or manifold) is used to supply water to multiple ground loops in a parallel
arrangement. Figure 7 illustrates the design for a 10-ton header, which supplies water to
10 wells, each with a depth of 200 feet. Bose (1988) discusses this header design and
others. Supply water enters from the left side and enters the branch line to each borehole
numbered 1 through 10. The supply pipe diameter is reduced as the water flow rate
decreases in the supply header from left to right.
The flow rates and pressure drops through the header during the purging process can be
analyzed by using one of the many commercial software packages for pipe networks. For
illustrative purposes the software program AFT Fathom® from Applied Flow
Technology is used here, but other software programs can be used. The pipe network in
Figure 8 is illustrated with the graphical interface inside of AFT Fathom®. The network
represents the supply header (Figure 7), ground loops and return header. The water
supply (40 gal/min) in the upper left-hand corner flows through the supply header across
the top. The ground loops are in the middle of the diagram, and the return header is at the
bottom. In the return header, 3/4 - inch pipe is on the left side and 2-inch pipe is on the
right side. The U-bend at the bottom of each ground loop is represented by two 90degree elbows.
As shown in Figure 9, the distribution of water velocity among the ground loops falls
between 2.0 and 2.4 ft/sec. Thus, a reasonably uniform flow distribution is achieved
among the loops.
The velocity along the length of the header is shown in Figure 10. The downward steps
correspond to take-off pipes to ground loops. The upward steps are associated with a
reducer in the header. Note the lowest velocity of 2 ft/sec occurs in the 1-1/2 inch and
3/4 inch pipes, which should be sufficient to purge air from the system. Figure 11 is a
similar graph for the velocity in the return header.
Static pressures along the supply and return headers are shown in Figure 12. The pressure
drop across the entire system (9 psi) is the difference between the pressure at the entrance
of the supply header (34 psig) and the pressure at the exit of the return header (25 psig).
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Dissolved Air in Water
Even if air bubbles are not present in the ground loops, some air is present in the loop as
dissolved air in water (or anti-freeze solution).
Consider an uncovered glass of water in a room. The interface between the liquid water
and the air in the room is obvious, but the interface is not an impermeable wall separating
all water molecules from air molecules. From our experience we know some liquid water
may evaporate and appear as water vapor in the air. Terms such as relative humidity are
used to quantify the amount of water vapor in air.
On the other hand, some air is also dissolved in the water. The volumetric concentration
(Lu and Likos, 2004) is one way of quantifying the amount of air in solution with water.
The volumetric concentration is expressed as
Va
= ha
Vl
(6)
where
Va = volume of dissolved air
Vl = volume of liquid
ha = volumetric coefficient of solubility (volume/volume)
At 68 ºF and one atmosphere of pressure ha is approximately 0.0187. That is, if the air is
taken out of solution, the volume of air originally dissolved in water can be as large as
1.87 percent of the volume of water. Consider a water-filled U-tube inside vertical
borehole with a depth of 300 feet. If all the air is taken out of solution from the water the
air could occupy as much as 1.87 percent of the U-tube length or 5.7 feet. Thus the
amount of dissolved air can be significant. Keep in mind the numbers written here are
for the maximum air that can be dissolved in water. Some of this air will nearly always
remain in solution.
The volumetric coefficient of solubility depends on temperature and pressure as shown in
Figure 13. The effects due to temperature changes are of particular interest in ground
loops. If the water temperature increases from 68 ºF to 104 ºF the coefficient of
solubility decreases from 0.0187 to 0.0141. Thus if the U-tube (depth of 300 ft) is filled
with water at 68 ºF and experiences this temperature increase, the air coming out of
solution could occupy as much as 0.46% percent of the length or 1.4 ft.
Of course, if air comes out of solution, the air will form in small bubbles much like
carbon dioxide gas appearing after a bottle of soda is opened. The carbon dioxide comes
out of the soda as the pressure is decreased on opening the bottle. This demonstrates that
the volumetric coefficient of solubility decreases with decreasing pressure. The same is
true for air as seen in Figure 13.
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Addition of Antifreeze
Once the air is purged from the system, the purging equipment is used to add antifreeze.
