REFRIGERANTS
What’s glide – and why do we care?
Professor Dick Powell of Refrigerant Solutions sheds some light on this often misunderstood aspect of refrigerant gases.
T
raditionally, the refrigeration industry preferred single fluids or azeotropes and, until ~30 years ago, was mainly dependent on the R12, R22, R502 and ammonia with R13B1, R500 and R503 for lower tonnage applications. Zeotropes – refrigerant blends evaporating and condensing over a temperature range at constant pressure – were limited to specialised systems. They were mooted by 1980s academics as a way of improving energy efficiency by exploiting their inherent ‘temperature glide’ in counter- current heat exchangers via the Lorentz cycle. But concern was expressed over possible differential leakage resulting in a composition change, the potential shift in the circulating composition caused by fractionation in flooded evaporators, which could cause excessive discharge pressures, and the effects of ‘glide’ in DX evaporators and condensers. Industry was reluctant to adopt them, but now the situation is different. Needing retrofit refrigerants for the ozone-depleting CFCs and HCFCs and now high GWP HFC blends, notably R404A and R507, the industry has had little option but to accept zeotropes. Although differential leakage has not proved a problem in practice, composition shift is still a contentious issue.
This article, however, focuses on the third perceived problem, ‘temperature glide’. So what is glide? I define it as the observed temperature range over which a zeotrope evaporates or condenses at constant pressure. In the condenser, the glide is the temperature difference between the dew point and the bubble point, in the evaporator between the entry temperature and the dew point. The assumption is often made that the only significant contributor to the observed glide is the thermodynamic properties of the zeotrope, but this is wrong. A refrigerant flowing through a heat exchanger is driven by a pressure difference, so its vapour pressure drops and consequently its evaporating or condensing temperatures; a refrigerant also experiences a ‘pressure-induced glide’ related to its flow. This applies as much to single fluids and azeotropes as to zeotropes, so ‘glide’ has been as integral an aspect of vapour compression refrigerant technology from the beginning, although perhaps unrecognised. How do the intrinsic ‘zeotrope glide’ (ZG) and ‘pressure-induced glide’ (PG) combine in an actual unit? In the condenser, the ZG and PG operate in the same direction so are additive. But, in the evaporator, they oppose each other and thus tend to be self-
34 August 2018
www.acr-news.com
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72
