Cold Storage Engineering · Practical How-To Guide

How to Build a −25°C Freezer Room: A Step-by-Step Guide for a 3 m × 4 m × 2.5 m Cold Storage Room

This guide walks through the full engineering path for a small walk-in freezer in Xi'an, China — from cooling-load calculation and compressor selection to panels, valves, controls, refrigerant and commissioning. Every figure below follows one worked example: a 30 m³ room held at −25°C for storing already-frozen goods.

3.0 × 4.0 × 2.5 m · 30 m³ · −25°C · R404A · 2.5 HP low-temp unit · DJ-2.5/15 evaporator

Project Brief

1. What we are building — and the assumptions behind it

Before choosing any equipment, the engineer must fix the project definition. A freezer designed for storage of frozen goods is very different from a room that must freeze fresh product inside it; confusing the two is the most common sizing mistake.

This worked example assumes the following conditions. If any of them differs on site, the load calculation and equipment selection must be repeated.

• Purpose: storage only. The room holds product already frozen to a core temperature of −18°C or below; it does not perform the freezing process itself.
• Location: the room sits inside a ventilated workshop, out of direct sun. The condensing unit is also sheltered and well aired.
• Goods flow: about 5% of room volume enters per day, at frozen temperature; normal, not heavy, door-opening frequency.
• Power supply: 380 V, three-phase, 50 Hz. A 220 V single-phase site would require a different single-phase low-temperature unit.
• Climate: a northern Chinese city (Xi'an / Zhengzhou type). Summer outdoor design dry-bulb around 35°C; the workshop air around 32°C.

Step 1 · Cooling Load

2. Size the cooling load before picking a compressor

The cooling load is the total heat that must be removed from the room every hour to hold −25°C. Chinese cold-storage design standard GB 50072-2021 breaks it into four components: heat leaking through the insulated envelope, heat from the goods, heat released by the evaporator fan motors, and heat from people, lighting and door openings.

Design working conditions

The evaporator does not run at room temperature — it must be colder than the air it cools. With a 6 K temperature difference between room air and evaporating refrigerant:

• Evaporating temperature T_e = −25 − 6 = −31°C
• Condensing temperature T_c ≈ 35 + 12 = 47.2°C (air-cooled condenser, 12 K rise above summer dry-bulb)

The four heat loads in this room

Adding the four components gives the cooling-equipment load Q_e ≈ 1,022 W (1.02 kW). Piping losses and a safety margin are then applied: a 15% margin gives 1,175 W, and a conservative 25% margin gives Q_m ≈ 1,277 W (1.28 kW). This 1.28 kW at the −31°C / 47°C working condition is the figure the compressor must deliver — not a nominal room-volume rule of thumb.

Step 2 · Condensing Unit

3. Select the condensing unit (compressor + condenser)

The condensing unit sits outside the room and carries the high-pressure side of the cycle: compressor, air-cooled condenser, fan, receiver and frame. For this room it must deliver at least 1.28 kW of cooling at −31°C evaporating and 47°C condensing on R404A.

Step 3 · Evaporator

4. Choose the ceiling evaporator (air cooler)

The evaporator sits inside the room, mounted to the ceiling. It absorbs heat from the air blown across its finned coil. For a −25°C room the coil operates near −31°C, so frost forms quickly and fin spacing matters: narrow fins clog with ice, wide fins keep airflow stable between defrosts.

Step 4 · Insulated Shell

5. Build the insulated room shell

The refrigeration machine only removes heat that reaches the coil; the panel envelope stops heat entering in the first place. For a −25°C room the shell must be a continuous thermal barrier with no cold bridges at corners, joints, door frame or floor.

Shell components for this room

• Insulated panels: 150 mm double-color-steel PU / PIR panels, foam density 40 ± 2 kg/m³, λ ≤ 0.024 W/(m·K); wall, ceiling and floor panels, about 59 m² of panel area in total. Sun-exposed or conservative projects thicken the panels to 180 mm.
• Door: one single-leaf half-buried hinged door (or sliding door for goods traffic), opening about 900 × 1,900 mm. It carries a door-frame heater and observation-window heater, EPDM seals, an inside-opening escape latch and a viewing window.
• Floor: 150 mm floor insulation under a reinforced-concrete wearing slab. At −25°C the ground below cannot sit directly against uninsulated concrete: a ventilated air gap or floor heating wires protect against frost heave.
• Pressure relief port: a panel-mounted pressure equalization port balances inside and outside pressure so the door opens easily and panel joints do not leak as temperature swings.
• Fasteners and seals: hangers, angle aluminum, silicone sealant, self-tapping screws and connectors, included in the panel package.

On-site panel assembly runs ceiling panels first, then walls and floor; corner joints are sealed, and evaporator hangers are embedded while the ceiling is being laid.

Step 5 · Valves, Piping & Insulation

6. Lay out the valves, copper piping and insulation

Between the condensing unit and the evaporator runs the refrigerant circuit: a liquid line carrying condensed refrigerant, and a suction line returning cold vapor to the compressor. Valves in this circuit meter the refrigerant, shut off flow on stop, remove moisture and protect the compressor.

