Primer on purging refrigeration systems

Key concepts Purgers are perhaps the least understood component in a large industrial refrigeration system.

09/01/2000


Key concepts&/HEADLINE>

Purgers are perhaps the least understood component in a large industrial refrigeration system.

Purging removes undesirable gases to enhance operating efficiency of compressors and condensers.

Locations where air is likely to accumulate must be identified before any air can be removed.

A few years ago, on the advice of an efficiency expert, a plant engineer for a mid-sized manufacturing firm sponsored a "purging party" and invited the entire staff to participate. The party's purpose was simple—spend one day purging files, throwing away unwanted or unnecessary documents, and eliminating related clutter.

Now, let's apply the same purging philosophy to large refrigeration systems. When unwanted and undesirable noncondensable gases are removed, the heat transfer efficiency of such systems improves greatly. When it comes to removing such gases from refrigeration systems, however, most people likely have more questions than answers.

This primer is designed to enhance the understanding of purging by reviewing the basic principles of why, how, and where to purge.

Understanding the system

Most large industrial size (150 tons or more) refrigeration systems reside in the food processing, refrigerated food storage, chemical, and pharmaceutical industries. Plant engineers in these facilities appreciate that failure to refrigerate properly usually results in economic disaster, high probability of food spoilage, and lost production.

The primary components of refrigeration systems are the compressor, condenser, receiver, evaporator, and purger (Fig. 1). Of these items, the purger is perhaps the least understood.



The purger removes undesirable gases (air) from the system to enhance the operating efficiency of the compressors and condensers. Regardless of the type of refrigerant used, removing air quickly and efficiently is essential. Air finds its way into any refrigeration system, no matter how carefully it is maintained. Air enters the system in several ways:

  • Leaking through seals and valve packing when suction pressure is below atmospheric conditions

    •  

      • When the system is open for repairs, coil cleaning, equipment additions, and the like

        •  

          • When refrigerant trucks charge the system

            •  

              • When oil is added

                •  

                  • Through the breakdown of refrigerant or lubricating oil

                    •  

                      • From impurities in the refrigerant.

                        • Why remove air?

                          Insulating properties of air are well known. Air molecules generated in the gas by the compressor accumulate on the inner heat transfer surface of the condenser. This accumulated air both insulates the transfer surface and effectively reduces the size of the condenser. (A good analogy is cholesterol and fatty deposits clogging arteries.) To offset this size reduction, the system must work harder by increasing the pressure and temperature of the refrigerant.

                          Air in the system typically causes excessive wear and tear on bearings and drive motors and contributes to a shorter service life for seals and belts. Plus, the added head pressure increases the likelihood of premature gasket failures. The most obvious reason to remove air is evident on the utility bill. For each 4 lb of excess head pressure caused by the air, the power cost to operate the refrigeration system compressor increases 2% and the compressor's capacity drops 1%. This reason alone makes it essential to choose the proper size and type of purger for the job.

                          Air in the system

                          The easiest way to determine the amount of air in a refrigeration system is to check the condenser pressure and the temperature of the refrigerant leaving the condenser. Then, compare the findings with the data found in a temperature-pressure chart (See Table I for an abbreviated chart. The complete table is contained in the ASHRAE 1997 Fundamentals Handbook . Information for obtaining the handbook is found in the More info box at the end of this article.)

                          For example, if the ammonia temperature is 86 F, the theoretical condenser pressure should be 154.5 psig. If the gauge reads 174 psig, the 20-psi excess pressure is increasing power costs 10% and reducing compressor capacity 5%.

                          Table II shows the annual dollar savings that can be achieved/100 tons based on 6500 hr/yr operation and the per kWh cost of energy. For instance, if the pressure is reduced 20 psi and the cost of electricity is $0.05/kWh, annual savings is $2600/100 tons.

                          Performing a purge

                          Air is removed from a system two ways: manually or automatically. When a system is purged manually, first a valve is opened by hand to let the air escape. Seeing a cloud of refrigerant gas discharging from the system does not mean the system has been purged.

                          Until the mechanical purger was introduced in 1940, manual purging was the only option available. However, manual purging wastes refrigerant, takes a lot of time, and does not totally eliminate air. It permits an escape of refrigerant gas that may be dangerous and disagreeable to people and the environment. Because of the drawbacks, manual purging is often neglected until the presence of air in the system causes problems. Therefore, automatic purging is preferred.

                          Choosing an automatic purger

                          Determining which automatic purger to use depends primarily on whether power is available at the purger location and safety considerations permit the use of electrical components. Let's consider two types of automatic purgers: nonelectrical automatic mechanical types and automatic electronic refrigerated types (single point and multipoint).

                          Nonelectrical automatic mechanical units are used primarily when there is no electricity at the point of use or in hazardous applications where electric components are not allowed. These units remove noncondensable gases from refrigeration systems by determining the density difference between the liquid refrigerant and gases. An operator opens and closes valves to start and stop the purging operation and ensure its efficiency.

                          Automatic electronic refrigerated purgers offer additional benefits when conditions permit their use. There are two types of electronic purgers: single point and multipoint. A single point unit performs a mechanical purge operation with a temperature/gas level monitor controlling the discharge to atmosphere. The purging sequence is done manually or tied to a PLC.

                          The multi-point purger handles a number of points from the same unit. However, each point is individually purged. The multi-point purger offers total automation and includes start-up, shutdown, and alarm features. This type of purger must be designed for the total tonnage of the system. Small purgers may cost less initially, but may adversely impact system efficiency and ultimately the payback period.

                          The most recent generation of multi-point purgers includes a microprocessor based, fully programmable controller. The controller learns as it cycles through the system. As the purger accumulates air and purges, the controller records and prioritizes each purge point in its memory, thus removing air more efficiently.

