All cool rooms and freezers use evaporators or coolers to help regulate temperature inside the room.

When people and equipment enter and exit a room they let varying amounts of ambient air into the space which needs to be brought back down to the regulated temperature. This air generally contains a significantly higher level of water vapour than the room. The water vapour or moisture in the air is naturally drawn to the coldest element in the room which just happens to be the fin and tube surfaces within the evaporator.

The coolant circulating through an evaporator or cooler is always colder than the air circulating around the room. Depending upon the criteria or special circumstances used by the installer to create your system, this coolant can be anywhere between 3°c and 8°c colder. After a period of time the accumulated ice and frost on these surfaces restricts the air flow over the unit and also acts as an insulator.

This significantly reduces the efficiency of your evaporator and creates a significant risk of consequential damage to compressors downstream by way of liquid refrigerant floor back and oil wash out. Both of these scenarios can have a measurable impact on the costs of operation.

In order to allow the refrigeration service to get back to working at optimal levels, you need to remove the frost build up as quickly and effectively as practical. There are many methods employed, all with their own strategic advantages and disadvantages.

OFF-CYCLE DEFROST:

The simplest and cheapest method is generally only applied to coolrooms operating above +2°c. In this mode any ice formed during the normal running time of the system is melted by the cyclic stopping of the cooling system, whilst maintaining the room air flow. The room air melts the ice which falls to the drop tray and away to waste. The number of stoppages and the length of the off cycle time could be either unforced or forced depending upon the room controls and system demands.

It is not uncommon for room temperatures to rise by several degrees under this methodology and so consideration needs to be given to the effect on product quality and storage certification needs.

ADVANTAGES: Cheap to install and operate. Simplest
DISADVANTAGES:

Little temperature control authority, suitable only for some cool rooms, risk of water droplet carry over into the room and product.

ELECTRIC DEFROST:

One of the most common methods in defrost management is to utilise AC electrical elements on or within the evaporator or cooler. This utilised a heating element within the coil block. Used predominantly in freezer rooms and cool rooms where off-cycle defrost is unreliable or too slow.

During a defrost cycle, determined by the automatic system controls or a manual initiation, the circulation fans are switched off and the heating element on.

The number of defrost cycles per day, length of defrost, defrosting temperatures, dropping time post defrost are determined by the system controls. These controls may employ the very simple time start: time terminate or the more complex system response controller.

ADVANTAGES:

 

Moderate in complexity and speed to defrost, good control authority, suitable for all small to medium sized rooms.
DISADVANTAGES:

Moderate cost to install, high energy cost to defrost and recover.

REFRIGERANT GAS DEFROST:

Another way of performing a defrost is through the use of the heated refrigerant already in your system.

“Hot Gas Bypass” systems can be found in both larger systems and small package equipment and will generally require the attentions of a refrigeration maintenance engineer to ensure it is both effective and reliable. These systems stop the cooler fans and divert a percentage of the hot discharge vapour from the refrigeration compressor outlet directly to the evaporator to life the temperature of the heat exchanger. The evaporator coils are heated from within, melting the ice and frost.

Like the electric defrost system, the number of defrost cycles per day, length of defrost, defrosting temperatures, dripping time post defrost are determined by the system controls. These controls may also employ the very simple time start: time terminate or the more complex system response controller.

Since this method also changes the refrigeration system complexity, careful consideration needs to be given to the system balance and lubrication issues that might be otherwise adversely affected.

ADVANTAGES:

 

Speed to defrost, good control authority, suitable for all small and larger sized rooms, lower energy cost to defrost and recover.
DISADVANTAGES:
High cost to install, moderate in complexity.

“REVERSE CYCLE”:

“Reverse cycle” systems are most often found in packaged equipment since they require the attentions of a refrigeration services designer to ensure it is both effective and reliable. These systems stop the cooler fans and literally reverse the system operation so the cooling is now performed outside and all the rejected heat is sent to the internal cooler. Since the cooler or evaporator fans are not running, the heat is almost wholly retained within the unit melting the ice and frost from within.

Like the electric defrost system, the number of defrost cycles per day, length of defrost, defrosting temperatures, dropping time post defrost are determined by the system controls. These controls may also employ the very simple time start: time terminate or the more complex system response controller.

Since this methods also changes the refrigeration system complexity, careful consideration needs to be given to the system balance and lubrication issues that might be otherwise adversely affected.

ADVANTAGES:

 

Highest speed to defrost, high control authority, suitable for package units only, lower energy cost to defrost and recover.
DISADVANTAGES:
Moderate cost to install, increasing complexity.

The important component of any of these three common methods of defrost management is to take the time to ensure that the system is correctly configured to suit your particular establishment.

Each and every defrost costs and the need for defrosting is a simple consequence of daily operation. A periodic review of effectiveness and bottom line impact can be a very worthwhile exercise and best results accomplished by engaging operational and refrigeration maintenance personnel.

If you are looking for assistance navigating these issues or looking for a defrost management solution for your refrigeration system please give us a call on 03 9793 6088 or contact us below an obligation free chat with one of our refrigeration maintenance technicians today.

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