Valve and protection package

Piping sizes and insulation

Suction and liquid lines are wrapped in B1 closed-cell rubber insulation at least 25 mm thick, with elbows, tees, joint sleeves, binding tape and sealant included; insulation must be continuous with no cold bridge. The internal drain line carries an electric heating cable and a U-shaped water seal, and slopes out of the room so meltwater cannot freeze and block the drain.

Step 6 · Controls & Heating

7. Wire the controls, door heaters and lighting

A low-temperature room is controlled by a dedicated cold-room electrical panel: it reads room temperature, starts and stops the compressor, schedules defrost, runs the fans, and trips out on high pressure, low pressure, phase loss, overload or alarm conditions. The panel also powers the anti-frost heaters that a −25°C room cannot do without.

• Control panel: Elitech ECB-5060 (or equivalent New Asia / Sanhe panel). Room thermostat, defrost by timer plus temperature cutoff, fan and condenser-fan control, high/low pressure protection, phase sequence and loss-of-phase protection, compressor overload, alarm output terminals; optional remote monitoring module.
• Interior lighting: two IP65 moisture-proof LED cold-room lamps, 20–30 W each, spaced to cover the room without blowing directly onto product.
• Door and window heaters: frame heater and observation-window heater, interlocked with the thermostat and door control. Without heat, the frame freezes solid, seals stick and the door cannot open.
• Cabling and trunking: power and control cables for compressor, fans, defrost heaters and lights; trunking, conduit and terminals; wire cross-section sized to each motor's rated current.

Step 7 · Refrigerant & Oil

8. Choose the refrigerant and lubricating oil

Low-temperature freezer systems of this class use R404A (R507A is an equivalent alternative). Every valve, seal, hose and oil selection must match this refrigerant; mixing refrigerants or oil families destroys compressor lubrication.

• Refrigerant charge: R404A, charged by weight according to the compressor nameplate and actual piping length. A small system like this typically references 2–4 kg; the real figure follows the completed circuit.
• Compressor oil: ester (POE) refrigeration oil matched to R404A and the compressor model, supplied pre-charged with the unit and topped up as specified.
• Hard rule: mineral oil and ester oil must never be mixed; R404A must never be blended with R22 or R134a in the same circuit.

Step 8 · Installation & Commissioning

9. Install, pressure-test, evacuate and commission

Mechanical installation is followed by a strict commissioning sequence. Skipping the pressure test or the vacuum step leaves moisture and non-condensable gas inside a low-temperature circuit, and that causes frosting, acid formation and compressor failure within weeks.

1. Erect the shell and hang the evaporator. Assemble panels ceiling → walls → floor, seal every corner joint. Embed and level the evaporator hangers while the ceiling goes up; direct outlet air away from dead corners around the product stack.
2. Brazed copper piping and electrical runs. Braze suction, discharge and liquid lines under dry nitrogen, fit the valve package, then run power and control wiring to the panel. Support lines on hangers with proper suction-line slope back toward the compressor.
3. Nitrogen pressure-hold leak test. Pressurize the circuit with dry nitrogen and hold pressure for the required period; watch the gauge for drift and check every joint with leak detector before venting.
4. Evacuate to vacuum. Pull the circuit down to the specified vacuum to remove air, moisture and non-condensable gas; break the vacuum only with the correct refrigerant.
5. Weigh in the R404A charge. Charge on the scale to the calculated weight, then verify liquid condition at the sight glass and adjust the expansion-valve superheat.
6. Pull-down, defrost and protection tests. Run an empty-room pull-down to −25°C, then test defrost start/stop, high- and low-pressure cut-outs, thermostat stop, door-heater function and alarm interlock. Tune the defrost cycle length to actual frosting before handover.

Equipment Bill of Materials

10. Full equipment list for this 30 m³, −25°C room

The table below consolidates the complete refrigeration package plus supporting shell and installation items grouped by system. Model classes are engineering references; final part numbers follow the manufacturer's catalog and site survey.

Before You Order

11. Questions to confirm before pricing or buying

The scheme above holds only under its stated assumptions. Confirm the following on site, and repeat the load calculation if any answer changes the picture.

1. Product and duty: what goods are stored, entry temperature, daily quantity, and does the room have to freeze product inside? Freezing duty changes the selection completely.
2. Placement: indoor or outdoor, shelter and ventilation for the condenser, and the actual piping distance between condensing unit and evaporator.
3. Power supply: 380 V three-phase or 220 V single-phase, and whether the supply capacity meets the rated current.
4. Door and floor: pedestrian door versus goods sliding door, door opening size, and whether floor anti-frost-heave, remote monitoring or an air curtain are required.

About this guide. It documents one worked engineering example for a 30 m³, −25°C storage freezer. Compressor, evaporator and valve model numbers are reference selections in the same performance class; final selection must be cross-checked against the manufacturer's catalog at the exact evaporating/condensing temperatures and refrigerant, and against actual site piping length, power supply and goods flow.

Standards referenced: GB 50072-2021 Code for Design of Cold Storage; GB/T 30134-2013 Code for Management of Cold Storage; GB 50736-2012 Design Code for Heating, Ventilation and Air Conditioning of Civil Buildings (outdoor design parameters).