RETA Breeze Sep-Oct 2022

According to Darcy’s law, cold air has a lower vapor pressure compared to warm air. Warmer air will move toward cooler air because of the driving forces of vapor differences between cold and warm air. RETA’s Industrial Refrigeration Book 2, Chapter 3 introduces the properties of air/water. Warm air has a higher vapor pressure which will force its way through a hole, a permeable layer, or move through earth and soil until the vapor reaches equilibrium or reaches a point where the temperature drops to 32° causing the water vapor to condense, form ice, or adhere to the underside of a frozen slab. Warm moisture in the soil will migrate to the cold at the bottom of the freezer slab. The temperature of the soil below the slab will form a gradient over time, gradually lowering the temperature and moving the freeze point deeper into the soil. Layer by layer the ice expands as it forms an ice lens forcing the floor up with extremely high pressure. In order to keep the vapor from freezing beneath the slab, the earth must be warmed above the freezing point restoring equilibrium. The freezer slab must be insulated to reduce the loss of heat from the slab and balance the vapor pressure with the warm ground below the freezer floor. The underfloor heating system beneath requires regular preventive inspection and maintenance. Proper precautions are required to prevent frost that can form in the sub-grade soils resulting in raised, cracked floors and a list of potentially serious problems, including safety concerns, operational issues, and significant structural damage. If the heating system malfunctions and the soil isn’t heated to above 32°, the extreme cold can penetrate through the concrete floor, past the insulation and through to the soil. This causes the moisture within the soil to freeze. Eventually forming an ice lens that pushes upward, heaving the concrete floor above. Given time, the lens will grow and, in nearly every case, will begin pushing the building foundations up. Heat flow is relentless and unstoppable. Insulation can only slow it down. The soil underneath freezer floors must be heated, ensuring it remains

above freezing temperature. This is achieved through floor heating systems, such as:

is easily installed, monitored, and repaired. The Major disadvantage is cost of electricity. Heated Hydronic (glycol or heat transfer fluids) The heated hydronic loop circulates a heated liquid through pipes, PEX or polypropylene tubing located under the insulation layer embedded in what is called the mud slab or compacted sand. This fluid can be heated in a number of ways including: • Heat exchanger from compressor discharge waste heat (desuperheater) is the most common heat source, • Waste heat reclaim, • Electric heaters, • Plant steam or boiler, • Solar energy. In all cases the circulating fluid’s level and chemistry must be monitored and maintained according to SOP and MP to prevent corrosion of the metal parts of the system. What can happen? An example of worst case what can happen when the floor heat system goes off-line. The first symptom was a gentle dome shaped rise in the floor, stacks of totes began to lean. Then came the cracks in the foundation.

• Static air vent tubes • Forced air vent tubes • Heated forced air vent tubes • Electric resistance heat • Heated hydronic

Natural ventilation air vent tubes Static air vent tubes utilize the natural prevailing winds to push air through tubes located in the soil below the insulation that run from one side of the building to the Forced air ventilation tubes Forced air vent tubes utilize a fan or blower to force ambient air through the under-floor tubes Heated forced air ventilation tubes Forced air vent tubes utilize a fan or blower to force heated air through the under-floor tubes located below the insulation layer. Electric Heat Electric resistance heating cable located in conduits or raceways embedded in the sub-slab or soil area below the insulation layer. Sub slab temperature is monitored by controls that power the self-regulating heaters ON/OFF as needed. Electric heat

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