Air is the enemy of any refrigeration system. Purging, whether manual or automatic, removes air and maximizes refrigeration system performance.

Table 1. Noncondensables force your refrigeration system to work harder. 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 those with theoretical values.


Figure 1. This schematic of a refrigeration system shows where a multipoint purger fits into the system.
Air in a refrigeration system robs it of its capacity to function, and failure to remove such air can be costly in terms of operating efficiency and equipment damage. Such damage is especially notable in the industrial-sized refrigeration systems commonly used in major cold storage facilities, food processing plants and some chemical plants.

Regardless of whether a system is charged with ammonia or a Freon refrigerant, the heat transfer efficiency of such systems will greatly improve when undesirable noncondensable gas (air) is removed. The process of removing air, which is colorless and odorless, is called purging.

Over time, this process has become increasingly automatic. But, it is important to understand why, where and how to purge the system before attempting to rely on an automatic purging system.

Table 2. Savings in compressor operating costs achieved by using a refrigerated purger to reduce excess high side pressure.

Why Purge

No matter how hard one tries to avoid it, air will get into the system and accumulate on the inner surface of the heat exchanger, essentially creating an insulating barrier. Air can enter a refrigeration system by several avenues:
  • When suction pressure is below atmospheric conditions, air can enter through seals and valve packing.

  • Air can rush in when the system is open for repair, coil cleaning or adding equipment.

  • Air can enter when the refrigerant truck is charging the system or when oil is being added.
This accumulated air insulates the transfer surface and effectively reduces the size of the condenser. To offset this size reduction, the system must work harder by increasing the pressure and temperature of the refrigerant. Therefore, removal of air as quickly and as efficiently as possible is essential.

Air in the system can result in excess 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. But, the most obvious reason to remove air can be seen on the utility bill: For each 4 lbs. of excess head pressure caused by air, the power cost to operate the compressor will increase by 2% and the compressor's capacity will be reduced by 1%. For this reason alone, it is essential to choose the proper size and type of purger for the job.

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 data provided in a temperature-pressure chart such as that published in the ASHRAE 1997 Fundamentals Handbook. Table 1 contains an abbreviated version of the ASHRAE table.

If, for example, the ammonia temperature is 86°F (30°C), the theoretical condenser pressure should be 154.5 psig. If your gauge reads 174 psig, you have 20 psi excess pressure. Under this condition, the power costs increase by 10% and the compressor capacity decreases by 5%. Table 2 shows the annual dollar savings per 100 tons at 6,500 hours per year, as determined by the per kWh cost of energy. As an example, if the pressure is reduced by 20 psi and the cost of electricity is $0.05 per kWh, the annual savings will be more than $2,600 per 100 tons.

Figure 2. When the condenser has a side inlet, incoming gas carries air molecules to the far end of the condenser, so purge from point X. If the purge point is located at point Y, air will not reach it until the condenser is more than half full of air.

How to Purge

Basically, there are two ways to purge a system of air: manual or automatic. To purge manually, a properly positioned valve is opened by hand, allowing the air to escape. It is a common misconception that when a cloud of refrigerant gas is seen being discharged to atmosphere, the system has been purged of air. Air can still be trapped in the system.

Besides wasting refrigerant, manual purging:

  • Demands the technician's time.
  • Does not totally eliminate the air.
  • Permits refrigerant gas, which may be dangerous and disagreeable to people and the environment, to escape.
  • Is easily neglected until the presence of air in the system causes problems.
Because of these reasons, many refrigeration system users prefer automatic purging.

Refrigeration systems include the compressor, condenser, receiver, evaporator and purger (Figure 1). Of these components, the purger is perhaps the least understood and appreciated. The purger's job is to remove air from the system, thus improving compressor and condenser operating efficiency.

Two types of automatic purgers are used: nonelectrical mechanical and automatic electronic purgers. Determining the type of automatic purger to use is a matter of whether electricity is available at the purger location and if it safe to allow electrical components to be used.

The nonelectrical mechanical units are used primarily in applications where electricity is not available at the point of use or in hazardous applications where electric components are not allowed. They remove air by sensing 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.

Electronic automatic refrigeration purgers are classed as single-point and multipoint purgers. The single-point electronic refrigerated purger has a mechanical-purge operation with a temperature/gas level monitor that controls the discharge to atmosphere. The purging sequence is performed manually.

A multipoint refrigerated purger will purge a number of points using the same unit. However, each purge point is purged individually, and the multipoint purger offers total automation, including startup, shutdown and alarm features. With this purger, it is important to choose a purger designed for the total tonnage of your system. Undersized purgers may cost less initially but may adversely impact the system's efficiencies and payback period.

Some multipoint purgers include a microprocessor-based programmable controller rather than a clock timer. The fuzzy logic controller can "learn" 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.

Figure 3. The high velocity of the entering refrigerant gas prevents air accumulation upstream of point X. So, it is not necessary to purge from point Y; instead, purge from point X.

Locating Purge Points

Before air can be removed, you must locate where it will collect. With multiple condensers and receivers, it can be difficult to determine the exact location of the air. Condenser piping design, component arrangement and operation affect the air location. Seasonal weather changes also affect where the air accumulates. It is important to frequently purge each suspected air purge point -- one at a time -- to ensure all the air is removed from every location.

As a rule, refrigerant gas enters a condenser at a high velocity. By the time it reaches the far, cool end of the condenser, its velocity is practically zero. This is where the air accumulates and where purge points should be located. Likewise, the purge point connection on the receiver should be located at the point furthest from the liquid inlet.

The best purger location depends on the specific system's layout. Based on years of experience with successful purger installations, here are some common configurations and the recommended purge point locations on each. In each of the following figures, the long red arrows show high gas velocity. Arrow lengths decrease as the gas velocity diminishes. Air accumulation is shown by black dots. Regardless of configuration, always locate the purge connection at the top of the pipe and above the discharge point of the liquid refrigerant.

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 (Figure 2). Purge from point X. If the purge connection is at point Y, air will 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.

Figure 4. The cloud of refrigerant gas that forms naturally near the inlet will prevent air from entering the condenser there, so a purge point is not necessary. Purge from point X, which is the point furthest from the inlet.

Evaporative Condensers

With an evaporative condenser (Figure 3), the velocity of entering refrigerant gas 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 point X. Therefore, do not try to purge from point Y at the top of the oil separator -- air cannot accumulate here when the compressor is running. Instead, purge from point X.

To ensure air is accumulated and moved into the purger, an air leg is recommended. When installing air legs, the rule of thumb is: For pipes up to 4" dia., the purge point connection should start with the same pipe diameter as the condenser outlet. If the outlet is greater than 4" dia., the air leg should be one-half the size at the outlet but never less than 4" dia. The larger accumulation leg will provide a place to catch the air and prevent siphoning of liquid into the foul gas line.

Receivers

As the liquid enters a receiver, a cloud of pure "flash" gas is formed near the inlet (Figure 4). This cloud will keep air away from point Y, so any purging at this point would be futile. The purge connection on a receiver should be at point X, the point furthest from the liquid inlet.

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