10.2 Purger Operation and Verification

Principles of Ammonia Purger Operation

To restore system efficiency and safety, non-condensable gases (NCGs) must be purged from the system. In standard refrigeration systems, purging can be accomplished either manually or automatically.

Manual vs. Automatic Purging

Historically, systems were purged manually, a process that is highly inefficient and dangerous:

• Manual Purging: A technician isolates a condenser or receiver, connects a hose to a purge valve at the top of the vessel, and runs the hose into a container of water. The technician slowly cracks the valve open to vent the gases. Because the gas inside the condenser is a mixture of ammonia vapor and NCGs, significant quantities of ammonia are vented and wasted. Manual purging is labor-intensive, leads to high refrigerant loss, and carries significant safety risks of ammonia exposure.
• Automatic Purging: Modern plants utilize automatic purgers, which operate continuously and autonomously. An automatic purger uses a small heat exchanger (evaporator) to cool the foul gas mixture down to low temperatures. Since ammonia condenses at typical system pressures when cooled, it liquefies and returns to the system. The NCGs, having boiling points below -100°F, remain as gases and are separated and vented. Automatic purging reduces ammonia loss by over 95% compared to manual purging, enhances plant safety, and maintains system efficiency continuously.

---

Piping and Connections of an Automatic Purger

For an automatic purger to operate correctly, it must be properly piped into various points of the refrigeration system. There are five main piping connections required for a typical automatic purger installation, as illustrated in the table below:

Safety Warning: Only one foul gas purge point should be active at any given time. If multiple purge points are open simultaneously, gas will migrate from the higher-pressure condenser to the lower-pressure condenser, causing liquid backups and rendering the condensers ineffective. Modern automatic purgers use automated solenoid valves controlled by a PLC to cycle through purge points sequentially.

---

The Automatic Purger Operating Cycle

The operation of an automatic purger is a continuous cyclic process that can be divided into four distinct phases:

1. Chilling Phase

The system's liquid feed solenoid opens, allowing liquid ammonia to flow through a metering valve (expansion valve) into the purger's cooling coil. The refrigerant boils inside the coil, absorbing heat and cooling the coil to a temperature matching the suction pressure (typically between -20°F and 10°F). The vapor is drawn out through the suction return line.

2. Condensing and Separation Phase

The foul gas solenoid valve opens, allowing the high-pressure gas mixture (ammonia and NCGs) to enter the purger shell. As this mixture contacts the freezing outer surface of the cooling coil:

• The ammonia vapor rejects its heat, condenses into liquid, and falls to the bottom of the shell.
• This condensed liquid drains back into the refrigeration system via the liquid drain connection.
• The NCGs (nitrogen, oxygen, hydrogen), which cannot condense at these temperatures, remain as gases and rise to the top of the purger shell.

3. Accumulation Phase

As NCGs continue to enter and accumulate at the top of the purger vessel, they occupy space and displace the liquid ammonia in the shell. The liquid level inside the shell drops.

• In mechanical float-type purgers, the falling liquid level lowers a float.
• In electronic purgers, a level sensor or temperature sensor detects that the cold liquid has been replaced by NCGs.
• This indicates that the purger shell is full of non-condensables and is ready to vent.

4. Venting Phase

Once the NCG concentration reaches the trigger point (sensed by the float switch or temperature controller), the automatic vent solenoid valve opens. The high system pressure drives the accumulated NCGs out of the top of the purger and down the NCG vent line into the water bubbler jar. As the NCGs escape, the liquid ammonia level inside the purger rises again, closing the vent solenoid and restarting the cycle.

---

Verifying Purger Activity: Bubble Analysis

Operators must visually verify that the purger is functioning correctly by observing the water bubbler jar (also called the bubble jar or water trap). The behavior of the bubbles and the sounds they produce provide immediate diagnostic feedback:

1. Quiet Bubbling (Normal NCG Venting)

When the vent solenoid opens and non-condensable gases (air, nitrogen, hydrogen) are discharged:

• Visual: Large, slow, distinct bubbles rise all the way from the bottom of the dip tube to the surface of the water, where they break and escape to the atmosphere.
• Audible: A soft, quiet, rhythmic bubbling or gurgling sound is heard.
• Indication: The purger is operating correctly, successfully separating and venting NCGs while retaining ammonia.

2. Crackling and Popping (Ammonia Venting / Malfunction)

If the purger is malfunctioning and venting active ammonia vapor:

• Visual: The bubbles exiting the dip tube collapse or shrink rapidly and disappear almost immediately before reaching the surface of the water.
• Audible: A loud, sharp, rapid crackling, popping, or snapping sound (frequently compared to the sound of "frying bacon" or "screeching") is heard.
• Indication: Ammonia gas is entering the water and dissolving instantly. This indicates a system malfunction: the cooling coil may not be cold enough (e.g., failed liquid feed or blocked suction line), or the vent solenoid valve is leaking or stuck open. The purger is wasting refrigerant and must be shut down for maintenance.

---

Maintenance and Safety Precautions

Venting NCGs requires strict adherence to safety standards, particularly regarding the water bubbler and waste disposal:

1. Water Saturation Limit

Ammonia is extremely soluble in water; at room temperature, one volume of water can absorb between 800 and 1,000 volumes of ammonia vapor. This reaction forms ammonium hydroxide ($NH_4OH$), a highly alkaline solution:

$NH_3 + H_2O \rightleftharpoons NH_4^+ + OH^-$

• As the water absorbs ammonia, it reaches a saturation point (typically around 28% to 30% ammonia by weight).
• Once the water is saturated, it cannot dissolve any more ammonia.
• If the purger subsequently vents ammonia, the gas will bubble through the saturated water without dissolving, rising to the surface and releasing toxic, high-concentration ammonia gas into the engine room.
• Action: The water in the bubble jar must be checked daily and replaced regularly (either manually or via an automatic water flushing valve on modern purgers). Operators should use pH paper (saturated water has a pH above 12) or monitor the odor.

2. Outdoor Venting Requirements

In accordance with IIAR 2 (Standard for Safe Design of Closed-Circuit Ammonia Refrigeration Systems), the discharge from the bubbler jar must not vent directly into the engine room or occupied spaces. The vent line from the top of the bubbler jar must be piped to a safe outdoor location, positioned away from building air intakes, windows, and personnel walkways, and equipped with a rain cap to prevent water ingress.

3. Safe Disposal of Saturated Water

The saturated water removed from the bubble jar is concentrated ammonium hydroxide. It is a hazardous chemical due to its high pH and toxicity to aquatic life.

• Disposal: Saturated water must never be poured down municipal storm drains. It must be neutralized (e.g., using a mild acid) or collected and disposed of in accordance with local environmental regulations (EPA and Clean Water Act requirements).