                          Locating purge points

                          Before air is removed from a system, the locations where it is likely to accumulate or collect inside the system must be identified. Multiple condensers and receivers make it difficult to determine the exact location of the air. Condenser piping design and component arrangement and operation affect the location of air. Seasonal weather changes also affect air location. Therefore, it is important to purge each purge point regularly and frequently one point at a time to ensure that all the air is removed from every possible location.

                          As a rule, refrigerant gas enters a condenser at a high velocity, but by the time it reaches the far (and cool) end of the unit, its velocity is practically zero. This point is where air accumulates and where purge points need to be made. Similarly, the purge point connection on the receiver should be made at the point farthest from the liquid inlet. Always locate the purge connection at the top of the pipe and above the discharge point of the liquid refrigerant.

                          The drawings illustrate several different system configurations and recommended purge point locations on each. On all the drawings, the long red arrows indicate high gas velocity. Arrow lengths decrease as gas velocity diminishes. Air accumulation is shown by black dots.

                          Evaporative condensers. Velocity of the entering refrigerant gas (Fig. 2) prevents any significant air accumulation upstream from point X. High velocity past point X is impossible because the receiver pressure is virtually the same as the pressure at this point. Therefore, purge from point X. Do not try to purge from point Y at the top of the oil separator because air cannot accumulate here when the compressor is running.


                          Installing an air leg is recommended to further ensure that the air is accumulated and moved to the foul gas lines and ultimately into the purger. As a general rule, the pipe diameter at the purge point connection should equal the pipe diameter of the condenser outlet for a diameter of up to 4 in. If the outlet is greater than 4 in., the diameter of the air leg should be half the size at the outlet, but never less than 4 in. The larger accumulation leg provides a place to collect the air and prevent the siphoning of liquid into the foul gas line.

                          Horizontal shell-and-tube condensers. When the condenser has a side (end) inlet, incoming gas carries air molecules to the far end near the cooling water inlet (Fig. 3).


                          Purge from point X. If the purge connection is at point Y, air does not reach the connection countercurrent to the gas flow until the condenser is more than half full of air. Therefore, there is no reason to make a purge connection at point Y.

                          Vertical shell-and-tube condensers. With this type of installation (Fig. 4), low gas velocity exists at both top and bottom of the condenser. Purge connections are desirable at both points X1 and X2.

                          Receivers. As the liquid enters a receiver (Fig. 5), a cloud of pure flash gas forms near the inlet. This cloud keeps air away from point Y, so purging here would be futile. Therefore, the purge connection on a receiver should be at point X, the point farthest from the liquid inlet.


                          How a purger removes air

                          Three basic steps describe how a refrigerated purger works (Fig. 6):


                          1. Priming the purger

                          2. Opening the purge point

                          3. Removing air and gas.

                            1. The purger is primed (filled with liquid) through P (A). At the same time, liquid flows through Tx (thermal expansion valve) to cool the purger. The float senses when the body is full and stops the filling process. When the purger is cooled, foul gas is allowed to enter the bottom of the purger from one purge point at a time (B). Subcooled liquid condenses the refrigerant gas and in the process noncondensable gas accumulates at the top of the purger and is vented to atmosphere (C).

                              —Edited by Jeanine Katzel, Senior Editor,

                              630-320-7142, jkatzel@cahners.com

                              Table I. Temperature-pressure relationship of saturated ammonia, R-12, and R-22

                              If the refrigerant temperature, F is:The gauge pressure should be:
                              For ammonia, psiThe gauge pressure should be:
                              For R-12, psiThe gauge pressure should be:
                              For R-22, psi

                              72

                              118.7

                              72.80

                              126.8

                              74

                              123.4

                              75.50

                              131.2

                              76

                              128.3

                              78.30

                              135.7

                              78

                              133.2

                              81.15

                              140.3

                              80

                              138.3

                              84.06

                              145.0

                              82

                              143.6

                              87.00

                              149.8

                              84

                              149.0

                              90.10

                              154.7

                              86

                              154.5

                              93.20

                              159.8

                              88

                              160.1

                              96.40

                              164.9

                              90

                              165.9

                              99.60

                              170.1

                              92

                              171.9

                              103.00

                              175.4

                              94

                              178.0

                              106.30

                              180.9

                              96

                              184.2

                              109.80

                              186.5

                              98

                              190.6

                              113.30

                              192.1

                              100

                              197.2

                              116.90

                              197.9

                              102

                              203.9

                              120.60

                              203.8

                              104

                              210.7

                              124.30

                              209.0

                              106

                              217.8

                              128.10

                              216.0

                              108

                              225.0

                              132.10

                              222.3

                              110

                              232.2

                              136.00

                              228.7


                              Table II. Compressor operating cost savings* (Annual dollar savings/100 tons operating 6500 hr/yr)

                              Pressure reduction, psiPower cost/kWh,
                              $ 0.03Power cost/kWh,
                              0.04Power cost/kWh,
                              0.05Power cost/kWh,
                              0.06Power cost/kWh,
                              0.08Power cost/kWh,
                              0.10Power cost/kWh,
                              0.12

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                              5

                              400

                              530

                              670

                              800

                              1070

                              1330

                              1600

                              10

                              800

                              1070

                              1330

                              1600

                              2130

                              2660

                              3200

                              15

                              1200

                              1600

                              2000

                              2400

                              3200

                              4000

                              4800

                              20

                              1600

                              2130

                              2660

                              3200

                              4260

                              5330

                              6390

                              *Savings in compressor operating costs (in U.S. dollars) achieved using a refrigerated purger to reduce excess high-side pressure.