To select a cold room evaporator, match its capacity to the room load at your chosen temperature difference (TD) between room air and evaporating temperature, pick fin spacing suited to the room temperature, check that air throw covers the room length, and choose a defrost method that fits the temperature. In practice that means DL-type coolers with fine fins for chillers, DD-type for −18 °C stores, DJ-type with wide fins for −25 °C and below, and electric defrost for any room below about +2 °C.
What do DL, DD and DJ evaporator types mean?
Many manufacturers in China classify cold room air coolers by temperature range. The letters identify the intended duty, which determines fin spacing and defrost arrangement.
| Type | Typical room temperature | Typical evaporating temperature | Typical fin spacing | Defrost |
|---|---|---|---|---|
| DL (high temperature) | About 0 °C (−2 to +10 °C) | About −10 °C | About 4.5 mm | Off-cycle or electric |
| DD (medium temperature) | About −18 °C | About −25 °C | About 6 mm | Electric |
| DJ (low temperature) | About −25 °C | About −32 °C | About 9 mm | Electric |
| Blast freezer coolers | −35 to −40 °C | About −42 to −45 °C | About 10–12 mm | Electric or hot gas |
Exact ranges vary between catalogs, so always read the rated conditions on the data sheet.
Why does fin spacing matter?
Frost forms on the fins whenever the coil surface is below 0 °C. Tight fin spacing gives more surface area in a compact coil, which suits chillers where frost is light. In freezers, frost builds faster and can bridge narrow gaps, choking airflow and cutting capacity between defrosts. Wider fins tolerate more frost but need a larger coil for the same capacity. Using a chiller coil in a freezer is a common cause of frequent defrost and rooms that drift warm.
How does temperature difference affect humidity and product quality?
The TD is the difference between room air temperature and the refrigerant evaporating temperature. A small TD means a larger coil with a warmer surface, which removes less moisture from the air. A large TD means a smaller, cheaper coil that dries the air and the product.
| TD | Approximate room RH | Suitable for |
|---|---|---|
| 4–6 K | 90–95% | Leafy vegetables, flowers, unwrapped produce |
| 6–8 K | 85–90% | Most fruit and vegetables, fresh meat |
| 8–10 K | 80–85% | Dairy, packaged food, general chillers, frozen storage |
| 10–12 K | 70–80% | Packaged goods, dry storage, some processing rooms |
The evaporator and condensing unit must be selected at the same evaporating temperature. If the coil is undersized, the system finds a lower balance temperature, widening the TD, drying the product and increasing frost.
How do you size and place the evaporator?
Capacity
Take the required refrigeration capacity from the load calculation and choose a cooler, or several coolers, that deliver it at your TD and room temperature. Most catalogs rate capacity at a standard TD such as 8 K or 10 K; apply the manufacturer's correction factors for other conditions.
Air throw and number of fans
Air throw is the distance the cooler can push air effectively. It should reach at least the far wall of the room. In long rooms, use two coolers facing the same direction or mounted at opposite ends. Avoid blowing directly onto product at close range, which dries the surface.
Placement rules
- Mount the cooler so air flows along the longest dimension of the room.
- Avoid mounting directly above the door, where it draws in warm, moist infiltration air and frosts quickly.
- Leave at least 300–500 mm between the coil inlet and the wall, or as specified by the manufacturer, for return air.
- Keep stacked product below the air stream with space at the top of the stack for circulation.
- Allow room for drain pan removal and fan maintenance.
Which defrost method should you use?
| Method | How it works | Best for | Notes |
|---|---|---|---|
| Off-cycle (air) defrost | Compressor stops; fans keep running and room air melts frost | Rooms above about +2 °C | Simple, no extra energy; not for freezers |
| Electric defrost | Heaters in the coil and drain pan melt frost | Freezers and most chillers below +2 °C | Reliable and easy to control; uses electricity |
| Hot gas defrost | Hot discharge gas is routed through the coil | Larger systems, blast freezers | Faster and energy-efficient; more complex piping and controls |
| Water defrost | Water spray melts frost on the coil | Large industrial coolers and blast freezers | Needs water supply and drainage; risk of freezing pipes |
How should defrost be controlled?
