S K Energy Consulting Ltd.

What is the optimized defrosting?
Defrosting with hot gas is the dominant method applied in industrial refrigeration, which has some advantages and disadvantages.

Advantages: Removing frost off a coil and removing oil out of the coil.

Disadvantages: Heating the coil, heating refrigeration space and blowing by hot gas.

In order to optimize defrosting we have to answer 3 questions:

A. What is the optimum frequency of defrosting?

Defrosting frequency should be chosen to keep plant efficiency at maximum. This frequency varies depending on the amount of moisture in ambient air, product activity and temperature difference between refrigerated space and evaporator coil temperature. Usually it has to be changed 2-3 times a year.

B. What are the right settings for defrosting?

There are three most important defrost settings:
head pressure
back pressure regulator setting
hot gas balancing valve opening

If too much hot gas is coming for defrosting, after 10-15 minutes non-condensed vapor may pass through the pressure relief regulator. This does not effect the defrosting of the coil, however, this blow by of hot gas will increase the compressors' load because the compressors are required to re-compress this gas. If too little hot gas is coming for defrosting the coil won't defrost properly. The right settings can keep refrigeration plant at peak efficiency during defrosting and after defrosting.

C. What is the optimum length of defrosting?

If defrosting is too long, additional heat is to be brought into the refrigerated space. If defrosting is too short, we have incomplete defrosting. Usually the length of optimized defrosting is 15-25minutes. Balancing defrosting settings and defrosting length can keep evaporator coils at peak efficiency and reduce the heat added into refrigerated space.

Some field tests and analyses of hot gas defrosting suggest efficiency (ratio of heat to melt frost to heat provided by hot gas) approximately 20%. Optimized defrosting can give efficiency 40-50%.