ASHRAE Journal - February 2013 - 17

commercial production, and they will be very expensive to
produce. It is unlikely that HFOs will develop into universal
replacements for the once ubiquitous R-22.
Carbon dioxide has been used in European supermarkets to
a significant extent, but it is inherently inefficient in use compared to R-22 except when used in cascade with another refrigerant. Subcritical carbon dioxide systems are less efficient
than ammonia systems, and transcritical systems are even less
efficient. It will be difficult to justify the use of carbon dioxide
as a general substitute for R-22 except in cool and temperate
regions.
Table 1 compares some physical properties of R-22 with
properties of other refrigerants that might be considered as a
substitute. All the refrigerants in the table are non-flammable
except for ammonia, which has a minimal fire risk when used
in small systems. R-23 is included for comparison although it
would not be a credible substitute.
The table shows that ammonia (R-717) is the only substitute
for R-22, having zero GWP. Ammonia also has a high critical
temperature that tends to make systems using ammonia more
efficient than systems using other refrigerants. Despite their
very different properties, the volumes of vapor to be pumped
to produce similar amounts of refrigeration are similar for ammonia and for R-22. This is not the case with the widely used
R-134a that requires about 30% more swept volume and has
significant global warming potential.

Challenges Presented by Ammonia
The main challenge presented by ammonia when used as
a refrigerant is its acute toxicity. Ammonia has a good safety
record in refrigeration. This is largely because of adherence
to well-established codes and practices, and because ammonia vapor in the atmosphere becomes very unpleasant at
concentrations on the order of 25 ppm, whereas it does not
become lethal until concentrations of more than 1,000 ppm
are reached. However, the unpleasant smell of ammonia could
cause panic within an occupied space and ammonia systems
should be kept well away from the general public.
Ammonia does not present a major fire hazard, even in
large systems, because it is difficult to ignite and will not
sustain combustion. However, precautions must be taken if
the charge of ammonia is sufficient to produce flammable
proportions in the air within any space into which the refrigerant might leak. If the charge is small and the high-pressure
side of the ammonia system is effectively in the open air,
then it usually will be found that risk of fire due to ammonia
leakage is negligible.
Ammonia has a great affinity for water. An ammonia system
contaminated with water will not freeze up as a halocarbon
system would do under similar circumstances. However, this
is a rather mixed blessing. Most ammonia refrigerating systems are contaminated with water to an extent that would be
unacceptable in a halocarbon system. The drawback of having water present within an ammonia system is that it tends
to concentrate in the low-pressure side of the system, where
February 2013

Ozone Depletion Global Warming Ct
(°C)
Potential
Potential

Boiling
Temperature
(°C)

R-22

0.055

1,700

–41

R-23

12,000

–82

R-134a

1,300

101

–26

R-404A

3,780

–47

R-410A

1,980

–51

R-717

133

–33

Table 1: Sample properties of several refrigerants.
it may form sludge with lubricating oil, and it will interfere
with the operation of any conventional thermostatic expansion valve. Thermostatic expansion valves are unpopular with
practical ammonia engineers and for good reason. The valves
tend to demand continual adjustment to compensate for the effects of water, with the result that the superheat is set to such a
low value that systems flood over and damage the compressor.
Ammonia is incompatible with copper and zinc. This rules
out the use of brazed copper tubing, which is cheap to install
and is reliable in operation. Larger ammonia systems tend to
use welded steel piping, which is more expensive to install
and is much less convenient. Smaller ammonia systems could
use aluminium or stainless steel pipe with special fittings that
could eliminate site welding. Such fittings are already in use
for carbon dioxide systems in supermarkets.
It is technically feasible to produce hermetic or semihermetic compressors for use with ammonia but, at present,
only a few are available. It remains to be seen whether small,
sealed, compressors will be developed for use with ammonia.
By coincidence, a refrigerating system was developed in
1972 that allows the challenges of applying ammonia to small
refrigerating systems to be overcome.1

Low-Pressure Receiver System
The low-pressure receiver system (LPR) comprises a circuit
that drains the condenser under some form of automatic control and feeds all refrigerant flow through the expansion device
to the evaporator from which it returns to the compressor via
a suction trap associated with a heat exchanger that subcools
high-pressure liquid by heat exchange with low-pressure liquid collecting in the bottom of the suction trap. By this means
the evaporator is slightly overfed with refrigerant while the
compressor is completely protected against liquid flood back.
Figure 1 is a simplified version of the circuit with a Mollier diagram of the system, which is self balancing. It can be
shown that the amount of overfeed is a function of the heat
exchanger surface, the latent heat of the refrigerant and the
condensing and evaporating temperatures.2
In an ideal world, the overfeeding of the evaporator would
produce high heat transfer coefficients without requiring the
high-temperature differences required for stable operation of
a thermostatic expansion valve as in a conventional system.
ASHRAE Journal

ASHRAE Journal - February 2013

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