When adding antifreeze to the purging tank, the antifreeze should enter the tank below
the water line. In this way less air is entrained with the added antifreeze.
Another issue is foaming when methanol is used as the antifreeze. Chen et al. (2007)
followed an ASTM test standard (D3601-88) to assess the foaming tendency of methanol
/ water mixtures. In this test procedure a mixture is placed in a bottle and shaken. Foam
heights are then measured. For a methanol concentration of 30% by weight, Chen et al.
observed foams that lasted for 20 to 25 seconds. For methanol concentrations of 50% or
greater by weight, they observed no foaming.
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References
ASTM Test Standard, D3601-88 (re-approved 1997).
Bose, J. E. 1988. Closed-Loop/Ground-Source Heat pump Systems: Installation Guide,
International Ground Source Heat Pump Association, Stillwater, OK.
Chen, G. X., Cai, T. J., Chuang, K. t., and Afacan, A., 2007, “Foaming Effect on Random
Packing Performance,” Chemical Engineering Research and Design, Transactions
IChemE, Vol. 85 (A2), pp. 278-282.
Ervine, D. A., 1998. “Air Entrainment in hydraulic Structures: A Review,” Proceedings
of the Institution of Civil Engineers. Water, Maritime and Energy, Vol. 130, pp. 142-153.
Escarameia, M. (Editor), 2005. Air Problems in Pipelines: A Design Manual, HR
Wallingford, Ltd., Howbery Park, Wallingford, Oxfordshire, UK.
Escarameia, M., 2007. “Investigating Hydraulic Removal of Air From Water Pipelines,”
Proceedings of the Institution of Civil Engineers. Water Management, Vol. 160, No. 1,
pp. 25-34.
Gardenberger, W., 1957. Uber die wirtschaftliche und Betriebssichere gestaltung von
fernwasserleitungen. R. Oldenburg Verlag, Munich.
Hesketh, R. P., Russell, T. W. F., Etchells, A. W., 1987. “Bubble Size in Horizontal
Pipelines,” AIChE J, Vol. 33, pp. 663-667.
Hinze, J. O., 1955. “Fundamentals of the Hydrodynamic Mechanism of Splitting in
Dispersion Processes,” AIChE Journal, Vol. 1, p. 289.
Kalinske, A. A. and Robertson, J. M., 1943, “Closed Conduit Flow,” Transactions of the
American Society of Civil Engineers, Vol. 108, pp. 1435-1516.
Kent, J. C., 1952. The Entrainment of Air in Water Flowing Through Circular Conducts
with Downgrade Slopes, PhD Thesis, University of California, Berkeley, 1952.
Levich, V. G., 1962. Physiochemical Hydrodynamics, Prentice Hall, Englewood Cliffs,
NJ.
Lu, L. and Likos, W. J., 2004. Unsaturated Soil Mechanics, John Wiley & Sons,
Hoboken, NJ.
Lubbers, C. L., 2007. On Gas Pockets in Wastewater Pressure Mains and Their Effect
on Hydraulic Performance, IOS Press, Deft University Press, The Netherlands.
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Martin, C. S., 1976. “Vertically Downward Two-Phase Slug flow,” Transactions of the
ASME Journal of Fluids Engineering, Vol. 98, No. 4, pp. 715-721.
Razzaque, M. M., Afacan, A., Liu, S., Nandakumar, K., Masliyah, J. H., and Sanders, R.
S., “Bubble Size in Coalescence Dominant Regime of Turbulent Air-Flow Through
Horizontal Pipes,” International Journal of Multiphase Flow, Vol. 29, pp. 1451-1471.
Wisner, P. E., Mohsen, F. N. and Kouwen, N., 1975. “Removal of Air From Water Lines
by Hydraulic Means,” Proceedings of the American Society of Civil Engineers, Journal
of the Hydraulics Division, Vol. 101, No. HY2, pp. 243-257.
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Table 1. Critical water velocity to remove air from HDPE (SDR-11) pipe of various
slopes and diameters.
