07 - Refrigerant Piping - A Trane Air Conditioning Clinic.pdf.pdf

Page 1 of 89

Air Conditioning

Clinic

Refrigerant Piping

One of the Fundamental Series

TRG-TRC006-EN

Page 2 of 89

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Refrigerant Piping

Page 4 of 89

Refrigerant Piping

One of the Fundamental Series

A publication of Trane, a business

of American Standard Companies

Page 5 of 89

Preface

ii TRG-TRC006-EN

Trane believes that it is incumbent on manufacturers to serve the industry by

regularly disseminating information gathered through laboratory research,

testing programs, and field experience.

The Trane Air Conditioning Clinic series is one means of knowledge sharing. It

is intended to acquaint a technical audience with various fundamental aspects

of heating, ventilating, and air conditioning (HVAC). We have taken special care

to make the clinic as uncommercial and straightforward as possible.

Illustrations of Trane products only appear in cases where they help convey the

message contained in the accompanying text.

This particular clinic introduces the reader to refrigerant piping.

Refrigerant Piping

A Trane Air Conditioning Clinic

Figure 1

Page 7 of 89

iv TRG-TRC006-EN

Page 8 of 89

TRG-TRC006-EN 1

notes

period one

Refrigerant Piping Requirements

The focus of this clinic is on the design and installation of the interconnecting

piping for vapor-compression refrigeration systems. Reviewing the physical

changes that the refrigerant undergoes within the refrigeration cycle will help

demonstrate certain demands that the piping design must meet.

This clinic focuses on systems that use Refrigerant-22 (R-22). While the general

requirements are the same for systems that use other refrigerants, velocities

and pressure drops will differ.

Figure 3 illustrates a basic vapor-compression refrigeration cycle. Refrigerant

enters the evaporator in the form of a cool, low-pressure mixture of liquid and

vapor (A). Heat is transferred to the refrigerant from the relatively warm air that

is being cooled, causing the liquid refrigerant to boil. The resulting refrigerant

vapor (B) is then pumped from the evaporator by the compressor, which

increases the pressure and temperature of the vapor.

period one

Refrigerant Piping Requirements

Refrigerant Piping

Figure 2

Vapor-Compression Refrigeration

expansion expansion

device

condenser condenser

compressor compressor

evaporator evaporator

A B

D C

Figure 3

Page 10 of 89

TRG-TRC006-EN 3

period one

Refrigerant Piping Requirements

notes

When a refrigeration system includes field-assembled refrigerant piping to

connect two or more of the components, the primary design goals are generally

to maximize system reliability and minimize installed cost. To accomplish these

two goals, the design of the interconnecting refrigerant piping must meet the

following requirements:

Return oil to the compressor at the proper rate, at all operating conditions

Ensure that only liquid refrigerant (no vapor) enters the expansion device

Minimize system capacity loss that is caused by pressure drop through the

piping and accessories

Minimize the total refrigerant charge in the system to improve reliability and

minimize installed cost

The first requirement is to ensure that oil is returned to the compressor at all

Refrigerant Piping Requirements

▲ Return oil to compressor

▲ Ensure that only liquid refrigerant enters

the expansion device

▲ Minimize system capacity loss

▲ Minimize refrigerant charge

Figure 5

Scroll Compressor

stationary stationary

scroll

driven

scroll

intake

discharge discharge

intake

discharge discharge

port

motor

shaft

seal

Figure 6

Page 14 of 89

TRG-TRC006-EN 7

period one

Refrigerant Piping Requirements

notes pressure drop in the suction line from 3 psi (20.7 kPa) to 6 psi (41.4 kPa)

decreases system capacity by about 2.5 percent and decreases system

efficiency by about 2 percent.

This reveals a compromise that the system designer must deal with. The

diameter of the suction line must be small enough that the resulting refrigerant

velocity is sufficiently high to carry oil droplets through the pipe. However, the

pipe diameter must not be so small that it creates an excessive pressure drop,

reducing system capacity too much.

The first three requirements have remained unchanged for many years.

However, years of observation and troubleshooting has revealed that the lower

the system refrigerant charge, the more reliably the system performs.

Therefore, a fourth requirement has been added for the design of refrigerant

piping: minimize the total amount of refrigerant in the system. To begin with,

this involves laying out the shortest, simplest, and most-direct pipe routing. It

also involves using the smallest pipe diameter possible, particularly for the

liquid line because, of the three lines, it impacts refrigerant charge the most.

The chart in Figure 11 shows that the liquid line is second only to the condenser

in the amount of refrigerant it contains.

This reveals another compromise for the system designer. The diameter of the

liquid line must be as small as possible to minimize the total refrigerant charge.

