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INSTRUMENTATION & LEAK DETECTION


the evaporator and reject it in the condenser. The change of state creates a change in the density of over 50 times which will result in that system operation is sensitive to changes in refrigerant charge.


Every design has a characteristic behaviour of parameters in the system. How the system responds to a leak of refrigerant is predictable and if the process is understood it can be detected by a 'clever' performance monitoring system.


The changes will be dependent on a number of factors, but to keep to keep it simple the discussions here are based on a system with an expansion device controlling the superheat in the evaporator to protect the compressor from liquid carry over.


As the same amount of refrigerant will take more than 50 times more space as vapour than as liquid. The result is that if superheat increase more surface in evaporator is required thus more vapour and less mass will be there. This refrigerant has to go to the condenser and will accumulate as liquid in the bottom of condenser.


H


The higher superheat will reduce the surface for evaporation which then requires the


evaporation to decrease to evaporate same capacity – this decreases the capacity. The increasing subcool increases capacity but it also reduces the surface for condensing thus forcing condensing up to transfer the same capacity. It is easy to see that the result is that there


is an optimum superheat and charge for each operating condition. So the refrigerant charge will always be a compromise to give the best operation as a whole over the intended envelope of a chiller or heat pump. Superheat and subcool will always vary over the envelope and it should be a requirement for all suppliers to document superheat and subcool at varying pressure ratios in operating and maintenance instructions. That these and other key performance


indicators (KPIs) are rarely documented in operating manuals is a major reason why maintenance so often leave systems far from their optimum. For technicians visiting systems with


hundreds of different models without the specification of KPIs for each particular design it becomes standard procedure to only look for flash gas in the sight glass as an indicator of


low refrigerant charge and best-case check that there is a superheat within a wide range fitting their rule of thumb. The introduction of continuous performance


monitoring of the refrigeration process changes the perspective as it documents all KPIs from the moment it is started and it becomes possible to monitor and alert if any of the KPIs change.


The changes of decreasing refrigerant charge for a typical system as described in this article are for example:  Subcool will decrease at a leak as less liquid is accumulated in condenser.


 Condensing temperature decreases at a leak as less liquid is accumulated in condenser.


 Superheat increases at a leak when the expansion valve is affected by low charge.


 Discharge temperature increases a result of increasing superheat and decreasing evaporation.


 Evaporation decreases when expansion valve is affected by low charge.


 COP decreases as evaporation decreases.  Energy consumption increases versus energy signature with correct charge when COP decreases.


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www.acr-news.com December 2021 31


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