This article explains how to improve plant efficiency through variable defrost timing and termination.
Most facilities have cool rooms or freezer rooms that use fan coil evaporator units. Over time, ice forms on these coils and in their condensate trays due to airborne moisture.
The rate at which ice is formed on a coil depends on several factors such as room temperature, cooling load, ambient conditions and number of door openings. As the ice layer on the coils increases, the coil’s heat exchanging capacity decreases. Hence, there is a need to defrost coils.
Conventional defrosting methods use either hot gas, electricity, air or water. Many ammonia plants use hot gas defrost as there is sufficient heat available from the refrigeration plant and it is cheaper than electricity.
The process of defrosting introduces heat into the refrigerated space, increasing the plant’s workload and energy consumption. The defrosting process also consumes energy. If defrosting methods are not optimised, refrigeration plant efficiency suffers.
Variable defrost timing and termination refers to managing the interval between defrosts, the duration of the defrosting process and termination of defrosts.
By convention, the frequency and duration of defrosts are fixed, regardless of room temperature and work load between defrosts. If there are too many defrosts or they last too long, heat will be needlessly added to the room. Hence optimised defrost management reduces overall refrigeration plant energy consumption.
The variable defrost timing and termination proposed involves monitoring the time over which the coil has been operating at full capacity. The interval between defrosts will be shorter if the coil has been running continuously at full capacity and longer if it has been running at lower capacity.
The capacity of the coil can be found by assessing the amount of cooling supplied to it over a period. In the case of a flooded ammonia evaporator coil, the capacity is the time for which the liquid solenoid valve is open.
Here are some of the benefits:
Managed defrosting intervals prevents wasteful defrosting during low-load periods.
Optimised defrost duration and termination of defrost prevents excess heat entering the cool room, easing the load on the refrigeration system and preventing energy waste.
Quick termination of defrost by electric defrosting units also saves unnecessary power consumption by the electric heaters.
Reduced need for defrosting during low-load situations means the refrigeration plant will be stable for longer periods of time. Similarly, with an ammonia evaporator coil, fewer defrosts means fewer suction pressure fluctuations, leading to more stable operations.
The amount of savings depend on:
the number, type and size of fan coil units in the room
the room temperature
the cooling load profile
the type of defrost: hot gas, electric, air or water
the number of defrosts and the defrost interval currently employed
the defrost relief point: low temperature, intercooler or economiser vessel.
Defrost relief piping to intercooler or economiser Implementation
You need to know the:
make and model of fan coil units in each of the rooms
defrost technique applied
coil arrangement within plant and circuit design
To facilitate a sound defrost management strategy you will need:
A temperature sensing device attached to the face of the evaporator coil. When coil temperature
reaches 10°C, the defrost is complete and can be terminated.
A means of tracking the room cooling load, such as a monitoring solenoid or control valve.
A high-level PLC (or remote control management system unit) which controls the defrost system.
This project is mainly achieved by implementing control algorithms. The only equipment required is a coil temperature thermostat or sensor to terminate the defrost cycle.
The estimated cost of equipment and initial definition of the control algorithm is approximately $1500 per evaporator, if the site has a control system to which the thermostat can be connected.
Optimisation costs would be additional based on the level of optimisation required.
* This information was sourced from “I am your industrial Refrigeration Guide” which was prepared on behalf of the NSW Office of Environment and Heritage by thermal engineering consultancy, Minus 40.