What a CA chamber is asked to do
A controlled atmosphere store holds fruit at low temperature with oxygen pulled down to 1 to 3 per cent and carbon dioxide held at 1 to 5 per cent, for months at a time. Apples stored this way keep their firmness and colour long past the end of a cold-store season. The regime works only if the chamber holds the atmosphere it is given.
What happens when it leaks
Air is 21 per cent oxygen. Every leak lets oxygen in. The nitrogen generator, sized to pull the chamber down once and then top it up, runs instead for hours a day. The CO2 scrubber cycles to keep pace. The energy bill climbs, the plant wears faster, and in a bad case the atmosphere never reaches its set point and the fruit keeps for no longer than it would have in a cold store that cost half as much. The owner sees a nitrogen plant that never stops and blames the machine. The machine is doing what a leaking box demands of it.
Where chambers leak
- Panel joints. The seams between insulated wall and ceiling panels, and the junction between wall and floor. Sealed as the panels go up, or never sealed properly at all.
- The vapour barrier. A continuous membrane on the warm side of the insulation. Any break lets moisture into the insulation, where it freezes, and lets gas out.
- Door seals. The largest single opening in the chamber. Gaskets, thresholds and the flatness of the frame they close against.
- Penetrations. Every cable, refrigerant line, sensor lead, drain and duct through the envelope. Each needs a sleeve, sealed on both sides. A chamber with forty unplanned penetrations does not hold gas.
- The floor. Floor insulation, its joint with the walls, and the slab beneath. Cracks in a slab are leak paths too.
Every item on that list is built, not bought. None of it is the refrigeration supplier's work. That is the whole argument: a CA store is a heavy industrial building with a sealed, refrigerated box inside it, and whether the box seals is decided by the people who build the building.
The standard
India's NHB and NCCD technical standards for CA stores reference ISO 6949:1988(E) Type-2, a pressure-decay test: the chamber is pressurised to a set point and the time it takes for the pressure to fall by a set amount is measured. It is a construction outcome with a number attached, which makes it the one line in a cold store specification a promoter can hold a contractor to. Ask for the test, ask for the result per chamber, and ask for it before the first bin goes in.
What to write into the specification
- A named gas-tightness standard and a pass criterion, per chamber, tested before commissioning.
- A penetration schedule agreed between the envelope, electrical and refrigeration contractors before the panels go up.
- A continuous vapour barrier detail at every junction, including the floor.
- Door specification with gasket type, and a flatness tolerance for the frame.
- Floor insulation, and sub-floor heating where any chamber will run below zero.
- One party responsible for the sealed envelope, whoever else is on site.
Why one contractor
Item six is the practical one. When the shell, the envelope, the electrical installation and the refrigeration plant are four contracts, the leak belongs to nobody. When they are one, it belongs to us. On 6 of SAI BA's 17 cold chain engagements all four disciplines were in the same scope, including both 5,000 MT stores. And the firm maintains the nitrogen plant, scrubbers and refrigeration of a 12,000 MT CA facility at Rai, which is where a builder learns what a leaking chamber costs its owner every hour.
How a CA store gets built puts the test in its place in the sequence.