However, the pipe diameter cannot be small enough to create an excessive

pressure drop that results in flashing before the liquid refrigerant reaches the

expansion device.

Minimize Refrigerant Charge

liquid line liquid line suction line suction line

condenser condenser

evaporator evaporator

filter

drier

compressor compressor

discharge discharge

line

Figure 11

Page 17 of 89

10 TRG-TRC006-EN

notes

period two

Suction Line

The first line to be considered is the suction line. Again, this pipe conducts low- pressure refrigerant vapor from the evaporator to the compressor.

Requirements for Sizing and Routing

The diameter of the suction line must be small enough that the resulting

refrigerant velocity is sufficiently high to carry oil droplets, at all steps of

compressor unloading. If the velocity in the pipe is too high, however,

objectionable noise may result. Also, the pipe diameter should be as large as

possible to minimize pressure drop and thereby maximize system capacity and

efficiency.

period two

Suction Line

Refrigerant Piping

Figure 14

suction line

Requirements for Sizing and Routing

▲ Ensure adequate velocity to return oil to

compressor at all steps of unloading

▲ Avoid excessive noise

▲ Minimize system capacity and efficiency loss

Figure 15

Page 21 of 89

14 TRG-TRC006-EN

notes

period two

Suction Line

The refrigerant velocity inside a pipe depends on the mass flow rate and

density of the refrigerant, and on the inside diameter of the pipe. The chart in

Figure 19 shows the velocity of R-22 inside pipes of various diameters at one

particular operating condition—40°F (4.4°C) saturated suction temperature,

125°F (51.7°C) saturated condensing temperature, 12°F (6.7°C) of superheat,

15°F (8.3°C) of subcooling, and 70°F (38.9°C) of compressor superheat. For an

example system with an evaporator capacity of 20 tons (70.3 kW), the

refrigerant velocity inside a 2 1/8 in. (54 mm)-diameter pipe at this condition is

about 1,850 fpm (9.4 m/s).

The easiest and most accurate method for determining refrigerant velocity is to

use a computer program that can calculate the velocity for various pipe sizes

based on actual conditions. However, if you do not have access to such a

program, a chart like this may be useful.

Assume that this example 20-ton (70.3-kW) system contains one refrigeration velocity, fpm (m/s)

evaporator capacity, tons (kW) evaporator capacity, tons (kW)

(3.5)

(17.6)

(35)

(176)

100

(352)

200

(703)

1,000

(5.1)

500

(2.5)

2,000

(10.2)

5,000

(25.4)

(70)

(7.0)

1/2 (12) 1/2 (12)

5/8 (15) 5/8 (15)

7/8 (22) 7/8 (22)

1 1/8 (28) 1 1/8 (28)

1 3/8 (35)

1 5/8 (42) 1 5/8 (42)

2 1/8 (5 2 1/8 (54)

2 5/8 (67)

3 1/8 (79) 3 1/8 (79)

3 5/8 (105) 3 5/8 (105)

R-22

suction line

Determine Refrigerant Velocity

pipe diameter, in. (mm)

3/8 (10) 3/8 (10)

4 1/8 (130) 4 1/8

3/4 (18)

(130)

Figure 19

suction line

Determine Refrigerant Velocity

velocity, fpm (m/s) velocity, fpm (m/s) pipe

diameter, diameter,

in. (mm) in. (mm)

20 tons 20 tons

(70.3 kW) (70.3 kW)

10 tons 10 tons

(35.2 kW) (35.2 kW)

1 1/8 (28) 1 1/8 (28) 7,000 (35.6) 7,000 (35.6) 3,500 (17.8) 3,500 (17.8)

1 3/8 (35) 1 3/8 (35) 4,600 (23.4) 4,600 (23.4) 2,300 (11.7) 2,300 (11.7)

1 5/8 (42) 1 5/8 (42) 3,250 (16.5) 3,250 (16.5) 1,625 (8.3) 1,625 (8.3)

2 1/8 (54) 2 1/8 (54) 1,850 (9.4) 925 (4.7) 925 (4.7)

2 5/8 (67) 2 5/8 (67) 1,200 (6.1) 1,200 (6.1) 600 (3.1) 600 (3.1)

3 1/8 (79) 3 1/8 (79) 850 (4.3) 425 (2.2) 425 (2.2)

Figure 20

Page 22 of 89

TRG-TRC006-EN 15

period two

Suction Line

notes circuit with two steps of capacity. Maximum system (evaporator) capacity is

20 tons (70.3 kW) and the circuit can unload to 10 tons (35.2 kW) of capacity.

Using the chart in Figure 19 on page 14, the refrigerant velocity at both

maximum and minimum capacities is determined for several pipe diameters.

