Refrigerant coil heat transfer
Computair’s WinCoil and WebCoil selection software supports several types of coil: hot water, cold water, steam, run around, direct expansion / DX / evaporating and condensing. Most of these use water (or water with additives such as glycol) and do not involve any phase change. That is the water remains as a liquid or as a gas as it passes through the coil and doesn’t change state between liquid and gas. In evaporating coils and condensing coils, the circulating fluid (refrigerant) either boils from liquid to gas or condenses from gas to liquid. The associated release or absorption of latent heat forms a large part or most of the heat exchange rather than just a change of temperature. When Nigel Taffs wrote Computair’s first coil performance calculations in 1980 only a small number of refrigerants were in use and it was common to use simple graphs or tabulated data to determine heat transfer coefficients and changes in enthalpy. But a lot has changed since then, largely because of government regulation of the use of some of the fluids which have historically been used as refrigerants. Computair’s coil performance calculations now use the best available and proven mathematical models of refrigerant flow and heat transfer.
Modelling refrigerant flow
To determine heat transfer between refrigerant and tube, Computair’s coil software models the flow of the fluid as it either boils or condenses. Our models are not specific to a particular fluid but are based on implementations of the best theory available and use our database of thermodynamic and transport properties for each refrigerant. Thus, we can easily add new fluids as regulatory changes and industry trends demand.
Modelling refrigerant flow through each coil circuit is complex, taking into account changes in vapour quality, flow regime and the geometry of the tube. For instance, as a refrigerant starts to boil it will be mostly liquid infused with bubbles of gas which will tend to be in the upper half of a horizontal coil tube. The refrigerant will continue to boil with more of the cross section occupied by gas until liquid exists only on the tube surface (annular flow). Eventually the refrigerant is almost entirely gas with entrained droplets (mist) before finally it has completely boiled and becomes superheated. Our coil software models all these kinds of flow in order to determine the heat transfer coefficient between the refrigerant and tube wall as well as the frictional pressure drop.
Composition (by mass for blends)
A few definitions
This is the heat of a refrigerant normally expressed as kJ/kg or BTU/lb. Normally a change in enthalpy a constant pressure will cause a change in temperature. In the saturation zone there is no change or only a small change in temperature due to temperature glide and frictional pressure drop. But the proportion of gas to liquid (vapour quality) will change as the refrigerant boils or condensing, either absorbing or releasing latent heat as it does so.
This is the ratio of gas to liquid in the saturation zone. It is normally expressed as a percentage. It can be given as mass basis (kg of gas to kg of liquid) or mole basis (moles of gas to moles of liquid). The vapour quality at the inlet of the DX coil is determined from the condensing coil outlet condition.
This is defined as a pressure and temperature. It is the point of highest pressure on the edge of the saturation zone.
This property of fluids determines how fluid it is. It is normally expressed as N·s/m2 or lb·s/ft2.
The mass of fluid in a given volume normally expressed at kg/m3 or lb/ft3. As a fluid changes from liquid to gas its density changes enormously causing a changing in the speed the fluid flows through a DX coil on condensing coil. This is a factor affecting heat transfer and frictional pressure drop.
FRICTIONAL PRESSURE DROP
The frictional pressure drop through the condenser and evaporator coils is part of the work the compressor has to do. It also affects the temperature of the refrigerant.
ISOBARIC SPECIFIC HEAT CAPACITY
This property describes the way temperate changes with respect to enthalpy when pressure is kept constant.
This a property of liquids and is an important part of determining flow type
This is a property of blended refrigerants. The components of the fluid may not evaporate on condense at the same rate causing a small change in temperature in the saturation zone when enthalpy changes. In DX coils this opposes the drop in temperature caused by the frictional pressure drop and may even overcome it. In condensing coils, it increases the temperature drop associated with frictional pressure drop.
States of fluid:
• Sub-cooled / liquid: This fluid state is characterised by incompressibility, surface tension, high density and viscosity
• Super-heated / liquid: This fluid state is characterised by compressibility, no surface tension, lower density and lower viscosity
• Saturated: The saturation zone is a range or pressure and enthalpy within which a fluid is composed of a mixture of liquid and gas. Changes to enthalpy can cause a change in the proportions of liquid and gas due to boiling or condensation
• Super-critical: Above the critical temperature and pressure a fluid exists in a state characterised by the density and viscosity of a liquid, but also the compressibility and no surface tension of a gas. It is typical for Carbon Dioxide (R744) based systems to take the fluid into this state.
Regulatory changes affecting refrigeration:
In the 1980’s a relationship between the depletion of the ozone layer and the release of chlorofluorocarbons (CFC’s) and hydrochlorofluorocarbons (HCFC’s) was discovered and international treaties were agreed which eliminated the use of these compounds. That meant that in the 1990’s CFC refrigerants such as R12 became obsoleten the 21st century HCFC refrigerants such as R22 started to be phased out. Other classes of compound including hydrofluorocarbons (HFC’s) were used as replacements.
In the 21st century further international action took place limiting the use of compounds with a high global warming potential (GWP). The GWP of a gas is defined as its contribution to global warming relative to carbon dioxide. For instance, R410A has a GWP of 2088 meaning that 1kg of R410A released into the atmosphere contributes as much to global warming as 2088kg of carbon dioxide. This has led to the progressive phase out of HFC’s.