Condensing set-evaporator match
The foregoing must be taken a step further by plotting the performance of the evaporator on the same co-ordinates as the condensing set. The intersection of the refrigeration-side characteristic of the evaporator and that of the condensing set will then tell us the actual performance of the combination. A pressure drop occurs in the suction line and it is conventional to regard this as the difference of two saturated pressures and to speak of a corresponding difference in saturated temperature. The suction line is usually sized for a pressure drop corresponding to a fall in saturated temperature of about 1°C. In Figure 12.7 this is assumed to be 1.2°C and thus a temperature of 5.6°C in the evaporator gives a suction temperature of 4.4°C. This allows the point Q to be established in Figure 12.7 at
52.2 kW and an evaporating temperature of 5.6°C. Referring to Figure 12.4 we see that this is also a point Q on the refrigerant-side characteristic, used in examples 12.2 and 12.3. From Figure 12.6 we can also plot curves for the condensing set performance at other air temperatures entering the condenser on Figure 12.7 and this will show how the match between the evaporator and the condensing unit changes as summer passes and winter approaches.
If the wet-bulb temperature on to the cooler coil falls the evaporator characteristic moves to the left (Figure 12.7) and the balance point with the condensing set shifts, with a reduction in evaporating and suction temperatures. Thus when the entering wet-bulb is 9.6°C the evaporating temperature is about +0.4°C (point W) and the suction temperature is about -0.8°C (point X), if the dry-bulb on to the condenser is 28°C but the balance drops
Saturated temperature, °C
Fig. 12.7 Evaporator/condensing set performance in terms of air dry-bulb temperature on to the
To about -1.2°C evaporating (point Y) and -2.4°C suction (point Z) when the air on the condenser is as low as 10°C (Figure 12.7). Low evaporating temperatures will give low surface temperatures with the risk of frosting on the DX coil if below 0°C. Frost forms rapidly because the latent heat of fusion is not great (235 kJ kg-1) and impedes the airflow, causing the evaporator characteristic to rotate to the left about the zero load point and so lowering the evaporating temperature and pressure even further. Hermetic compressors rely on the mass flow of suction gas to cool their stator windings and with a fall in suction pressure, density and mass flow rate also fall. Hence the risk of motor burn-outs at low load and the need to determine the risk of this by establishing where the evaporator — condensing set balance occurs at the lowest anticipated load.
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