After these velocities have been determined, the largest acceptable pipe

diameter is selected to minimize the overall pressure drop due to the suction

line.

When this system operates at maximum capacity, use of either the 1 1/8 in.

(28 mm)- or the 1 3/8 in. (35 mm)-diameter pipes results in a refrigerant velocity

that is greater than the recommended upper limit of 4,000 fpm (20 m/s). Again,

these high velocities may cause objectionable noise, so these pipe sizes should

probably not be considered.

Figure 21 shows the minimum allowable refrigerant velocity, for both a vertical

suction riser and a horizontal (or vertical drop) section of suction line, for each

standard pipe diameter. As mentioned earlier in this period, the minimum

allowable velocity in a suction riser depends on the diameter of the pipe. The

minimum velocity for a horizontal, or vertical drop, section is 75 percent of the

minimum allowable velocity for a vertical riser of the same diameter.

The minimum velocities listed in this table assume a worst-case operating

condition of 20°F (-6.7°C) saturated suction temperature. This provides a safety

factor, because a system will probably operate at this type of condition at some

time in its life.

riser horiz/drop

3/8 (10) 3/8 (10) 370 (1.9) 275 (1.4)

1/2 (12) 1/2 (12) 460 (2.3) 350 (1.8)

5/8 (15) 5/8 (15) 520 (2.6) 390 (2.0)

3/4 (18) 3/4 (18) 560 (2.8) 420 (2.1)

7/8 (22) 7/8 (22) 600 (3.1) 450 (2.3)

1 1/8 (28) 1 1/8 (28) 700 (3.6) 525 (2.7)

1 3/8 (35) 1 3/8 (35) 780 (4.0) 585 (3.0)

1 5/8 (42) 1 5/8 (42) 840 (4.3) 630 (3.2)

2 1/8 (54) 2 1/8 (54) 980 (5.0) 735 (3.7)

2 5/8 (67) 2 5/8 (67) 1,080 (5.5) 810 (4.1)

3 1/8 (79) 3 1/8 (79) 1,180 (6.0) 885 (4.5)

3 5/8 (105) 3 5/8 (105) 1,270 (6.5) 950 (4.8)

4 1/8 (130) 4 1/8 (130) 1,360 (6.9) 1,020 (5.2)

suction line

Minimum Allowable Velocities

pipe diameter, pipe diameter, minimum velocity, fpm (m/s)

in. (mm) in. (mm)

Figure 21

Page 32 of 89

TRG-TRC006-EN 25

period two

Suction Line

notes

After this short horizontal section, the suction line should drop vertically

downward to allow the evaporator, and the section of pipe with the TXV bulb

attached, to drain freely when the system is operating.

If the suction line leaves the evaporator and then must rise immediately, this

can be accomplished by using a small trap at the end of the horizontal section

of pipe, just before the suction line rises. The purpose of this trap is to provide

free drainage from the evaporator and the section of pipe to which the TXV

bulb is attached. This ensures that the TXV bulb is not the “low spot” in the

piping where oil or liquid refrigerant could be trapped. The purpose of this trap

is not to drain the suction riser.

The suction line must then rise above the height of the evaporator coil. This

prevents refrigerant and oil inside the evaporator from free draining into the

suction line, and toward the compressor, when the system is off.

The configuration in Figure 32 is typical for a system with an indoor air handler,

where the refrigerant piping is routed along the ceiling and must drop down to

the evaporator.

single distributor on circuit

Evaporator Coil Connection

TXV

bulb

must rise above must rise above

height of evaporator height of evaporator

must drop below must drop below

header outlet header outlet

suction header suction header

Figure 32

Page 36 of 89

TRG-TRC006-EN 29

period two

Suction Line

notes

The compressor is designed to compress refrigerant vapor only. A suction- line accumulator is a device that attempts to prevent a slug of liquid

refrigerant or oil from causing damage to the compressor. The accumulator

allows liquid refrigerant and oil to separate from the refrigerant vapor, and then

be drawn into the compressor at a rate that will not cause damage.

Oversizing the accumulator, however, can cause inadequate oil return due to

low refrigerant velocity through the accumulator. An accumulator also

increases the refrigerant charge of the system and increases the pressure drop.

Some refrigeration systems that use a flooded evaporator may require a

suction-line accumulator for freeze protection or other functions. Check with the

equipment manufacturer to determine if a suction-line accumulator is required,

recommended, or discouraged.

Suction-Line Accumulator

▲ Check with

equipment

manufacturer to

determine if required,

recommended, or

discouraged

photo provided by Henry Technologies photo provided by Henry Technologies

Figure 37