Choosing a 3–20 kW Condensing Unit for a Small Cold Room

After more than ten years in the cold room business, the question we get asked most is: how big a unit does my small cold room need?

The answer up front: selection takes three steps — calculate the load first, then fix the room temperature duty, and only then choose air-cooled, water-cooled or inverter. Get the order wrong and start comparing prices and brands, and nine times out of ten the job has to be redone.

Why? Two units both labelled 20 kW can differ several fold in cooling capacity depending on the duty. Buy one for an ambient duty, pull it down to -18°C and find it is not enough, strip it out and reinstall, and both money and schedule are gone. Each step is broken down below, with real numbers.

Calculate the Load, Fix the Duty, Then Choose the Model

Step one is the load. Room volume, what is stored, daily turnover, how often the door opens and how hot local summers get all determine the cooling requirement. A small cold room does not need a design institute, an estimate is enough, but a room with frequent loading has a far larger load than one kept permanently full, so do not apply a single factor across the board.

Step two is the evaporating temperature. This is where most people go wrong. A fresh-keeping room runs 0–5°C, a chilled room around -18°C and a deep-freeze room below -25°C. A difference in evaporating temperature costs a large amount of cooling capacity, so when you read a parameter table you must look at the column for your room temperature.

Step three is the model. Take LZAC-12: 17.53 kW at a 20°C duty and only 4.8 kW at -20°C, a 3.6 fold difference on the same machine. LZAC-20 is even clearer, falling from 27.9 kW to 7.0 kW, nearly 4 times. So never look only at the big number on the nameplate — check the data against your room temperature.

Air-Cooled, Water-Cooled or Inverter: The Real Numbers Side by Side

In the LZ series, LZAC is air-cooled fixed speed with seven models covering 3.0–83 kW, LZVAC is inverter with three models and LZWC is water-cooled with eight models. The temperature range is -35°C to +20°C and refrigerants are R410A and R404A. Three representative units compared:

The unit cooler supplied with the LZAC-12 is the LZAE-4002-25, with an air flow of 3800 m³/h, which is enough for small and medium rooms.

Match the Model to the Application

Air-cooled fixed speed: the default for most cases. Produce fresh-keeping rooms, meat chillers and seafood rooms with outdoor space are simple with air cooling — connect power and it runs. The LZAC-12 leaves the factory pre-charged with 5 kg of R410A, so after connecting the pipework and evacuating, the machine can be started without adding refrigerant.

Water-cooled: for plant rooms indoors, where there is no outdoor unit position, or where noise is sensitive. The LZWC-12 uses a shell and tube condenser at 63 dB(A), noticeably quieter than the 68 dB(A) of air cooling, and installs indoors. The price is the cooling water: 3 m³ per hour, with water charges, pipework and pumps to add to the cost. Factories that already have circulating water find it economical.

Inverter: for places with strict constant temperature requirements, such as mushroom rooms, cooked food pre-cooling rooms and laboratory cold rooms. The LZVAC-12 uses a DC inverter scroll compressor with PID control, three modes of cooling, heating and constant temperature, a control accuracy of ±0.5°C and an ambient range from -35°C to +48°C; it works even where winter temperatures drop to minus thirty or forty degrees, and the heating mode prevents the room temperature falling too low. An ordinary freezer room does not need that precision — fixed speed is enough and the savings buy a fair amount of stock.

Pairing with a Vacuum Cooling Line

This part is directly connected to our own projects. We held a round of food vacuum chiller project reviews on 12 August; for customers in braised products and cooked food, the product leaves the pot at 80–90°C and goes through a vacuum chiller first, where the core temperature drops quickly to the 0–8°C end point, and then straight into the cold room to continue the cold chain. That is when the work of the cold room condensing unit truly begins: the vacuum chiller pulls the temperature down fast and the cold room unit holds the chain, so the core temperature of the product does not rebound while the room temperature stays stable. Vacuum cooling delivers speed, the cold room delivers stability, and you need both.

One reminder: do not confuse a produce pre-cooler with a food vacuum chiller. Produce at 25–30°C uses a produce pre-cooler before storage, while hot food at 80–90°C uses a food vacuum chiller, and the cold room unit finishes the job for both lines. If it is unclear, ask rather than guess.

Frequently Asked Questions

Q1: How big a cold room will a 3–20 kW unit serve?
It cannot be stated directly by floor area; it depends on room temperature and insulation. The same LZAC-12 gives 17.53 kW at a 20°C duty and only 4.8 kW at -20°C. Fix the room temperature first and calculate from the load; the rule of thumb of so many square metres per horsepower is ambient air conditioning logic and does not apply to cold rooms.

Q2: What is the difference between air-cooled and water-cooled?
Air-cooled is simple: put it outdoors, connect power and it runs. Water-cooled is quiet and installed indoors but needs cooling water; the LZWC-12 needs 3 m³ per hour, which is not a small cost over time. Water cooling is worth it only when there is no outdoor unit position or noise is a hard requirement.

Q3: The inverter unit costs more than fixed speed — is it worth it?
It depends on the application. For rooms needing tight temperature control, yes. The LZVAC-12 holds ±0.5°C and has a heating mode, which suits mushroom rooms, cooked food pre-cooling rooms and laboratories. An ordinary freezer room is fine with fixed speed.

Q4: What does factory pre-charged refrigerant mean?
The refrigerant is charged at the factory before shipping, for example the LZAC-12 with 5 kg of R410A, so on site you connect the pipework, evacuate and open the valves to start, without charging refrigerant yourself. If the pipe run is very long, a small top-up is added during installation.

Q5: Only 4.8 kW at the -20°C duty — is that a false rating?
No. The lower the evaporating temperature, the more cooling capacity falls; that is a physical characteristic of the compressor and applies to every manufacturer. Selection means matching the nameplate data to your room temperature duty; once the duty matches, the number is real.

Q6: How loud is it?
The air-cooled LZAC-12 is 68 dB(A) and the water-cooled LZWC-12 is 63 dB(A). If the room is close to housing or needs to be quiet at night, prioritise water cooling or add acoustic treatment to the outdoor unit.

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