Above Image: Heater mat being covered by a thin concrete layer at a newly built freezer store in the UK.
In a newly commissioned Danish freezer some years ago, a Storax mobile refused to move. After some initial panic, it was found that the obstacle was a ridge in the concrete floor. As the floor had been checked for level after initial curing, the newly formed ridge was surprising.
The floor had a glycol heater mat beneath the underfloor insulation and every circuit had a flow meter outside the building so that movement of the glycol mixture heated by the refrigeration system condensers could be easily checked.
A bumblebee had become lodged in a flow meter obstructing the flow of glycol and allowing ice to form in the subfloor and creating the problematic ridge through “frost heave”. The bee was removed, the ice melted and the ridge disappeared without cracking the concrete floor.
In Australia, 200mm pipes are laid under freezer floors and air pumped through them to stop ice from forming in the subfloor. At one freezer the pipe did not have sufficient fall, condensation puddled and froze obstructing the pipe eventually causing the cold store floor to rise. At one local store, similar air vents were blocked to prevent rat ingress. That floor rose by 900mms……..
Right: Heat exchanges next to the condensors on the cold store roof when heating the glycol
In South Africa, while some recent stores have installed glycol systems, most rely on electrical heater mats which normally consist of three circuits of wire a little like a hot blanket on a bed. The wires lie in a sand bed which may or may not be held together with a weak cement mix. The circuits end in a control box which turns them on only if the temperature beneath the cold store floor dips beneath 4 degrees C.
4 degrees C is interesting as water with impurities can freeze at higher temperatures than zero. Some chill stores have found this out to their cost, as heater mats were considered unnecessary. As the business changed temperatures were dropped to minus 0.5 degrees C and the rooms kept running year round instead of being switched off in the summer months. After several stores experienced frost heave, heater mats and underfloor insulation have now become part of the building design.
While it may sound obvious, check that the heater mats are on when the chamber is commissioned. We have seen at least three stores where the floor rose through frost heave because the heater mats had not been turned on. In one instance the floor subsided. In the others the movement had already cut the wires.
I have visited one store where the heat mat cables had been cut by accident and attempts were made to drill right under the floor in the insulation to replace the wires. I don’t know if the plan worked.
What I have found somewhat surprising is the number of times when management don’t immediately know where the heater mat control panel is situated and how it works. Only last week we had to help a customer who found that his heater mat control panel needed fixing.
This was only identified after the floor began to rise.
It is critical that heater mats get checked regularly and that management understands their purpose. In freezer stores, especially those with mobile racking systems, keeping the floor level is so essential that annual floor level checks are recommended.
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