A frosty purger tells when it's working

June 1, 2000
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Thousands of large industrial-size (150 tons or more) refrigeration systems need constant attention to ensure their peak performance.

Concentrated largely in food processing and food storage industries, each of these systems represents a significant investment. Failure to refrigerate properly can result in economic disaster with a high probability of food spoilage.

If it is your job to service one of these refrigeration systems, you will find this article of value. It will address how to make these systems more efficient, and how to troubleshoot for conditions that are costly in terms of labor, energy, and maintenance.

The primary components of a refrigeration system are the compressor, condenser, receiver, evaporator and purger (see Figure 1, page 48).

The purger is perhaps the most critical but least understood component of the system. Its function is to remove undesirable gases (air) from the system, thus enhancing the operating efficiency of the system’s compressors and condensers.

Regardless of whether ammonia or Freon refrigerant is used, removal of air is absolutely essential.

Air: the enemy

The insulating properties of air are well known. Air molecules in the gas from the compressor accumulate on the inner heat transfer surface of the condensers.

This accumulated air not only insulates the heat transfer surface, it effectively reduces the size of the condenser. It’s like 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. This is an expensive luxury.

Air in the system is expensive for a number of reasons. The most obvious can be seen on the utility bill. For each 4 lb 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%.

The easiest way to determine the amount of air in a refrigeration system is to check the condenser pressure and temperature of the refrigerant leaving the condenser. Then, compare the findings with data provided in a temperature-pressure chart, (see ASHRAE 1997 Fundamentals Handbook). An abbreviated version is shown in Table 1, page 48.

If, for example, the ammonia temperature is 86ÞF, the theoretical condenser pressure should be 154.5 psig. If your gauge reads 174 psig, you have 20-psi excess pressure that is increasing power costs 10% and reducing compressor capacity by 5%.

Table 2 (page 49) shows the annual dollar savings per 100 tons, at 6,500 hr 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.

Other costs of air in the system include wear and tear on bearings and drive motors, and a shorter service life for seals and belts. Increased head pressure increases the likelihood of premature gasket failures.

Purging your system of air

There are two basic ways to get the air out of your system, manually or automatically.

When purging manually, a valve is opened by hand, allowing the air to escape. It is a false assumption that when you see a cloud of refrigerant gas being discharged that the system has been purged, but that’s how it was done in the years prior to the 1940 patent of the mechanical purger.

Besides wasting refrigerant, manual purging:

  • Takes a lot of valuable time;

  • Does not totally eliminate air;

  •  Permits the escape of refrigerant gas that may be dangerous and disagreeable to people and the environment and may also be illegal; and

  • Is easily neglected until the presence of air in the system causes problems.

Mechanical purgers are designed to remove noncondensable gases from refrigeration systems by the density difference between the liquid refrigerant and gases. They operate mechanically — no automation, no electronic controls.

They also require an operator to open and close valves in order to start and stop the purging operation and ensure its efficiency.

While this type of purger has been the “workhorse” of the industry for decades, today it is used primarily in applications where there is no electricity at the point of use, or in hazardous applications where electric components are not allowed.

Don’t discard these mechanical purgers — they can be retrofitted with up-to-date components, making them function as an electronic purger. Such upgrades are economically attractive over replacing them entirely.

Automatic purgers: There are two types of electronic purgers, the single point and the multi-point. The single point is a mechanical purge operation with a temperature-gas level monitor controlling the discharge to the atmosphere. The purging sequence is still done manually.

The multi-point purger purges each purge point individually, but offers total automation including startup, shutdown, and alarm features.

The most recent generation of multi-point purgers includes a microprocessor-based, fully programmable controller. This has an advantage over clock timers; the 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.

Do you have a purging problem?

How do you know if there is a problem with your refrigeration system and, more specifically, with your purger?
  •  If your purger is not cold or does not frost over, there may be air, dirt, or oil in the system.

  •  If your compressor runs more frequently and for longer periods, you may have air in your system. You may note this in the increased cost to operate the compressor.