Slope From
Horizontal
(degrees)
Nominal
Size
(in)
Inside
Diameter
(in)
Water
Velocity
(ft/sec)
Volume
Flow Rate
(gpm)
0
0
0
0
0
0
0
50
50
50
50
50
50
50
3/4
1
1-1/4
1-1/2
2
4
6
3/4
1
1-1/4
1-1/2
2
4
6
0.860
1.077
1.358
1.554
1.943
3.682
5.421
0.860
1.077
1.358
1.554
1.943
3.682
5.421
0.9
1.0
1.1
1.2
1.4
1.9
2.3
1.5
1.7
1.9
2.0
2.3
3.1
3.8
1.7
2.9
5.2
7.2
13
60
160
2.8
4.8
8.6
12
21
100
270
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CL/GS System Configuration
(Typical Residential)
Pumps and Service valves
located externally
Pump Module may be located inside
heat pump cabinet
Figure 1. Closed loop configuration (Bose, 1988).
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Figure 2. Header configurations (Bose, 1988).
Figure 3. Flow pattern in sloping pipe (Lubbers, 2007).
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Pressure Loss (ft of water)
3.5
3
2.5
2
Increasing
Fraction of Air
1.5
1
0.5
No Air
0
0
1
2
3
4
5
6
Velocity of Water (ft/sec)
Figure 4. Pressure (head) loss in 10-degree sloping pipe over ranges
of water and air flow rates (Lubbers, 2007). Diameter of pipe is 8.7 inches.
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Pressure Loss (ft of water)
8
6
4
Increasing
Fraction of Air
2
0
0
1
2
3
4
5
6
Velocity of Water (ft/sec)
Figure 5. Pressure (head) loss in vertical pipe over ranges of water
and air flow rates (Lubbers, 2007). Diameter of pipe is 8.7 inches.
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1.2
1
Froude Number,
0.8
Garenberger (1957)
0.6
Kent (1952)
0.4
Kalinske & Robertson (1943)
Martin (1976)
0.2
Kalinske & Martin
0
0
20
40
60
80
100
Angle From Horizontal (degrees)
Figure 6. Velocity, Uc, required to move air down sloping pipe (Ervine, 1998).
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Figure 7. Supply / return header for a 10-ton system.
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J1 v
P1
P20
P15
P65
J57
P22
P66
J46
P21
P27
J58
P67
P33
J47
P5
J59
J48
P68
P69
P6
P38
J50
P37
J34
P36
J33
P35
J61
P7
P70
J17
P34
J7
Return Header
J60
P32
J49
P31
J32
P30
J31
P29
J16
P28
J6
Supply Header
Ground
Loops
P26
J30
P25
P4
J5
P8
J62
J63
P48
J52
J38
P46
P72
J64
P73
P11
J65
P74
P54
J53
J20
P49
J11
P53
J40
P52
J39
J22
P50
P10
J37
J10
P51
J51
P43
P9
P45
P47
P71
J19
P44
J9
P42
J36
P41
J35
P40
J18
P39
J8
P12
J66
P75
P59
J54
P58
J42
P57
J41
P56
J21
P55
J12
Figure 8. Pipe network for modeling supply header, ground loops
and return header.
J56
P17
J45
P16
J28
J27
J25
J26
P24
P19
P14
J29
J15
J14
P23
J13
P3
J4
P18
P2
J3
P13
J2
J67
P76
P64
J55
P63
J44
P62
J43
P61
J24
P60
J23
J68
Velocity (ft/sec)
2.4
2.2
2
1.8
10
9
8
7
6
5
4
3
2
1
Ground Loop Number
Figure 9. Distribution of water velocity through ground loops.
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Velocity (ft/sec)
8
6
4
2
0
0
50
100
150
200
Flow Length (ft)
Figure 10. Water velocity along the length of the supply header.
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Velocity (ft/sec)
8
6
4
2
0
0
50
100
150
200
Flow Length (ft)
Figure 11. Water velocity along the length of the return header.
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Static Pressure (psig)
40
35
Supply Header
30
Flow Direction
Return Header
25
20
0
50
100
150
200
Flow Length (ft)
Figure 12. Static pressure along the length of the supply and return headers.
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Volumetric Coefficient of Solubility
0.14
0.12
0.1
60 psig
0.08
30 psig
0.06
0.04
0 psig
0.02
0
0
50
100
150
200
250
Temperature (ºF)
Figure 13. Volumetric coefficient of solubility of air in water.
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