A good defrost cycle follows a sequence:
- Initiation by timer at a set interval, typically 3–6 times per day for freezers and 2–4 for chillers. Demand-defrost controllers can start defrost based on coil condition.
- Pump-down or solenoid closure, so the compressor stops and refrigerant is held in the receiver.
- Heating with coil heaters or hot gas, with fans off to keep heat on the coil.
- Termination when the coil sensor reaches about 10–15 °C, with a maximum time of about 20–40 minutes as a safety limit.
- Drip time of 2–5 minutes so water drains completely.
- Fan delay after refrigeration restarts, until the coil is below about 0 °C, so water droplets are not blown into the room and heat is not pushed onto product.
What about the drain line?
In freezers, defrost water must leave the room before it refreezes. Use a heated drain pan, a self-regulating heater cable inside or along the drain line through the cold zone, a slope of at least 1:50 toward the outlet, and a trap outside the room to stop warm air entering. Ice in the drain is one of the most common freezer service calls.
Worked example: selecting an evaporator for a −18 °C store
Assume the load calculation for a 6 × 5 × 3 m frozen store gives a required capacity of 5.0 kW at −25 °C evaporating, which is a TD of 7 K below the −18 °C room temperature.
- Type: a DD-type cooler for −18 °C duty, with fin spacing around 6 mm and electric defrost.
- Rated condition: suppose the catalog rates capacity at a TD of 10 K. Capacity at 7 K is roughly 70% of the rated value, because capacity is roughly proportional to TD for a given coil and airflow.
- Required catalog capacity: 5.0 ÷ 0.7 ≈ 7.1 kW at the 10 K rating.
- Air throw: the room is 6 m long, so the cooler's throw should be at least 6 m, ideally with some margin. A two-fan model mounted on the 5 m wall, blowing along the 6 m length, fits well.
- Defrost: check heater power and voltage match the supply, and plan a heated drain line through the room.
If the catalog's correction method differs, use the manufacturer's selection software or tables. The key point is to compare coolers at the same TD as the condensing unit selection.
What maintenance keeps an evaporator efficient?
- Inspect the coil for ice build-up, especially after door-heavy days or power failures.
- Clean the coil and drain pan periodically with a suitable non-corrosive cleaner.
- Check that drain heaters and defrost heaters draw current and that termination sensors are secure.
- Listen for fan bearing noise and check fan blades for ice damage.
- Confirm superheat during routine service, because a drifting expansion valve wastes capacity.
Which other features should you check on a data sheet?
- Coil material: copper tubes with aluminum fins are standard; coated or epoxy fins suit salty or acidic environments such as seafood rooms.
- Fan motor type and voltage: standard external-rotor axial fans, or EC fans for lower energy use.
- Casing material: painted steel, aluminum or stainless steel for washdown areas.
- Heater power and voltage for electric defrost.
- Refrigerant and expansion valve type, external equalizer and distributor size.
Our DL, DD and DJ air cooler evaporators include electric defrost on freezer models and use YWF external-rotor axial fans. Pair them with a matched condensing unit or send your room data for a selection.
Frequently asked questions
What is the difference between DD and DJ evaporators?
DD coolers are designed for about −18 °C rooms with fin spacing around 6 mm, while DJ coolers are designed for about −25 °C rooms with wider fins around 9 mm to tolerate heavier frost.
How often should a freezer evaporator defrost?
Most freezers defrost 3–6 times per day, adjusted for door traffic and humidity. Each defrost typically lasts 20–30 minutes and should end when the coil sensor reaches about 10–15 °C.
Does a chiller room need electric defrost?
Rooms above about +2 °C can usually use off-cycle defrost. Chillers held near 0 °C or with low evaporating temperatures often need electric defrost to keep the coil clear.
Why is my evaporator icing up?
Common causes are failed defrost heaters or sensors, too few defrost cycles, a blocked drain, door left open, low refrigerant charge, or a coil with fin spacing too tight for the room temperature.
