REFRIGERANTS
c ancelling. Indeed, a zeotrope can show zero observed glide while a single fluid or (near-) azeotrope can have a significant glide. This point is illustrated by the table that s ummarises cycle calculations for R404A and i ts lower GWP, retrofit replacements, R464A a nd R442A, using the same evaporator dew p oint and condenser bubble point. With a (hypothetical) zero pressure drop, all glides are positive, indicating the temperature increases along the coil. But with increasing pressure drop, t he observed glides decrease and progressively b ecome negative, indicating that the temperature d ecreases along the coil. Glide can thus be p ositive or negative relative to the direction of the refrigerant flow.
Typically, an engineer sets up an evaporator containing R404A by measuring the suction pressure, reads the dew point from the saturation table and makes the usual assumption that t he dew point is the same as the evaporation temperature since R404A has a small zeotropic glide. An assumed -30°C dew point/evaporating temperature would be achieved by adjusting the operating parameters to achieve the c orresponding suction pressure. However, this ignores the effect of the pressure induced
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glide (PG). If this is 0.4 bar drop the effective evaporating temperature is -28°C, not the desired - 30°C. Ironically, the combined PG and ZGs of the t wo zeotropes means they are likely to be closer t o the desired midpoint evaporating temperature than R404A. As the pressure drop increases, the R404A glide becomes more negative and numerically exceeds the R442A glide at 0.2 bar p ressure and the R442A glide at 0.3 bar pressure d rop. The R442A and R464A glides are negligible a t 0.3 and 0.5 pressure drops respectively. Z eotrope producers typically provide guidance calculations for engineers so they can determine the operating pressure for a desired midpoint evaporating temperature, but ignore PG. A lthough the methodology is consistent with t hat for single fluids/(near-)azeotropes, the s ample calculations provided in the table raise a fundamental question about the validity of this approach. Ironically, an engineer who ignores all the advice and treats zeotropes as he has been taught to treat single fluids/(near-) azeotropes, m ight be closer to his target evaporator operating c ondition.
I ntroducing PG as a parameter complicates the assessment zeotrope containing units, since pressure drops across heat exchangers
are not usually measured. However, values may b e available from manufacturers’ literature. Furthermore, the glide can depend on operating conditions, especially in units where the flow rate is varied to adjust the cooling capacity; a lower flow rate will have a lower pressure drop and hence a lower PG. The quantity of oil circulating with the refrigerant can also have a significant e ffect.
Zeotropes will continue to play a significant role in the refrigeration industry. A new unit can be optimised around the specific properties of a zeotrope, a freedom not available when the refrigerant is being retrofitted into an existing m achine replacing a single fluid such as R22 or (near-) azeotrope such as R404A. Assessing the merits, or demerits, of a zeotrope as a retrofit by focusing just on its intrinsic glide, can be misleading.
Despite past scepticism about the efficacy, often based on very simplistic cycle calculations, c onsiderable experience has accumulated over the past 20 years confirming that zeotropes can be e ffective retrofits, as well as OEM fluids. In many cases, zeotropes have performed better than predicted – perhaps pressure induced glide has made an unrecognised, but positive, contribution?
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