  •  Do you notice excessive wear of the belts resulting in shorter service life? If so, your system may be generating excess head pressure due to air in the system.

  •  Finally, are gasket failures increasing? It could be caused by increased pressure because the system is compensating for the air molecules insulating the inside surface of the condenser.



Troubleshooting tour

A frosty purger tells you that the system is transferring refrigerant and making surface contact. But that may not be the whole story.

Let’s go on a little troubleshooting tour. We’ll check out some of the more common questions and explore what may be contributing to the problem.

The purger will not discharge air: If the top of the purger is frosted (or cold) and the liquid level is well up in the gauge glass, but no air is discharged to the water jar, check the following —

  • All the air may have been removed from the receiver or condenser. Try closing the valve from the condenser and opening the valve from the receiver (or vice-versa).

  • There may be no more air in the system. The fact that no air is discharging indicates that there is no air in the gas flowing to the purger.

  • If the head pressure is higher than it should be, perhaps the purge point connection is improperly located or the condenser tubes may be plugged.

  • There is no flow of gas to the purger. If the liquid level is very high or rises out of sight in the gage glass, and if you cannot hear gas entering the purger, then you may have:

    — A liquid seal in the purge gas line;

    — Liquid pressure in the purger that may be higher than the foul gas pressure; this may indicate that you need a differential valve or that the differential valve (K-3 in Figure 1) is not working properly; or

    — Dirt or oil film over the vent of the inverted bucket.

    Clean the purger, strainer, and bucket vent, and check the condition of the scrub wire that keeps the vent free of plugging material.

    The liquid level falls in gauge glass: If the purger remains frosted but liquid will not remain in gauge glass, there may be an obstruction in the air discharge. This condition may be caused by —

  • Valve D not completely opening;

  • Dirt in the air discharge valve seat; or

  • Friction that causes the air-release mechanism to hang up.

Another reason the liquid level may fall is that the liquid discharge valve and seat may be leaking and require replacement. Or, the upper gauge glass valve may be closed, or gauge glass packing is wedged beyond the glass, thus plugging the valve.

The purger will not chill: When the purger will not chill or frost over, the liquid level in the gauge glass will drop, causing the float to sink and open the air valve. This allows the refrigerant gas to escape.

If this appears to be the problem, the refrigerating coil may be starved due to —

  • Dirt in the strainer;

  • Dirt in the expansion valve;

  • Oil freezing in the expansion valve; or

  • A build-up of oil in refrigerating coil.

Another reason the coil may be starved is that there is not enough refrigerant in the system.

If none of these conditions appear to be the problem, check to see if there is insufficient suction pressure on the coil. You should check to see if the valve may be only cracked open — always keep the suction line valve wide open when purger is operating. This could also be caused by too small of a suction line, or the line may be improperly located.

The purger is losing excessive refrigerant gas: If a lot of refrigerant gas passes through valve D along with the air, then —

  • The purger will not be sufficiently chilled and the liquid level in the glass will be lower than normal.

  • There may be a defect or wear in the liquid trap mechanism. The valve and seat assembly may require replacement.

  • The float may have collapsed.

  • The air discharge valve leaks. You may need to replace it.

  • If leak persists, the cause may be a small leak between the liquid valve and seat. If this happens, replace with new parts.



Repair, replace, and upgrade

It is relatively easy to replace purger internal components or upgrade a mechanical purger with new electronic components, making it either a single- or multi-point purger.

Furthermore, the cost to upgrade a purger is substantially less than buying a new unit.

Repair parts to existing mechanical purgers come complete with installation instructions that are easy to follow. The key is to be sure that the parts are undamaged before installing them, and to clean out the purger body before you reassemble.

Retrofit kits for electronic upgrades are probably easier to install than the repair parts. These kits come with preassembled assemblies and clearly written installation instructions to help make the process easier.

The result of the electronic upgrades is more efficient, cost-effective purge operation.

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