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Refrigeration Piping

Refrigeration Piping

An often overlooked, but very important part of any refrigeration system, is the piping to connect all the components of the system. Improper piping can be the cause of overall poor performance and compressor failure.

The most critical line is the suction line. It carries refrigerant vapor from the evaporator back to the compressor. It must also have high enough velocity to carry oil back, yet have a minimum pressure drop.

Note: The usual total pressure drop used to size suction lines is a DP that would impose no more than a 2°F temperature penalty.

Gas velocity in horizontal lines should be a minimum of 500 feet per minute to move oil. In vertical up flow suction lines, the velocity should be at least 1500 ft./min. Vertical risers should be checked for part load minimum tonnages and the velocity determined at this condition. If the minimum load results in velocities below 1500 ft./min., a double riser should be used. This will be described later. The maximum velocity in a suction line should be held below 3000 ft./min. to prevent excessive noise.

Liquid line sizing is less critical than sizing other lines in a system. Since it carries liquid refrigerant, oil is carried along without any problems. Pressure drop has little effect on the overall system performance, but high DP will reduce the pressure available at the inlet to the TXV, and consequently, may affect its capacity. Velocity in liquid lines should be below 300 ft./min. to prevent liquid hammer due to operation of solenoid valves, noise, and vibration. It is generally considered good practice to limit pressure drops in liquid lines to 2 to 3 lbs.

Cost must be taken into consideration when sizing lines. Proper line sizing not only will result in good system performance, but also will be cost effective.Many companies such as DuPont, Tecumseh, and Copeland, print charts to assist in choosing line sizes. The attached set of pressure drop charts are produced by DuPont that show pressure drops for 100 ft. equivalent line lengths and velocities in lines.

Remember that line sizing is a bit of an art. For a particular job, one picks line sizes that look like the sizes that will work, and then checks those sizes. An example is shown on the chart for R-12 for a 5.5-ton capacity evaporator at -40°F with an 85°F condensing temperature and a 100-foot equivalent line length.

To convert pressure drop to temperature penalty, use a Temperature Pressure Chart. To size the suction line, enter the chart at the top at 5.5 tons. Draw a line vertically down to intersect the -40°F evaporator temperature line. From that intersection, draw a horizontal line to the left to intersect various line sizes. From the intersection of a line size selected, go vertically down the chart to find the pressure drop at 85°F condensing temperature. Let’s try 2-1/8 O.D. This results in about a 1.4 to 1.5 lb. pressure drop, a 3°F temperature penalty. The temperature penalty is too high.

Note: A one-pound change in pressure for R-12 is equal to about 2°F. A two-pound change in pressure is equal to 2°F for R-22 and R-502.

Trying 2-5/8 O.D. results in a pressure drop of .49 lbs., about a 1°F temperature penalty. This is acceptable. When checking a 2-5/8 O.D. pipe on the velocity chart for R12, we’ll find just about 3000 ft./min. This is not acceptable since it would cause excessive noise. Trying a 3-1/8 O.D. pipe results in .19 lbs. pressure drop and 2000 ft./min. velocity, both acceptable figures. Our choice for the suction line size should be 3-1/8 O.D. Sizing the liquid line follows the same procedure. Don’t forget you are using equivalent line lengths. See the chart in Info-Tec 10 for equivalent feet figures for fittings, valves, and other components.

Proper piping practices are very important when running refrigeration lines. How the lines are run from the condensing unit to the evaporator is as important as what sizes they are. The spacing of pipe supports, location of equipment, valves, and other components should be laid out, even if only mentally for small systems, on paper for larger systems.

Since the design and selection of liquid lines are not concerned with oil circulation problems, we need only be concerned with maintaining a solid column of liquid at reasonable velocity and working temperature to the TXV.

If the evaporator is located above the receiver, precautions should be taken to make sure the liquid line would not exceed a 15-foot vertical rise. If the total vertical rise of the liquid line exceeds 15 feet, re-calculate the line size to reduce pressure drop due to friction to overcome the effect of the static head added. The static head will induce flash gas in vertical risers over 15 feet. If this is not taken into consideration when originally line sizing, line sizing will have to be re-done, or devices used to sub-cool the liquid to overcome the static head. Any liquid line that runs through a space that can get hotter than the design temperature of that liquid line should be insulated. If soft copper tube is used for a liquid line, make sure it is not kinked or flattened. 90° bends should have radii equal to or greater than long radius elbows of the same O.D. If using hard drawn copper tubing, use only long radius ells.

How suction lines are run is much more important than how liquid lines are run. Let’s first dispose of the question of trapping suction lines. There are only three proper applications in a single evaporator system where a suction line should be trapped. They are:

1. After the TXV bulb on a horizontal suction line. See Figure 1.

Figure 1. Proper trap after TXV bulb on horizontal line

2. At the evaporator to prevent draining to the compressor on the off-cycle. See Figure 2.

Figure 2. Proper piping of evaporator to prevent liquid draining into compressor during the off-cycle

3. On a double-suction riser. See Figure 3.

Figure 3.

