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the evaporator. In practice, this manifests as fluctuating suction pressures and inconsistent system performance. Hunting is particularly common in systems where the valve is oversized or where load conditions change rapidly, such as in blast freezers or high-traffic cold rooms.
The use of different bulb charge types, including cross charges and MOP( maximum opening pressure) charges, as well as MOP with ballast, helps mitigate some of these issues. MOP valves, for instance, are designed to limit suction pressure and protect the compressor from overload by keeping it within its own safe envelope. However, this protection comes at the cost of flexibility, as it can restrict system capacity during high-load conditions.
These trade-offs become especially pronounced in applications where the refrigeration load varies significantly over time. In such cases, selecting a single valve that performs optimally across the entire operating range becomes a compromise.
Selecting the correct expansion valve has traditionally been a detailed and methodical process. Engineers would rely on manufacturer tables, identifying the appropriate refrigerant, evaporating temperature, condensing temperature and pressure differential before determining the required valve and orifice size. Subcooling factors and system capacity further influenced the final selection.
While modern tools such as Danfoss
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Coolselector
app have
simplified
this process
considerably,
the underlying
principles
remain
essential
knowledge.
Understanding
how pressure
drops and
temperature
relationships
influence valve
performance
is critical not
only for design,
but also for
troubleshooting
and system
optimisation.
The Cape Town presentation.
EEVS: A SHIFT IN CONTROL PHILOSOPHY
EEVs represent a fundamental shift away
from purely mechanical control. Rather than
relying on pressure balances within the
valve itself( a‘ brain’), EEVs are controlled
by external processors that continuously
monitor system conditions through pressure
transducers and temperature sensors. This
allows for real-time calculation of superheat
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and precise modulation of refrigerant flow.
Two primary types of EEVs are commonly used. Pulse-type valves operate in a manner similar to high-speed solenoids, rapidly opening and closing to regulate flow. Their robust design and responsiveness make them well suited to demanding applications, although care must be taken to avoid liquid line hammering. Stepper motor valves, by contrast, provide smoother and more continuous control, using a motor-driven mechanism to adjust the valve position incrementally. While slightly slower to
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respond than an AKV( Pulse), they offer a wider operating range.
A key advantage of EEVs is that their behaviour can be programmed. Functions such as MOP control, which are fixed in mechanical valves, can be dynamically adjusted in electronic systems. This flexibility allows designers to tailor system performance to specific operational requirements.
Perhaps the most compelling argument for EEVs lies in their ability to improve system efficiency.
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Supplied by SAIRAC Cape Town Centre |