Problem
Cold room temperature won't pull down, and most people's first reaction is that the compressor is broken. In our business with LZ series cold room units, the situation we encounter is exactly the opposite: the compressor is rarely the problem. It is the copper tube between the condensing unit and the cold room ceiling-mounted unit cooler. Pipe diameter, pipe length, and routing determine how much refrigerant actually reaches the evaporator.
Cause
Every LZ series unit ships with connection pipe sizes marked. These numbers are not reference values; they are derived from pressure drop calculations based on the model's refrigerant charge and cooling capacity.
The air-cooled fixed-speed LZAC series has seven models:
| Model | Refrigerant | Liquid line / suction line | Factory charge |
|---|---|---|---|
| LZAC-12 | R410A | 12.7mm / 19.05mm | 5kg |
| LZAC-20 | R404A | 15.88mm / 22mm | 7kg |
| LZAC-25 | R410A | 19.05mm / 28mm | 10kg |
| LZAC-30 | R410A | 19.05mm / 28mm | 11kg |
| LZAC-35 | R410A | 19.05mm / 28mm | 12kg |
| LZAC-50 | R410A | 22mm / 35mm | 19.5kg |
| LZAC-65 | R410A | 22mm / 35mm | 25kg |
The water-cooled LZWC series follows the same logic, but is smaller. LZWC-07 connections 12.7mm / 15.88mm, charge 2.2kg; LZWC-12 connections 12.7mm / 19.05mm, charge 3.7kg. Water circuit connection DN25; the shell-and-tube condenser requires 2 to 3 m³/h of circulating water.
Principle
There are two hard rules for refrigerant piping.
First, liquid line pressure drop must be controlled within 0.5 bar. If the liquid line is undersized, flash gas will appear before the electronic expansion valve. If EEV liquid supply is insufficient, the evaporator is starved of refrigerant, and the room temperature will never reach the setpoint. The compressor will show high discharge temperature and continuous hot running, but actual mass flow will keep decreasing.
Second, suction line pressure drop must not cause the evaporating temperature to drop by more than 1 to 2°C. An oversized suction line is also a problem—flow velocity is too low, oil return is poor, oil accumulates in horizontal pipe sections, and the compressor becomes oil-starved.
A vertical suction riser requires an oil return trap every 4 to 6 m of rise. This is standard practice, and it is also the step most often omitted. The oil return trap collects oil and lets the gas carry it up in batches, keeping the compressor oil level normal.
Solution
LZ units use ball valve connections, not brazed stub tubes. This is an advantage, but it must be used correctly. Ball valves can isolate the unit separately, so service does not require recovering the entire system's refrigerant, and pressure leak testing is also convenient. But conversely, it requires the installer to braze the field joints before opening the valves. If welding with the valve open under pressure, there is no nitrogen protection inside the pipe, oxide scale will form inside the copper tube, and this oxide scale goes directly into the EEV and compressor. Most of the first-month failures we encounter in trade after-sales occur at this step.
Three things must be done right on installation day:
- Nitrogen purge throughout the entire brazing process. When brazing each joint, flow low-pressure nitrogen through the pipe to prevent oxide scale formation and keep the pipe interior clean.
- Respect the factory charge. The factory refrigerant is only enough for the condensing unit plus a short section of connecting pipe. If the unit is more than about 5 m from the cold room unit cooler, refrigerant must be added. Weigh in additional refrigerant according to the added pipe length; do not guess based on gauge pressure.
- Insulate the suction line and add oil return traps. Insulation keeps superheat stable, and vertical riser oil return traps ensure compressor oil level.
Real Data
Cooling capacity varies greatly with room temperature. LZAC-12 single-EEV condition: 17,530W at +20°C room temperature, 8,550W at 0°C, and only 4,800W at -20°C. The same machine, when used as a freezer, has only about one-fourth the cooling capacity of the high-temperature condition. Therefore, selection must be back-calculated from the target room temperature; do not order based on the largest number on the nameplate.
Large model LZAC-50 single-valve condition delivers 70,500W at +20°C and 18,800W at -20°C; the condensing unit weighs 420kg, and the ceiling-mounted unit cooler has two DN32 drain ports. LZAC-65 achieves 83,125W at +20°C, the unit weighs 630kg, and the R410A charge is 25kg. Mounting brackets, service access, and pipe supports must all follow these numbers. A 630kg condensing unit cannot be supported by wall brackets; a steel frame must be used.
Installation Sequence
- First place the condensing unit on the steel frame and level it. Check the condenser fan discharge direction; do not face it into the prevailing wind, and keep the discharge side unobstructed.
- The ceiling-mounted unit cooler drains naturally by gravity; connect it to the DN32 drain port. The LZ series, from LZAE-4001-15 to LZAE-5504-125, all use DN32 drains; large models use two. If the drain pipe has no slope and no trap, condensate will back up onto products in the cold room.
- Route the liquid line and suction line. Keep the liquid line short and straight; slope the horizontal suction line slightly toward the compressor.
- Nitrogen-purge braze each joint; do not open the ball valves yet.
- Pressure test with nitrogen to the nameplate test pressure and hold, then evacuate to below 500 microns before opening the valves.
- Open the ball valves; check the sight glass if there is one; add refrigerant if the piping is long; start up.
Common Questions
Q1: Can the copper pipe be one size smaller than the connection pipe size? No. The connection size is the minimum requirement for that model. A smaller pipe increases pressure drop, generates flash gas, and reduces effective cooling capacity. For longer piping, use the same pipe diameter and add refrigerant; do not reduce the diameter.
Q2: What happens if the piping is longer than the factory charge covers? The system will be short of refrigerant. Suction pressure will be low, the evaporator will frost unevenly, and pull-down will be very slow. Weigh in additional refrigerant according to the added length. R410A must be charged as liquid, not as vapor.
Q3: Does summer high temperature affect an outdoor condensing unit? Yes. LZAC air-cooled units have a maximum design ambient temperature of 48°C. Above this, condensing temperature rises and cooling capacity drops. In hot regions, LZWC water-cooled units are more suitable: lower condensing temperature and noise of 62 to 63dB(A), but a water source is required.
Q4: How often should the ceiling-mounted unit cooler drain be cleaned? Check at least monthly. A clogged DN32 drain is the most common cause of water dripping onto goods in the cold room. Clean the drain pan and flush the drain pipe with warm water.
Q5: Do freezer rooms and preservation rooms require different units? The same physical unit can perform both +20°C preservation and -20°C freezing, but check the cooling capacity at the target room temperature. LZAC-12 has only 4,800W at -20°C and 17,530W at +20°C. Select based on the freezing condition, then verify whether the room heat load matches.
Summary
Copper pipe specifications and installation quality determine whether the cold room can pull down properly on day one. The LZ series provides clear connection pipe sizes, factory charges, and ball valve service points. Follow the specifications, purge nitrogen during brazing, add oil return traps to risers, and get drainage right; a 630kg LZAC-65 installed on a qualified steel frame and connected with clean piping can run stably in a -20°C freezer for many years. The same machine, with undersized pipe and no nitrogen purge, will need repair within a month.
For complete parameter tables for LZAC, LZVAC, and LZWC, contact Yuanxian Machinery sales to request cold room unit specifications.
Author: Yuanxian Engineering Technical Team | 2026-08-13