Superheat eCts alva ActOnS Superheat acts ave ctos gure model actually two, The hysteresis band load increase starting from the vaLve apening cur Superheat ieCts alva ActonS hysteresis. Valves with low intenal resis at all, the opening and alve to be too n become unstable. with the valve possibly going into a hunting mode. WiatlammossnsdoteErnorter More unstable zone. evaporator char che MsS ourve) ceasing the statc superheat untl stabilty s just3 Superheat Affects Valve Actions1 Friction, which results in resistance to the movement of the pushpins, causes hysteresis. All TEVs are affected by hysteresis. A certain amount is needed, as we will see, but more than that is definitely detrimental to the refrigeration system's efficiency. Figure models the action of a typical TEV. Although valve response is often represented by a single curve, there are actually two, one for valve opening and the other for closing. The area between the two curves is called the hysteresis band. The hysteresis band Superheat Affects Valve Actions2 A load increase starting from the valve opening curve. Let's say we're on the valve opening curve and the load on the evaporator increases, in turn requiring an increase in cooling capacity (Figure 3). The superheat increases, raising the bulb pressure, which opens the valve. The capacity delivered by the valve changes almost instantly. Superheat Affects Valve Actions3 If we're on the opening curve and the load decreases, the story is different (Figure 4). Before the valve can start closing, superheat must decrease by the distance between the opening curve and the valve closing curve. With any smaller decrease in superheat, we will still be in the hysteresis band, and the valve will not begin to close. Only after reaching the closing curve is the valve ready to begin closing with any further decrease in superheat. The relationships between superheat changes and valve actions are similar if we start on the valve closing curve. There, a decrease in flow through the valve occurs without hysteresis (Figure 5), but since an increase in refrigerant flow requires crossing the hysteresis band, superheat must increase by the distance between the two curves, overcoming the valve's hysteresis (Figure 6). Capacity can only increase on the opening curve, and can only decrease on the closing curve. Figure 4. A load decrease starting from the valve opening curve. Figure 5. A load decrease starting from the valve closing curve. Figure 6. A load increase starting from the valve closing curve. Hysteresis can't be computed. Valve manufacturers make laboratory measurements over the valve's capacity range to determine the curves for a given design. At Danfoss, this is done using precision automatic measuring and recording instruments. The process takes place under standardized, controlled conditions. The design of a valve determines its internal friction, and therefore its hysteresis. Valves with low internal friction have correspondingly low hysteresis. But if there were no hysteresis at all, the opening and closing curves would become one, and the valve would react instantaneously to any change in load, even extremely small changes. While that might sound great at first, it would cause the valve to be too sensitive. The system could then become unstable, with the valve possibly going into a hunting mode. What Happens inside the Evaporator? Figure 7 shows the behavior in an evaporator at a given capacity. Given a negligible pressure drop across the system, if we place temperature probes (T1 and T2) at the inlet and outlet, we can determine the superheat across the entire evaporator (T2-T1). By moving T2 closer (to T2A), the temperature difference drops as we get closer to the liquid front. Moving T2 even closer (to T2B), we begin to see temperature fluctuations caused by T2B sensing both liquid droplets and vapor. The point just before the fluctuations can be seen is the minimum stable superheat (MSS) point. At MSS, the highest efficiency is achieved for the given load condition. The evaporator is most efficient at the MSS point because this is the point at which all of the refrigerant has finished evaporating and the evaporator is fully utilized. Figure 7. Refrigerant behavior in an evaporator, showing the MSS point and the changes in the quality of refrigerant. More… A curve can be graphed by determining the MSS point at different loads. This is shown by the red curve in Figure 8. For every evaporator, the MSS curve characteristic is unique. This curve can have many shapes, as it is primarily a function of evaporating temperature, airflow, and coil design. The gray area to the left of the MSS curve represents an unstable zone where liquid and gas coexist. When a system operates within this region, liquid refrigerant escapes at the evaporator outlet and overall system efficiency falls. The further we go into the unstable zone, the greater the potential for liquid slugging, which in turn leads to serious compressor damage. To the right of the MSS curve there is only superheated gas. Moving too far into this region also reduces system efficiency because the evaporator is not being fully utilized. In practice, achieving minimum stable superheat for optimum system performance may involve first decreasing the TEV's static superheat setting until the system begins to become unstable, then slightly increasing the static superheat until stability is just reached. Figure 8. A graphical representation of the action in an evaporator, showing the evaporator characteristic (the MSS curve)