Refrigerant cylinders: types, filling, storage and transport

Northfield's shop has a cylinder cage at the back, chained and labelled, with three separate sections: new refrigerant, recovery cylinders waiting to go to the reclaimer, and empty cylinders. Every Friday afternoon, one technician does the cylinder check: weighing every recovery cylinder, checking test dates, reading every label and moving full cylinders to the shipping pallet. This week it was Theo's turn, and Dana went through it with him. "More technicians are hurt by cylinders than by anything else in refrigeration," she said. "Dropped cylinders, overfilled cylinders, cylinders in a hot van. A cylinder is a pressure vessel. Treat it like one."

This lesson covers the containers refrigerant lives in when it is not in a system: disposable cylinders, refillable cylinders and recovery cylinders. It explains how they are built and marked, how full they may be filled and why, how to inspect, store and secure them, and the basic rules for carrying them in a service vehicle.

Types of cylinders

Disposable cylinders, also called non-refillable cylinders, are the familiar small steel cylinders that new refrigerant has often been sold in. They are built to a transport specification for single use, such as TC-39M in Canada or DOT-39 in the United States, and may never be refilled: they are thin-walled, have no provision for retesting, and refilling them is prohibited. Their valves include a pressure relief device and, on newer cylinders, a check device that resists refilling. Ontario's rules require that containers filled with refrigerant by technicians be refillable and recyclable, and several provinces discourage or restrict disposable cylinders, so many suppliers now sell refrigerant in returnable cylinders.

Refillable cylinders are heavier cylinders built to specifications such as TC-4BAM or TC-4BWM, which allow them to be refilled and periodically retested. Suppliers use them for bulk refrigerant and return them for refilling, usually with a deposit. Ontario's rules require a deposit of at least $25 on refillable refrigerant containers, refunded when the container comes back.

Recovery cylinders are refillable cylinders built for recovered refrigerant. They have two valves, one connected to a dip tube that reaches the bottom of the cylinder for liquid, and one at the top for vapour. Many also have a port for a float switch, a level sensor that connects to the recovery machine and shuts it off when the cylinder reaches its safe fill level. Recovery cylinders have traditionally been grey with a yellow top or shoulder, and many cylinders for flammable refrigerants carry a red band. A recovery cylinder must be rated for the pressure of the refrigerant it will hold: high-pressure refrigerants such as R-410A and R-32 need cylinders with a service pressure of at least 2,760 kPa (at least 400 psig).

Large quantities move in ton cylinders and cylinders of several hundred kilograms on supermarket and industrial jobs, and bulk refrigerant ships in tank trucks and rail cars. Those are handled with lifting equipment and procedures of their own.

Reading the stamped markings

The markings stamped into a refillable or recovery cylinder's shoulder or collar identify the cylinder and its status. They normally include:

• the specification, for example TC-4BAM followed by the service pressure, or a DOT specification on older cylinders allowed in Canada;
• the serial number and the manufacturer's mark;
• the date of manufacture, as month and year;
• the water capacity, WC, the mass of water the cylinder holds when completely full, and the tare weight, TW, the empty weight of the cylinder with its valve;
• the date and the mark of each retest, added by the facility that performs it.

Refillable refrigerant cylinders must be requalified, normally by a hydrostatic pressure test or another approved method, at the interval set for their specification, which for common refrigerant cylinders is every five years. Transport Canada regulates these containers, and retesting is done at a facility registered with Transport Canada. A cylinder whose retest date has passed must not be refilled until it has been retested; it may be emptied and shipped for testing.

How full is full? The fill limit

Lesson 3 explained hydrostatic pressure: a cylinder that becomes completely full of liquid can rupture from a small temperature rise because liquid refrigerant expands as it warms. To prevent this, a cylinder is never filled to more than a set fraction of its volume, leaving a vapour space that can absorb the expansion. The widely used rule for recovery cylinders is that they must not be filled to more than 80 per cent of their volume with liquid.

Because cylinders are filled by weight on a scale, the 80 per cent volume limit has to be turned into a mass. The water capacity stamped on the cylinder gives its volume: a cylinder with a WC of 21.6 kg (47.7 lb) holds that mass of water, so its volume is about 21.6 L (5.7 US gallons). The mass of refrigerant that fills 80 per cent of the volume depends on the density of the liquid refrigerant, which is usually expressed as its specific gravity, the ratio of its density to that of water.

The catch is that liquid refrigerant gets less dense as it warms. A cylinder filled to 80 per cent at room temperature will be fuller on a hot day, and for refrigerants whose density falls steeply with temperature, such as R-410A and R-404A, a cylinder filled using room-temperature density can come close to liquid-full at the temperatures a closed van reaches in summer. Many companies therefore use the conservative field rule of filling to no more than 80 per cent of the water capacity by weight, which is the same as assuming a specific gravity of 1. Northfield's policy is that rule, combined with a float switch on every recovery cylinder that has a port for one.

