Operational definitions
The superheat is the difference between the real temperature of the steam and the saturation temperature corresponding to the pressure detected at the same point. Indicates how much steam is located beyond the saturation condition. An adequate margin for the evaporator output helps to prevent liquid refrigerant from reaching the compressor.
The subcooling is the difference between the saturation temperature corresponding to the liquid pressure and the actual temperature of the liquid line in the same section. Indicates how much liquid is below its saturation temperature and helps to assess the availability of stable liquid to the expansion organ.
Using a pressure detected away from the temperature temperature probe introduces the effect of load losses. In addition, the dew and bubble references indicated by the instrument manufacturer or cooler must be correctly applied.
Before numbers: identifies architecture
The same SH/SC pair changes meaning between capillary or fixed orifice, TXV, EEV, circuit with receiver and critical charging system. Identifies the organ of expansion, presence and level of the receiver, strategy of the controller and point of measurement. On a receiver system, for example, the subcooling does not allow itself to obtain the total charge; with a capillary the quantities react to the balance of the whole system.
How to build a credible measure
- Refrigerant confirmation and composition selected in the instrument.
- Wait for a sufficiently stable condition, unless the transition is the object of the analysis.
- Prepare the pipe: clean surface, good contact, temperature probe suitable for diameter and protection from air influence when necessary.
- It detects pressure and temperature in the same functional tract, annoting the exact point.
- Enclosure, room temperature, setpoint, load, ventilator status, circuit architecture and valve opening percentage if available.
- Compare the result with specific documentation and control of the unit.
Read the grandeur together
The following combinations do not identify a cause. They indicate what data they collect after recognizing architecture, operation phase and load conditions.
| OBSERVED PANEL | QUESTIONS TO INVESTIGATE | ADDITIONAL DATA |
|---|---|---|
| High, low SC | Is stable liquid available to the expansion organ? Is the circuit a critical charge or does it have a receiver? | Documented load, leaks, receiver/spia status if required, evaporator load |
| Low SH, high SC | Is the evaporator supercharged or does the data reflect reduced load, control or position of the measurement? | Valve control, bulb/son, liquid return risk, receiver level and condensation |
| High SH, high SC | Is there an abnormal fall along the liquid line or an unconsistent expansion command? | Temperatures and pressures upstream/valley, receiver, filter, solenoid, valve and control signal |
| SH and SC both low | Are the conditions stable and the correct points? How are loads, air and expansion device? | Absolute, flowing presses, frost, architecture, environmental conditions and historical plant |
Electronic and control valves
With an electronic expansion valve, the superheat is also the result of sensors, transducers, logic and parameters. Before acting on the charge, check plausibility and position of probes, coolant selection, controller configuration, valve control and application limits. The Danfoss library, CAREL library and the Manual library lead to official documents of indexed families.
Four frequent errors
- decide the charge by observing only the pressure of suction;
- measure the temperature with a temperature probe not adherent or influenced by the air;
- ignore glide, reference point or selected refrigerant;
- compare values taken in very different load conditions.
The target depends on application, expansion body, refrigerant, project and control strategy. Follow the manufacturer data and do not change setpoint or charge to chase a generic number.
Consistent instruments
Compatible pressure measures and temperature probes are necessary to the point: pipe clamps, thermocouples, surface probes or digital systems with registration. The temperature catalogue REFTHE distinguishes the function of the instrument from its commercial offer.
Official sources consulted
- Danfoss — Thermostatic Expansion Valves TUA/TUAE and TCAE (data sheet) )
- Danfoss — Diagnostics of thermostatic valves in the system a
- Danfoss — TXV Superheat Tuner
- Danfoss — Ref Tools
REFTHE original editorial synthesis; the documentation of the equipment always prevails for values and procedures.