Warm Glycol Defrosting

Hot Glycol Defrost System for Freezer Room Evaporators            

The hot glycol defrost system has been developed by Strathbrook Industrial Services and uses a non freezing brine or glycol to introduce heat into the coil block to effect a defrost of the evaporator coils of a CO2 or conventional Freon system.

The glycol is heated in a heat exchangers by harvesting heat from the discharge of either a CO2, ammonia, HFC (or other) compressor.

The Glycol is held in an insulated tank until a defrost is required.  A glycol pump delivers the hot glycol from the tank to the evaporator that is being defrosted, which is selected by opening a solenoid valve in the glycol delivery pipe work on the coil block.

The warm glycol is delivered into the dedicated glycol tubes in the coil block, which are positioned throughout the coil block so that the heat can transfer into the coil block to effect a defrost.  The heat spreads from the tube into the coil fins and end plates (plus any coil block dividers that may be installed) so the heat can spread to all areas of the coil block.  The drip tray is also warmed by a glycol circuit piped to the glycol headers, which ensure the drip tray is able to drain freely and that no ice can form over an extended time period.



When CO2 is used as the refrigerant, the vapour in the coil block assists in transfusing the heat into the coil block and return bends as the CO2 is very effective in transfusing heat even in the vapour phase due to the high vapour density.

Once the coil block is defrosted the defrost solenoid is closed and the glycol sits static in the coil block where it acts as a heat transfer medium to increase the capacity of the evaporator coil.  The refrigerant cools the coil block and the glycol in the coil block and cold glycol provides some cooling effect as warmer air is circulated over the coil block.

When the next defrost start the cold glycol is replaced by warm glycol for the next defrost.  The glycol pump can be stopped between defrosts to reduce power usage.  The cooler defrosting temperatures that are required to effect a defrost provide a number of benefits.

The lower temperature reduces the chimney effect which is caused by using the high temperatures that electric or hot gas coil deliver.  This reduces the amount of heat introduced into the room and reduces the warming of the room or fixture.

This reduces the time and energy that is required to return the room or fixture to its design temperature after the defrost is complete.

The lower temperature also reduces the formation of water droplets which can collect on the underside of the ceiling and walls.  When the defrosting is done at these lower temperatures, the warm glycol is not hot enough to vaporize the water dripping from the coil block.  This is very common with electric defrost system and you will often hear the water being vaporized as it drips onto the very hot heater elements towards the end of the defrost cycle.  If vaporization occurs the water vapour will be carried into the room by the hot air rising out of the evaporators, and will spread the water droplets over the ceiling and pallet racking, where it will freeze and form ice.  Drips formed on the ceiling are blown around when the evaporator fans restart forming drips that fall to the floor and form Ice.  Ice on the floor is a serious OH&S issue and a costly and time consuming problem, if staff are required to chip the floors clean at regular intervals.

The glycol can be heated either in a heat exchanger and transferred to the storage tank, or the heat transfer can be done in the tank which reduces the need for a pump or additional pipes and controls.  In tank heating is normally done on smaller projects.

The inclusion of the glycol heater also reduces the temperature of the discharge vapour being delivered to the condenser or gas cooler which effectively allows more of the condenser or gas coolers area to be used as condenser delivering a lower refrigerant exit temperature, which increases system efficiency.

This system will reduce the electrical energy required to operate the system and should also eliminate the need to run expensive defrost heater cables, contractors and overloads, or equally expensive hot or cool gas pipe and valve network.

The hot glycol system also has very few moving parts.  One pump plus one solenoid valve per evaporator.  These parts would be relatively easy to service compared to a hot gas system which are more complex or electric defrost systems which require replacement and repairs to heaters, overloads, contactors and wiring.

In 2009 Strathbrook Industrial Services installed this system into a large distribution centre in western Sydney (180kw freezer capacity) and has now been in operation since that time.

The freezer is extremely dry and has no frost or ice formations in the room at all!

The removal of the electric defrost heaters has reduced the power consumption of the site significantly as the eight evaporators would normally have drawn 88maps each during defrosting, for each of the 30 min defrosts.  The glycol defrost system is defrosting for 15 min per cycle with one coil defrosting per hour, giving each coil three defrosts per day.

The power saving is in the order of $15,000.00 per year for defrosting alone.  The room also does not suffer from a temperature spike after each defrost, so the need to recover the room temperature after a defrost is almost totally eliminated, further reducing the power used by the system.

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