# Maximum fill of a recovery cylinder and how full it becomes when hot
# Approximate liquid specific gravities (density relative to water) for teaching
sg_21c = {"R-22": 1.21, "R-134a": 1.22, "R-410A": 1.08, "R-404A": 1.06, "R-448A": 1.12}
sg_54c = {"R-22": 1.07, "R-134a": 1.09, "R-410A": 0.91, "R-404A": 0.90, "R-448A": 0.98}
wc_kg = 21.6        # water capacity stamped on the cylinder, kg
tare_kg = 12.0      # tare weight stamped on the cylinder, kg

for ref in sg_21c:
    m_density = 0.8 * wc_kg * sg_21c[ref]       # 80 % by volume using room-temperature density
    m_field = 0.8 * wc_kg                       # conservative field rule
    full_hot_density = m_density / (wc_kg * sg_54c[ref]) * 100
    full_hot_field = m_field / (wc_kg * sg_54c[ref]) * 100
    print(f"{ref:7s} density rule {m_density:5.1f} kg -> {full_hot_density:4.0f} % full at 54 C;"
          f" field rule {m_field:5.1f} kg -> {full_hot_field:4.0f} % full at 54 C")
print(f"Gross weight at the field-rule limit: {tare_kg + 0.8 * wc_kg:.1f} kg on the scale")

R-22    density rule  20.9 kg ->   90 % full at 54 C; field rule  17.3 kg ->   75 % full at 54 C
R-134a  density rule  21.1 kg ->   90 % full at 54 C; field rule  17.3 kg ->   73 % full at 54 C
R-410A  density rule  18.7 kg ->   95 % full at 54 C; field rule  17.3 kg ->   88 % full at 54 C
R-404A  density rule  18.3 kg ->   94 % full at 54 C; field rule  17.3 kg ->   89 % full at 54 C
R-448A  density rule  19.4 kg ->   91 % full at 54 C; field rule  17.3 kg ->   82 % full at 54 C
Gross weight at the field-rule limit: 29.3 kg on the scale

The output shows why the conservative rule exists. Filled using room-temperature density, a cylinder of R-410A or R-404A would be about 95 per cent full at 54 °C (130 °F), a temperature a van parked in the sun can reach, with little margin left. Filled by the field rule, every refrigerant in the table stays at or below about 90 per cent full when hot.

Always weigh: the cylinder's appearance tells you nothing about how full it is, and a gauge on a cylinder shows only the saturation pressure, which is the same whether the cylinder is almost empty or almost full as long as some liquid is present. Write the tare weight and the fill limit on the cylinder tag, and update the contents weight on the tag every time refrigerant goes in or out.

Inspecting a cylinder

Before filling or using any cylinder, inspect it. Reject a cylinder, mark it and set it aside if you find:

• dents, gouges, cuts or bulges in the wall, or any sign of fire or heat damage, such as burnt paint;
• heavy rust or pitting, especially around the foot ring and the bottom, where water collects;
• a damaged, bent or leaking valve, a missing valve guard or cap, or damaged threads;
• missing, worn or unreadable stamped markings, or a retest date that has passed;
• a label that is missing or does not match the refrigerant you intend to put in.

A recovery cylinder that has never been used, or that has been emptied and opened, should be evacuated before use so that air and moisture are not mixed with the recovered refrigerant. Northfield evacuates every empty recovery cylinder in the shop to a deep vacuum, labels it with the refrigerant it is dedicated to, and keeps it closed until it goes on the van.

Storage

Store cylinders in a cool, dry, well-ventilated place, away from heat sources, direct sun, exits and electrical panels. Keep them upright and secured, with chains, straps or racks, so they cannot fall. Keep valve caps or guards in place when the cylinder is not connected. Most refrigerant SDSs say cylinders should not be stored where they can exceed about 52 °C (125 °F). Keep full, empty and recovery cylinders separate and clearly marked, and keep flammable refrigerant cylinders away from ignition sources and in the quantities the fire code allows.

Never store cylinders in living spaces, below grade where leaking vapour could collect, or in a closed vehicle on a hot day. Never use a cylinder as a roller, a support or a hammer, and never alter or repair a cylinder or its valve and relief device.

Carrying cylinders in a service vehicle

Refrigerant cylinders are dangerous goods under the federal Transportation of Dangerous Goods (TDG) Regulations: most refrigerants are in class 2.2, non-flammable, non-toxic gases, while A2L and A3 refrigerants are in class 2.1, flammable gases. The regulations include exemptions for carrying limited quantities of gases in cylinders, such as for a service technician's own work, provided conditions are met: the cylinders must be built and marked to an approved standard, protected against damage to the valve, and secured so they cannot move. Larger quantities and shipments to a reclaimer require full compliance, including safety marks, documentation and a trained person. Northfield's office handles shipments, and every technician holds TDG training for the cylinders they carry.

Inside the van, cylinders ride upright in a rack or strapped to a bulkhead, never loose on the floor and never in the cab with the driver. The cargo area should be ventilated, and the van should not be left in the sun with cylinders inside any longer than necessary. Recovery cylinders are tagged with their contents before they leave the job site.