Figure 4 illustrates a non-recommended use of multiple trapping of suction risers. It is not an effective method.

Figure 4.

In properly sized suction lines, neither oil nor refrigerant in liquid state will be found sitting on the bottom of a line when the compressor is on. Velocity will be great enough to prevent this. The basic problem of poor line sizing cannot be corrected by adding traps to a vertical suction riser. Adding traps only compounds the problems of poor suction line design. Traps add resistance to a system. Overcoming the added resistance requires increasing the suction line size, adding cost, and worsening the problem. The theory is that added resistance creates a higher velocity before a riser, which aids oil return. If the trapping requires the next larger suction line size to compensate for the added losses, velocity goes down again. Oil will not return, even if the riser is trapped, if the velocity is not high enough. If the velocity is high enough for good oil return, riser traps are not needed. Oil return is not a question of traps; it is a function of proper line sizing!

To provide for oil return during reduced capacity, consequently reducing velocity on a tall suction riser, a double riser system should be employed, as shown in Figure 3. The two pipes should be sized with a combined capacity for full load to maintain 1500 ft./min. velocity. The smaller pipe is selected to maintain capacity and velocity at minimum load.

The double riser works because on low load, as the suction volume is reduced, so is the velocity in a certain size suction pipe. Oil in the suction gas, with the reduced velocity, will separate out in the trap and seal it, causing the refrigerant vapors to pass up the small pipe. As the load increases, the suction pressure builds up, creating sufficient pressure difference on each side of the trap, the trap seal will be broken and the vapors passing over the oil in the trap will gradually pick up all the oil in the trap and return it to the compressor.

To size the lines of a double-suction riser, you need to know full load line size and the low load line size. Let’s assume we need to construct a suction riser that requires a line size of 1-5/8 O.D. to handle the capacity of the system and has the velocity at full load to get good oil return, but at minimum load the velocity in the 1-5/8 O.D. line is too low for oil return. At the low load, a 7/8 O.D. line will have the needed velocity for oil return and will pass the capacity of minimum load. The double riser needs to be constructed of two lines, which together will have a capacity rating of the single 1-5/8 O.D. line. To do this, we need to know the cross sectional area of the lines.

1-5/8 O.D. is 1-1/2 nominal. The formula for calculating the area of a circle is pr2. The inside diameter of 1-1/2 nominal type L copper tube is actually 1.505 inches. The radius is half the diameter, or .753 inches. We will give p a value of 3.1416. The radius squared is .753 x .753, or .567. Solving pr2: 3.1416 x .567 = 1.781 square inches. Refer to Figure 5.

Figure 5.

The minimum load line size of 7/8 O.D. has an area of .484 square inches. Since we need to have the two lines total area equal to approximately the 1-5/8 O.D. line area, we subtract the 7/8 O.D. line area from the 1-5/8 O.D. line area and pick a line size that gets closest in area to the answer. 1.78 - .484 = 1.296. 1-3/8 O.D. has an area of 1.26 square inches. Therefore, our double suction riser will consist of a 7/8 O.D. line and a 1-3/8 O.D. line. For systems with controlled suction capacity and a short suction line to just above the evaporator, minimum velocity for proper oil return can be maintained by reducing the riser to a size equal to the size pipe required for minimum velocity at the lowest capacity of the compressor. In making the turn at the top of this riser, use an elbow the same size as the riser; then increase the piping to the full suction line size.

On systems with multiple evaporators using a common suction line, there are some piping practices that require special care. Suction lines for stacked coils are frequently piped wrong. See Figure 6.

Figure 6.

Often the TXV feeding the lower coil is condemned as bad in this situation. Here’s why (see Figure 7).

Figure 7.

The shaded area represents small quantities of semi-vapor and liquid refrigerant. As this mixture flows from the top coil toward the lower coil, it tends to flow into the tee branch, following the wall of the tubing and spreading out as shown in cross section A-A, thus causing an effective temperature change of the T.E.V. bulb, and in turn affecting the proper functioning of the valve itself.

Multiple adjoining coils are often piped wrong. See Figure 8.

Figure 8.

If the suction piping were installed in the manner shown, it would interfere with the proper operation of the TXV’s.

The solution to this problem is shown in Figure 9.

Figure 9.

The branch suction lines from each blower coil have been connected into the top of the main suction line, thus preventing the draining of refrigerant from one coil into another.

We have not discussed the vagaries of discharge gas lines from compressors to remote condensers and lines from condensers to receivers, as we deal mostly with condensing units where these lines are already there. For those of you interested, Witt’s catalog 105-2 “Air-Cooled Condensers” is an excellent source on these subjects. (A. H. Witt Inc., 435 Washington Street, Collierville, TN 38017, telephone 901/853-2770)

Chart 1. Pressure Drop for “Freon” 12

Chart 2. Pressure Drop for “Freon” 22

Chart 3. Pressure Drop for “Freon” 502

Chart 4. Line Velocity for “Freon” 12

Chart 5. Line Velocity for “Freon” 22

Chart 6. Line Velocity for “Freon” 502

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