Walk-In Filter Drier and Sight Glass: What They Tell You

The filter drier and sight glass are two small parts in the liquid line, usually at or near the condensing unit. They cost little, but a clogged drier can starve the whole walk-in, and a misread sight glass is behind a lot of unnecessary refrigerant top-offs.

What each part does

The filter drier is a sealed canister filled with desiccant (a drying material) and a filter. Liquid refrigerant passes through it on the way to the expansion valve. It does three jobs: it absorbs moisture, it neutralizes some of the acid that forms when moisture and heat break down oil, and it catches debris such as metal shavings, flux and dirt left from installation or a failed part.

Moisture matters because water in a refrigeration system can freeze at the expansion valve, form acid with the oil, and damage compressor motor windings. Debris matters because the expansion valve has a very small opening that is easy to plug.

The sight glass is a small window in the liquid line. It shows whether the refrigerant passing by is solid liquid or a mix of liquid and vapor (bubbles). Most sight glasses on commercial systems also include a moisture indicator, a small element in the center that changes color when moisture in the refrigerant goes above a safe level. The color key is usually printed on the sight glass ring or cap. Read the key on your part rather than assuming, because colors vary between manufacturers.

Some systems combine the two in one fitting, and some smaller or newer systems have no sight glass at all.

How a filter drier fails

A drier doesn't wear out in the usual sense. It fills up.

• Saturated with moisture. Once the desiccant has absorbed all the water it can, moisture passes through. The moisture indicator shows wet.
• Restricted by debris or sludge. As the filter traps particles, it becomes harder for refrigerant to pass. After a compressor burnout, acid and carbon can plug a drier quickly.
• Restriction from the start. An undersized drier, or one installed backward on a one-way drier, restricts flow from day one.

What a restriction looks like

When the drier is partly plugged, refrigerant drops in pressure as it squeezes through. That pressure drop also drops its temperature, so:

• the outlet side of the drier feels noticeably colder than the inlet, and in bad cases frost or sweat forms at the outlet or just downstream;
• bubbles may appear in a sight glass located after the drier;
• the evaporator is starved, so the box warms up while the compressor runs, and suction pressure runs low;
• on pump-down systems the low-pressure control may trip, causing short cycling.

These symptoms overlap with low refrigerant charge and with a stuck or restricted expansion valve. That is exactly why a technician measures before adding refrigerant.

What bubbles in the sight glass can mean

Bubbles mean vapor is in the liquid line. That happens when the liquid refrigerant is not fully "subcooled" (cooled below its boiling point at that pressure). Causes include:

A clear glass does not always mean full charge, either. A system can be badly overcharged or have a flooded condenser and still show clear liquid. The only reliable charge check uses pressures, temperatures, superheat and subcooling measured against the manufacturer's specifications. Heatcraft's installation manual, for example, notes that systems with a design TD of 10°F should generally have 6° to 10°F of evaporator superheat.

Tip: If a technician says "bubbles in the glass, it's low," it is fair to ask what the superheat and subcooling were. See hiring a refrigeration technician.

Checks staff can do safely

These are observation only. Do not open panels or touch lines.

1. If the sight glass can be seen without removing a panel, look at it while the compressor is running. Note whether it is clear, has occasional bubbles or a steady stream, and what color the moisture indicator shows.
2. Note whether you can see frost on any small canister or fitting in the liquid line on the outdoor or rooftop unit.
3. Check the box temperature with an independent thermometer and write it down with the time.
4. Take a photo of the sight glass and the nameplate for the technician.

Heatcraft's maintenance guidance for air-cooled condensing units includes a quarterly visual, leak and sight-glass check, which is a good habit whether or not you have a service contract.

Safety: Liquid lines can be hot or cold enough to burn, and condensing units contain live electrical parts and moving fans. Look, photograph and report. Leave the rest to a licensed technician.

What a technician will check and do

• Pressures and temperatures: suction and head pressure, superheat at the evaporator, subcooling at the condenser outlet.
• Temperature across the drier: a thermometer or clamp probe on each side. A meaningful drop points to restriction.
• Moisture indicator: a wet reading usually means a drier change and a hunt for how moisture got in (a leak on the low side, a poor evacuation after a past repair).
• Replacement: the refrigerant is recovered or isolated, the old drier is cut or unscrewed out, a new one of the right size and type goes in, and the system is evacuated and recharged. Only an EPA Section 608 certified technician may do this work.
• After a burnout: a technician may install a suction-line filter drier designed to catch acid, and check an oil sample for acid. Those driers are typically removed or changed after a set period, per the manufacturer.

Most technicians replace the liquid line drier any time the system is opened, such as for a leak repair, an expansion valve replacement, or a compressor change. The drier costs little compared with the damage moisture can do, and an opened drier starts absorbing moisture from the air.

Symptoms this part can cause

A restricted or saturated drier can look like several other problems. If you are working from a symptom, these pages cover it in full:

• Fans running but not cooling
• Walk-in cooler short cycling
• Evaporator coil frozen
• Low head pressure in winter

For a plain-English list of terms like superheat, subcooling and head pressure, see the walk-in cooler glossary. For how the whole refrigeration circuit fits together, see how walk-in coolers work.