Page 40 - Climate Control Magazine September 2018

Chillers
PRINCIPLE AND
EQUIPMENT
The function of TXVs in DX vapour compres- sion refrigeration systems is to regulate the refrigerant flow through the system while maintaining a minimum level of superheat in the refrigerant gas leaving the evaporator. This ensures that there are no droplets of un- evaporated refrigerant that could cause dam- age to the compressors. TXVs are mechanical devices and, as such, their degree of control is limited. EEVs are more sophisticated and can perform the task of regulating refrigerant flow more precisely and with a smaller mar- gin of the superheat required for the protec- tion of the compressor from ‘liquid slugging’. This improves the efficiency of the compres- sor and reduces energy consumption.
MINIMUM REQUIRED INFORMATION
The minimum required information for retrofit- ting EEVs includes:
• age of equipment or system
• system refrigerant type and expected life
• system capacity
• system operating parameters – evaporating
and condensing
• pressure drop across EEV
• whether the refrigeration system operates
on a pump-down cycle
• interfacing EEV controls with BMS for
monitoring.
MINIMUM REQUIRED EQUIPMENT
The minimum equipment includes:
• field sensors (temperature, pressure)
• controllers and data processors
• EEV.
Figure 11: Thermostatic expansion valve is replaced with an electronic expansion valve.
CURRENT PRACTICE
Most new AC systems have EEV devices, while many older systems (i.e. older than 5–10 years) typi- cally have TXVs. Wear will degrade the operation of TXVs through control drift while superheat is often set conservatively, erring on safety; both of which will compromise the energy efficiency of the device.
OPPORTUNITY FOR OPTIMISATION
The following control strategies and parameters are recommended in order to maximise the en- ergy efficiency of refrigeration systems without compromising their performance:
ENERGY-SAVING POTENTIAL, COSTS, BENEFITS AND RISKS Retrofit of EEVs can reduce energy consumption of AC compressors by up to 15 per cent. Cost-ef- fectiveness can vary, with payback periods de- pending on the cost of the new valves.
Benefits arising from the use of EEVs include: • more reliable operation of refrigeration
systems
• reduced energy cost
• reduced GHG emissions
• reduced maintenance costs.
APPLICATION NOTES
Typically, EEVs can be effectively retrofitted to vapour compression refrigeration circuits em- ployed with larger AC units (typically reciprocat- ing AC compressors).
GETTING STARTED
Facility managers or building owners/opera- tors will need to establish the age of the cur- rent refrigeration equipment/systems, the re- frigerant type, the remaining economic life of the equipment, and the system capacity, pri- or to the retrofit of an EEV. Interfacing with the current BMS or controller will also be re- quired for monitoring of the EEV. ✺
ABOUT THE AUTHOR
Information for this feature has been sourced from the HVAC Optimisation Guide jointly compiled by the NSW Office of Environment and Heritage’s (OEH) Energy Efficient Business (EEB) team and the Australian Institute of Refrigeration, Airconditioning and Heating (AIRAH).
To view the publication go to: https://www. environment.nsw.gov.au
• •
Retrofit EEV instead of TXV for larger refrig- eration circuits controllinsuperheating in the region of 2–3°C, see Figure 11.
Where it is not cost-effective to retrofit EEVs, adjust TXV to control superheating in the re- gion of 5–7°C.
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