Optimal defrost technique leads to 5% energy saving in ammonia systems

By Janet Thompson, Feb 22, 2013, 13:20 2 minute reading

In a recent paper, titled “Using Hot Gas Defrost in Low Temperature Refrigeration Evaporators with Natural Refrigerants,” Anatolii Mikhailov and Joris Kortstee from Danfoss compare different hot gas defrost strategy control methods and make suggestions for ammonia system improvements. The authors found that when defrost is performed optimally, a 5% savings in energy consumption is possible.

Frost, if it is not melted, will severely impact the performance of an air-cooler and eventually completely block the air flow. One of the most efficient and cost effective means of melting the frost that forms on an evaporator is hot gas. However, hot gas techniques are not without problems. Based on an article originally presented at the 2012 IIR Gustav Lorentzen Conference on Natural Refrigerants in Delft, Netherlands, this paper examines several valve and control configurations for hot gas defrost systems using ammonia, or other natural refrigerants like CO2, to determine which strategies are most efficient.

Control Groups

The paper compares four different hot gas defrost strategy control possibilities, considering both the hot gas as well as the condensate drain. Each option is analysed and preferable valve and control configuration techniques that optimise the hot gas defrost process and cut energy consumption are indicated.
  • Liquid feed strategy
  • Hot gas line
  • Condensate line
  • Wet return line
Calculations of defrost efficiency

To demonstrate the efficiency improvement of the optimised defrost control against a conventional one, the study includes calculations for an example industrial refrigeration cold store. Efficiency is measured based on the energy necessary for melting the ice and removing the water and the calculation is valid for an ammonia system.


The study finds that if defrost is performed in an optimal way, meaning it is done quickly and efficiently, energy savings of 5% or more can be achieved. Energy savings for the worst case examples greatly exceeded the average. Practical tests, which were not yet finalised at the time the article was released, are now being conducted to confirm these theoretical conclusions.

Read Paper: “Using Hot Gas Defrost in Low Temperature Refrigeration Evaporators with Natural Refrigerants”

About Danfoss

One of the largest Danish companies operating in refrigeration, air conditioning, heating and water and motion controls, Danfoss is a leader within research, development and production, sales and service of mechanical and electronic components. The company specialises in automatic controls, compressors and electronic sensors. Danfoss has a long standing commitment for social responsibility towards the environment.
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Danfoss Partner Page

Danfoss Flexline – optimising hot gas defrost


By Janet Thompson

Feb 22, 2013, 13:20

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