13.2 Evaporators, Defrost Methods, and Defrost Controls
Key Takeaways
- Off-cycle (air) defrost works only above about 34 degrees Fahrenheit box temperature, where box air itself is warm enough to melt frost.
- Electric defrost energizes calrod heaters in the coil while the evaporator fans are stopped, and it is the most common method in walk-in freezers.
- Hot gas defrost routes discharge gas through the evaporator, making it the fastest and most efficient method but requiring a suction accumulator and careful control of returning liquid.
- A defrost termination thermostat ends defrost on temperature, typically 50 to 55 degrees Fahrenheit, while the timer fail-safe ends it on elapsed time if the thermostat does not close.
- Fan delay after defrost holds the evaporator fans off until the coil refreezes, preventing the box from being blasted with warm, wet air and steam.
13.2 Evaporators, Defrost Methods, and Defrost Controls
The Commercial Refrigeration sheet names "describing the purpose and applicability of a defrost cycle," "describing defrost cycle initiation and termination," "defrost heater," "defrost terminator," "mechanical or electronic defrost timer," "installing a low temperature evaporator with electric defrost," "describing a drain and drain pan heater and their operation," and "describing dry type evaporators and their operation." More service calls trace back to defrost than to any other refrigeration subsystem.
1. Why Defrost Is Mandatory
Any evaporator with a surface below 32°F collects frost from the moisture in the box air. Frost is a two-stage disaster:
- Insulation. Frost is an excellent insulator. As it builds, heat transfer falls, evaporator temperature drops further, and the coil frosts faster — a self-accelerating cycle.
- Airflow blockage. Frost bridges between fins and eventually blocks the coil face. Airflow collapses, box temperature rises, and suction pressure falls until the compressor trips on the low-pressure switch or runs continuously in a deep vacuum.
The visible symptom is deceptive: a completely iced coil in a warm box, with the compressor running continuously. Technicians new to refrigeration often diagnose a low charge. The correct diagnosis is a defrost failure.
Fin spacing is the first defense. Air conditioning coils use 10–14 fins per inch. Medium-temperature refrigeration uses 4–6. Low-temperature coils use 3–4 fins per inch, deliberately sacrificing surface area so frost has room to accumulate between defrost cycles.
2. Evaporator Construction and Airflow
A unit cooler (the evaporator assembly in a walk-in) consists of a finned-tube coil, one or more propeller fans, a drain pan, an expansion valve and distributor, and — on freezers — defrost heaters.
- Dry-type (direct-expansion) evaporators are the norm: refrigerant flows inside the tubes, superheats before leaving, and air passes over the fins. Contrast with a flooded evaporator (Section 12.2), where the tubes are submerged in liquid refrigerant.
- Airflow direction is normally horizontal, throwing air the length of the box and returning under or around the product. Correct throw matters: short-throw coils leave dead zones where product warms.
- Air velocity in a produce box is kept moderate to avoid dehydrating product, and higher in a freezer where product is packaged.
- Distributor and feeder tubes (Section 12.1) must be equal length so each circuit loads equally.
- The drain line must slope, be trapped outside the box (a trap inside a freezer freezes solid), be heat-traced through a freezer wall, and terminate with an air gap over a floor drain.
- Drain pan and drain line heaters are mandatory on freezers: melted frost that refreezes in the pan or line backs water into the box and onto the floor, producing an ice hazard and a coil that floods during the next defrost.
3. The Four Defrost Methods
| Method | How it works | Applicability |
|---|---|---|
| Off-cycle (air) defrost | The compressor stops; box air above freezing melts the frost as the evaporator fans continue running | Only above ≈ 34°F box temperature. Coolers only, never freezers |
| Electric defrost | Calrod heating elements inserted in the coil (and in the drain pan and drain line) energize while the compressor and fans are off | The standard for walk-in freezers; simple and reliable, but adds heat directly into the box |
| Hot gas defrost | A solenoid routes compressor discharge gas through the evaporator, warming it from the inside | Fastest and most efficient; requires multiple evaporators or a hot-gas source, plus an accumulator |
| Water defrost | Water is sprayed over the coil | Large industrial coils and blast freezers; not typical in light commercial |
Off-cycle defrost
The simplest possible strategy: schedule the compressor off for a period each hour or use a defrost thermostat that stops the compressor and lets the coil rise above freezing. Fans keep running so box air does the melting. It costs nothing and adds no heat, but it only works when the box air is warmer than 32°F — which is why every freezer needs something else.
Electric defrost
- The defrost timer or controller initiates: compressor off (or the liquid-line solenoid closes for a pump-down), evaporator fans off, defrost heaters on.
- Heaters warm the coil, melting frost, which drains through a heated pan and drain line.
