Frost heave happens when moisture in the subfloor freezes due to defective underfloor insulation. As ice occupies more volume than water something must give normally, meaning that the cold room floor rises. I have been in a cold store where in places the floor had risen by 900mms. In such situations, no racking can function safely. For mobile racking height changes of as little as 15 mm can negatively affect the operation of the bases.
As additional protection against frost heave, the Normal South African practice is to install heater mats in a sand bed normally about 30mms thick under the subfloor insulation. This is done primarily in freezer stores but increasingly in chill stores too as ground water can freeze at temperatures above zero Celsius as several deciduous fruit stores have found to their cost. The heater mats come on and start distributing heat like a heater blanket on a bed when a thermometer placed at the same level as the heater mat records temperatures somewhere between 4 and 7 degrees Celsius.
Some of the problems experienced with heater mats.
- Not correctly installed so that they either do not come on or only heat a portion of the cold room floor.
- Staff do not understand how heater mats work and either turn them off or remove the control panel, seeing it as a forklift hazard.
- The cables get cut during maintenance operations.
- Insufficient temperature sensors under the floor. If subfloor temperatures go too low on one side of a store the heater mat may not come on as the temperature by the probe is higher. It is good practice to have multiple probes under large floors.
- Heater mats are set to come on at temperatures that are too low. At 4 degrees Celsius, you could just about have some freezing already taking place. It is better to make the set temperature higher.
- The old practice of placing underfloor insulation so that the gaps between the panels in each of the layers are not in the same place but staggered may not always be observed. This means that in places the subfloor temperature can get into the danger zone within a fairly short time. Polystyrene panel does gradually degrade so temperature reductions in the sub-floor can be expected.
How do other countries deal with the danger of frost heave?
In Europe, the preferred method is via glycol piping where the glycol water mixture is heated via an exchanger with the heat given off by the condensers. The glycol circuits need to have easily seen flow meters which can be checked for flow on a regular basis. Hence underfloor temps are continuously kept at up to 12 Degrees C. The glycol pipes are at the same level as the heat mats but are normally encased in a cement mixture. They do need to be pressure-checked before the final wearing slab is poured.
In Australia, the preferred method is air pipes laid in the subfloor about 200mms apart. They go from one side of the store to the other. The pipes are laid at a sufficient angle so that any moisture flows out at one side. Otherwise, a pipe can eventually become blocked with Ice and stop airflow. Such pipes are increasingly being fed with hot air from the condensers.
Other methods include constructing the cold store floor off the ground. In some places, the gap is made to accommodate cars! But it’s an extremely expensive option.
In South Africa, blast freezers are especially at risk of frost heave given their lower temperatures, (down to minus 40 Degrees C.) if they contain spiral freezers then the risk is even greater. They should have thick high-density underfloor insulation, glycol, and possibly heater mats in a belt and braces approach.
In freezer stores higher switch-on temperatures should be investigated together with thicker and staggered insulation. An additional glycol or pipe arrangement could also be included where the subfloor conditions are particularly wet.