After the Montreal Protocol
several new classes of refrigerants such as HFCs, FCs
and interim blends of HFCs and HCFCs were developed.
Mineral oils and alkylbenzenes were ineffective with
the new refrigerants. Synthetic lubricants such as Polyol
esters (POE) and Polyalkylene glycol's (PAG) were found
to work effectively. The primary difference with these
new lubricants is that they are miscible with the new
refrigerants.
Miscibility and solubility
between the refrigerant and lubricant determine how the
two will behave throughout the system. Miscibility is
defined as the ability of two liquids to mix together
to form a single liquid phase. Solubility is the ability
of a gas to dissolve into a liquid.
Miscibility is a function
of temperature and concentration. Over the operating
range of a system, a lubricant may exhibit one of three
different forms of miscibility. Completely miscible is
where the refrigerant and oil mix to form a single phase
over the applied temperature range. Partially miscible
is where the lubricant and liquid refrigerant form a
single phase over a limited temperature range. The maximum/minimum
temperature at which the lubricant and the refrigerant
separate into two phases is called the critical solution
temperature (CST). Immiscible is when the oil and refrigerant
do not mix and stay in distinct two phases.
Miscibility Examples
Complete |
Freon
R-12(CFC) / BVA 3 (MO)
Suva
134a (HFC) and RPOE LT 32 (POE) |
Partial |
Freon
R-22 (HCFC) / BVA 4 (MO)
Suva
R-134A (HFC) / RPOE 68 (POE). |
Immiscible |
Suva
134A (HFC) / BVA 3 (MO) |
Miscibility plays a critical role in VLT
applications and using the wrong lubricant can create
many problems over the life of the system. In the past
the only lubricant used for VLT applications was alkyl
benzene. Alkyl benzene is a fairly specific composition
and the various refrigeration grades commercially available
exhibited the approximately the same miscibility characteristics
with low temperature refrigerants. The same cannot be
said about POEs.
Every manufacturer of
POEs uses different building blocks and additive packages
in the formulation of their product. Miscibility characteristics
of the same ISO grade with low temperature HFC refrigerants
can range from immiscible with one manufacturer to completely
miscible with the other.
The
problem with immiscible and insoluble lubricants seems
to be more pronounced in flooded liquid
evaporators then in direct expansion or dry evaporators.
In flooded evaporators, the turbulence may be enough
to move the oil back to the crankcase. At the same time
this turbulence may result in the immiscible lubricant
being brought into contact and freezing on the surface
of the evaporator tubing.The formation of an immiscible
lubricant film greatly reduces heat transfer by creating
a thin coating on the evaporator wall.
In DX evaporators an immiscible fluid
will separate into two layers if the product does not
efficiently flow out of the evaporator at the same rate
which it enters, heat transfer will be affected as well
as oil return. . In addition to oil logging reducing
heat transfer, it may also starve the compressor of lubricant.
Using a lubricant such as RPOE LT 32 with Suva 95 (R408B) in
VLT applications is a excellent way to eliminate potential
low temperature problems. |
