Import & Export Guides
Mini Cold Storage Evaporator Selection: Fin Spacing, Air Throw and Humidity Explained
Oct
Most buyers of a mini cold storage spend their attention on the compressor and the insulated panels. The evaporator, the fan-and-coil box hanging from the ceiling inside the room, usually gets accepted as whatever comes in the kit. That is a mistake, because the evaporator is the part that actually touches your product’s air. It decides how evenly the room cools, how much moisture is pulled out of your potatoes or fish, and how often the system has to stop and defrost. Two rooms with an identical compressor can perform very differently purely because of the evaporator inside them.
This guide explains what to look for when choosing or checking an evaporator (also called a unit cooler or air cooler) for a small cold room in Bangladesh, and how to read the datasheets Chinese manufacturers send with their quotations.
What the evaporator actually does
Inside the evaporator, liquid refrigerant boils at a temperature below the room setpoint. Room air is pulled through a block of thin aluminium fins bonded to copper tubes, gives up its heat to the boiling refrigerant, and is blown back into the room colder than it arrived. Two things happen at the same time: the air is cooled (sensible cooling) and some of the water vapour in it condenses or freezes on the cold fins (latent cooling, or dehumidification).
That second effect is why the evaporator matters so much for fresh produce. Every gram of water that ends up as frost on the fins came either from outside air entering through the door or from your product itself. If the coil pulls too much moisture, leafy vegetables wilt, fruit loses weight and shines less, and potatoes shrivel. Weight loss is money loss when produce is sold by the kilogram.
Temperature difference (TD): the number that controls humidity
The single most important design idea for a produce room is the temperature difference, usually written TD or DT1, between the air entering the coil and the refrigerant evaporating temperature inside it. A coil running with a small TD is large relative to the load and works gently: the fin surface is only a little colder than the room air, so less moisture condenses out and relative humidity stays high. A coil running with a large TD is small relative to the load and works hard: very cold fins, more condensation, drier air.
In practical terms:
- Rooms for fresh fruit, vegetables, flowers and eggs benefit from a generously sized coil running at a low TD, because high humidity protects the product.
- Rooms for frozen fish, meat or ice cream are less sensitive, because the product is already frozen and packed, so a higher TD is acceptable.
The exact TD that suits a particular crop depends on the product and how long it is stored, so this is a conversation to have with whoever sizes the system. The key point is that a cheaper, smaller coil is not just a smaller version of the same thing; it changes the humidity your product lives in. Our separate guide on humidity control in mini cold storage goes deeper on this.
Fin spacing: why frozen rooms need wider gaps
Fin spacing (fin pitch) is the gap between the aluminium plates, normally quoted in millimetres on Chinese datasheets. Tight spacing packs more surface area into a small box, which is efficient when the coil stays free of frost. Wide spacing gives less surface area for the same size but leaves room for frost to build up before the airflow chokes.
For a chiller room running above freezing, frost is light and melts off easily, so closer fin spacing is normal. For a freezer room, every door opening brings humid Bangladeshi air inside, and that moisture freezes onto the fins almost instantly. A tightly finned coil in a freezer room blocks with ice quickly, airflow drops, the room warms, and the system defrosts so often that it struggles to hold temperature. Blast freezers and rooms receiving warm, wet product (fresh fish, for example) need the widest spacing of all.
When you compare quotations, check that the fin pitch matches the room’s duty. A supplier who offers the same evaporator model for a chiller room and a freezer room without changing the fin spacing has probably not thought about your application.
Air throw and coil placement
Air throw is how far the fan can push a usable stream of cold air before it slows and mixes with the room. Manufacturers list it on the datasheet. If the throw is shorter than the length of the room, the back corner becomes a warm pocket where product spoils first, even though the controller near the coil shows the correct temperature.
Some placement rules that apply to almost every small room:
- Mount the evaporator on the short wall and blow along the long dimension, so the throw covers the whole room.
- Do not mount it directly above the door. The fan will pull warm air in every time the door opens, and the coil will frost faster.
- Leave clear space behind the coil for air to return. Stacking cartons right up against the back of the evaporator starves it of air.
- Keep the air path above the stack clear. Product stacked to the ceiling blocks the throw entirely.
Our article on airflow, stacking and clearance covers the stacking side in more detail.
Defrost, drain line and the freezer room trap
All evaporators below about freezing need a defrost method. Small rooms usually use electric heaters fitted inside the coil, timed by the digital controller. Above-freezing rooms can often defrost simply by stopping the compressor and letting the fan run. The defrost cycles guide explains the options.
The detail that catches many installations is the drain. Melted frost has to leave the room through a drain pipe. In a freezer room, that pipe passes through air below freezing, so the water can refreeze inside it, back up into the drip tray and form a block of ice that eventually lifts the tray or spills onto product. Freezer evaporators therefore need a heated drain line, a correct slope, and a trap located outside the cold space. Ask the supplier specifically whether a drain line heater is included, because it is easy to leave out of a kit price.
How to compare evaporator datasheets from Chinese suppliers
Chinese factories quote evaporator capacity in watts or kilowatts, but that number is only meaningful together with the conditions it was measured at: the room air temperature, the evaporating temperature and therefore the TD. One supplier may rate a coil at a large TD, which makes the capacity look high, while another rates a comparable coil at a smaller TD and looks weaker on paper. Comparing the two headline numbers directly is misleading.
| What to check | Why it matters |
|---|---|
| — | — |
| Rated capacity and the TD it was rated at | Capacity at different TDs cannot be compared directly |
| Fin pitch in mm | Must suit chiller or freezer duty |
| Air volume and air throw | Throw must cover the full room length |
| Fan motor voltage and phase | Must match the supply at your site |
| Defrost heater power and drain heater | Freezer rooms need both |
| Coil material and fin coating | Coastal sites may need corrosion-resistant fins |
| Refrigerant the coil is designed for | Must match the condensing unit |
If you are importing a room as a package, the evaporator, condensing unit and expansion device should be selected together as one balanced system. If you are buying an evaporator on its own to replace or upgrade a coil, send the supplier the room size, setpoint, product, door opening pattern and existing condensing unit details, and ask them to state the rating conditions in writing.
Common mistakes and what they cost
- Undersized coil in a produce room: the system runs at a large TD, humidity falls, and stored vegetables lose weight week after week. The loss is invisible until you weigh the stock out.
- Chiller-spec coil in a freezer room: fins block with ice, defrosts become frequent, and the room swings in temperature.
- Coil above the door: frost builds rapidly, and humid outside air is pulled straight across the coil on every opening.
- No drain heater on a freezer room: ice blocks the drain, water overflows the tray, and product below gets ice damage.
- Coastal installation with plain aluminium fins: salty air corrodes the fins over time, reducing capacity. Ask about coated fins for sites near the coast, such as in Cox’s Bazar or Khulna.
- Ignoring fan voltage: a fan rated for a different supply runs hot or fails early, and the replacement may need to be imported.
Before you approve the order
Ask for the full datasheet of the exact model, and ask how the coil is protected for shipping, because bent fins reduce airflow. Pricing, capacity and lead time all depend on your room size, product and site conditions, so there is no single answer we can give here; ask us for a quote built around your specific room and we will compare options for you.
Planning a new mini cold storage or upgrading an existing one? DE International sources complete rooms and individual components from vetted Chinese manufacturers and handles shipping and clearance to Bangladesh. See our services, browse equipment in our shop, use our China sourcing and buying agent service, or contact us with your room details. For the bigger picture, read our heat load and sizing guide and compressor types explained.
