Defrost Thermostat Systems
What is a Defrost Thermostat?
Defrost thermostats are a common feature in many refrigeration devices and process, from domestic appliances to industrial cold storage warehouses. The build-up of frost on evaporators creates a layer of insulation that reduces the efficiency of the system, meaning either increased running costs to keep temperatures down or the possibility of damage to products that are not being kept at the correct temperature.
A defrost thermostat works separately to the refrigeration system and is responsible for stopping frost forming on evaporators, in order to avoid the increased costs of less efficient refrigeration. It does this by activating either an electrical heating element or a hot gas valve to increase the temperature across the evaporator and melt the frost.
Benefits of a Defrost Thermostat
In any refrigeration process or application the heat being transferred can cause condensation to form on the evaporator. If the temperature is low enough the gathered condensation will freeze, leaving a frost deposit on the evaporator. The frost will subsequently act as insulation on the evaporator pipes and reduce the efficiency of heat transfer, which in turn means the system needs to work harder to cool the environment sufficiently, or that the fridge cannot reach the setpoint at all.
This has repercussions on either the product not being kept or cooled to the correct temperature, which can increase instances of faulty product, or it means more energy is expended trying to maintain the correct temperature, increasing running costs. In either case there is a loss to the business due to wastage or higher overheads.
Defrost thermostats combat this by periodically melting any frost forming on the evaporator and allowing the water to drain away, keeping the moisture level in the environment as low as possible.
How do Defrost Thermostats Work?
Defrost thermostats work as part of a process control loop in which the defrost thermostat measures a variable and is set to activate the heating element once the variable reaches a certain point.
There are several potential variables for a defrost thermostat to measure and activate according to:
- Time – the defrost thermostat activates at specific time intervals, regardless of the level of frost
- Temperature – the defrost thermostat measure the temperature of the evaporator, activating once it reaches a set point to warm and defrost the evaporator
- Frost thickness – an infrared sensor is used to measure how much frost built up and activate the heating element once it reaches a certain thickness.
Once the measured variable reaches the specified point, whether it be a time period, temperature or frost thickness, the defrost thermostat shuts down the compressor and, if one is installed, activates the heating element.
The defrost thermostat will have a second setpoint at which to cut off in a similar way to the activation setpoint. This ensures the heating element isn’t running any longer than necessary to bring the refrigerator or freezer back to peak efficiency.
Electric vs Hot Gas Defrost Thermostat Control
If using an active heating element with a defrost thermostat there are two options available, either an electrical element which is switched on, or hot gas which is released in to the evaporator using a valve.
Electrical defrost thermostat systems are cheaper to install and simpler to operate, due to the lack of mechanical parts involved in the system and because they are installed adjacent to the evaporator, but remain separate. However the downside of this is that because the electric heating element is installed in the refrigeration area itself it can result in more heat being transferred to the environment, rather than the evaporator. It will subsequently take longer to bring the refrigerator back down to the setpoint.
Conversely hot gas defrost systems work inside the evaporator by using a valve to allow high pressure, high temperature gas from the compressor to flow through the evaporator and warm the frost from inside. This heats the frost more precisely and melts it more efficiently than an electric heater, as well as resulting in less heat potentially being pushed in to the refrigeration area. The downsides to this are the increased cost and complexity of installation, the issue of wear and tear on mechanical parts that will require more regular maintenance, and, additionally, the increased potential for thermal shock damaging the evaporator as hot gas flows through it when it has been cooled to below 0°C.
Managing the Defrost Thermostat
There is an associated running cost with having the additional defrost thermostat system, which can be mitigated in several ways.
If the level of frost is low it is possible to defrost the evaporator during a compressor off cycle. This means the device isn’t actively cooling so the temperature will begin to rise toward ambient. As no additional heating element is required the running costs are therefore minimal, commonly just the cost of keeping the fan operating to help move the condensate away from the evaporator towards a drain, to remove moisture and reduce future frost build up.
If the refrigerator temperature setpoint is very low and simply turning the compressor off is insufficient to raise the temperature enough for the frost to melt then it is necessary to include a heating element in to the system. These systems will have a higher running cost than relying on the off cycle, but will remove larger frost deposits far more effectively which will improve overall system efficiency longer term.
In cases where a heater is present the most efficient way to defrost is when a specified variable that measures the level of frost reaches a setpoint. For an infrared system this would be when there is no frost tripping the sensor and for a temperature controller system it would be when the temperature of the evaporator has risen to a predefined temperature.
Finally there is the option of having timed defrost at regular intervals which last for an amount of time specified by the user. These intervals need to be long enough to remove the accumulated frost, but not so long as to as unnecessarily warm the environment.
This method is cheaper and easier to install and set up than solutions involving an additional sensor, however the timed element does not guarantee the same efficiency as the other methods and also requires greater input from an operator in terms of changing the settings to work out what the optimum length of time is for the defrost system to operate. As such over the lifetime of the defrost thermostat system the lower efficiency and higher running costs can outweigh the higher initial cost of a more complicated sensing system in particularly sensitive environments.
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