- Termination occurs on temperature or, failing that, on time.
- A fan delay holds the fans off until the coil has refrozen.
Fans must be off during electric defrost. If they run, they blow the heaters' output straight into the box, raising product temperature, wasting energy, and preventing the coil from ever reaching termination temperature.
Hot gas defrost
The reversing-valve concept applied to refrigeration: a solenoid opens a bypass so hot discharge gas enters the evaporator, condensing inside the coil and giving up its latent heat to the frost.
- Advantages: very fast (often 5–10 minutes versus 20–30 for electric), and the heat comes from work the compressor already did rather than from resistance elements.
- Requirements: a suction accumulator, because the gas condenses in the evaporator and liquid will return to the compressor; careful attention to where the condensed refrigerant goes; and either multiple evaporators (so one can defrost on the others' load) or a supplemental heat source.
- Reverse-cycle defrost on a heat pump (Section 9.2) is the same principle implemented with a four-way valve.
4. Defrost Initiation and Termination
The initiation and termination decisions are separate, and understanding that separation resolves most defrost troubleshooting.
Initiation
| Method | Description |
|---|---|
| Time clock (mechanical or electronic) | Fixed number of defrosts per day at set times; typically 2–6 per day for a freezer |
| Accumulated run time | Initiates after a set number of compressor run hours |
| Demand (pressure differential) | Senses air-side pressure drop across the coil, which rises as frost blocks it |
| Demand (coil temperature/airflow) | Compares coil temperature to box temperature; a widening spread indicates frost |
Demand defrost is more efficient because it defrosts only when needed, but time-clock initiation remains the most common in light commercial work because it is cheap and needs no sensors.
Termination — this is the critical distinction
| Method | How it ends defrost |
|---|---|
| Temperature (defrost termination thermostat) | A bimetal or thermistor clamped to the coil closes at a set temperature — commonly 50–55°F — signaling that all frost has melted |
| Time (fail-safe) | The timer ends defrost after a maximum elapsed period, typically 20–45 minutes, regardless of temperature |
| Pressure | Used on some hot-gas systems: rising evaporator pressure indicates the coil is clear |
Temperature termination is the intended path; time termination is the backup. A coil that is clear of frost warms rapidly once the ice is gone, so the thermostat closes and defrost ends early — often in half the allotted time. If the thermostat fails open, the timer still ends defrost at the fail-safe limit, so the box does not stay in defrost forever.
The diagnostic value of this pair is high. A system that always runs the full defrost period and terminates on time is telling you the termination thermostat has failed open, is not making contact with the coil, or the heaters are not clearing the frost. A system that terminates almost immediately is telling you the thermostat is stuck closed or is mounted somewhere that gets warm too quickly.
Fan delay
After defrost ends, the coil is warm and wet. If the fans start immediately, they blow warm humid air and water droplets into the box, raising product temperature and depositing moisture that becomes frost within minutes. A fan delay switch (a separate bimetal, typically closing near 25–30°F) or a controller-based delay holds the fans off until the coil has refrozen and any residual water has frozen in place or drained.
A missing or jumpered fan delay produces a distinctive symptom: fog or "steam" rolling out of the unit cooler after defrost, wet floors, and a box that recovers slowly.
5. Defrost Troubleshooting Patterns
| Symptom | Likely cause |
|---|---|
| Coil solidly iced, box warm, compressor running continuously | Defrost not occurring at all: failed timer/controller, open heater circuit, blown defrost fuse, or failed defrost contactor |
| Coil iced only on the entering-air face | Insufficient defrost duration or frequency; check termination and initiation settings |
| Defrost always runs to the time fail-safe | Termination thermostat failed open, loose from the coil, or heaters not producing full output (one element open) |
| Box temperature spikes during every defrost | Defrost too long or too frequent, fans running during defrost, or fan delay defeated |
| Ice building in the drain pan and on the floor | Drain pan or drain line heater failed, drain line trapped inside the box, or drain line slope inadequate |
| Fog rolling out of the unit cooler after defrost | Fan delay failed closed or jumpered |
| Frost only on the suction line, not the coil | Overfeeding metering device or low load — a refrigerant-side problem, not a defrost problem |
Measure the heaters. An electric defrost assembly is a set of parallel elements. One open element reduces total wattage and lengthens defrost without eliminating it. Measure resistance of each element and amperage during defrost, and compare against the nameplate.
A walk-in freezer's electric defrost always runs the full 30-minute timer period and terminates on time rather than on temperature, and light frost remains on the coil. What is the most likely cause?
Why must the evaporator fans be off during an electric defrost cycle?
Why is off-cycle defrost unsuitable for a walk-in freezer?