Download - Australian Institute of Refrigeration Air Conditioning and ...

26
FORUM
Ammonia Absorption
Refrigeration Plant
D W Hudson, Gordon Brothers Industries Pty Ltd
ABSTRACT:
The application of ammonia absorption systems offers many advantages and this paper will review the basic
operating cycle, and investigate performance and economic benefits.
Keywords: Ammonia, absorption, refrigeration cycle.
INTRODUCTION
Refrigeration plants using absorption principles have been
around for many years with initial development taking place
over 100 years ago.
Although the majority of absorption cycles are based on
water/lithium bromide cycle, many applications exist where
ammonia/water can be used, especially where lower
temperatures are desirable.
In both systems water is used as working fluid, but in quite
different ways: as a solvent for the ammonia-system, and as
THE OFFICIAL JOURNAL OF AIRAH - august 2002
refrigerant for the lithium bromide system. This explains that
the lithium bromide absorption system is strictly limited to
evaporation temperatures above 0ºC. On the other hand,
water as a solvent in an ammonia absorption system has a
vapour pressure that requires rectification, whereas LiBr - a
hygroscopic salt - is non-volatile and the desorbed water
vapour is free from solvent without the need for purification.
The main industrial applications for refrigeration are in the
temperature range below 0ºC, the field for the binary system
ammonia-water.
A typical ammonia absorption system is shown above.

Operation of the refrigeration cycle is conventional with high
pressure liquid entering the liquid receiver from the condenser
before passing to the evaporator where heat is absorbed from
the process.
The remaining items in the system replace the conventional
compressor to achieve “thermal” compression in three steps.
1. Absorption of the ammonia vapour in a weak ammoniawater
solution at evaporation pressure.
2. Transport of the strong ammonia water solution from
evaporator pressure to condenser pressure.
3. Removal of the ammonia from the ammonia-water solution
(Desorption) at condenser pressure, together with the
purification of the ammonia by heat energy.
High pressure ammonia leaving the fractionator column
passes to the condenser for the cycle to continue.
It is important to note that the system must maintain a
thermal balance with total heat input balancing with the
total heat rejected. This provides a simple check on the
plant, as a variation would indicate a design error.
A description of each component in the plant follows:
a. A conventional evaporative condenser is used which in
fact is slightly smaller than for a conventional plant.
Ammonia vapour leaving the fractionator column is not
highly superheated which allows the condenser to operate
without the need for desuperheating.
b. The liquid receiver is conventional providing for variations
in refrigerant volume flowing in the system.
c. A plate heat exchanger is used for glycol chilling duties as
in a conventional plant.
d. The suction liquid heat exchanger E3 provides subcooling
of the liquid and superheating of the suction gas. This
heat exchanger is particularly important to improve the
overall plant efficiency. The effect of a highly superheated
gas at the absorber is not critical.
e. The absorber is a critical plant item allowing the ammonia
vapour to be absorbed into an ammonia water solution. It
is a combined heat and mass transfer problem where a
considerable quantity of heat is liberated which must be
rejected to the ambient air or a cooling water system.
Careful design of the absorber is important and thin film
techniques over a tube bundle are normal to handle the
large differences in flow volumes between the ammonia
vapour and the ammonia water solution.
Evaporating pressure is set by the absorber and if it fails
to reject the required heat of solution under maximum
operating conditions, then evaporator pressure and
temperature will rise accordingly.
Total heat rejected by the absorber is much greater than
for the evaporator duty and this depends to some extent
FORUM
on the COP of the plant. Typically it can be over twice as
much as the evaporating duty.
f. The absorbate receiver collects the strong ammonia water
solution from the absorber for pumping.
g. A positive displacement pump or a high head centrifugal
pump is used to lift the ammonia water solution from
evaporator pressure to condenser pressure. The power
required for this pump is only small as the volume flows
are relatively low for the system. For industrial plants, the
power consumed is negligible and can be minimised by
optimisation of the plant design.
h. The Aqua Heat Exchanger E1 reduces the temperature of
the ammonia water mixture from the fractionator column
and preheats the feed to the column. This heat exchanger
is also important to increase the overall cycle efficiency.
In addition, for the absorber to work at peak efficiency,
the weak ammonia-water solution must be as cool as
possible.
Heat exchanger E2 removes heat from the steam
condensate to add further heat to the column feed.
i. The fractionator column accepts the preheated feed from
the Aqua-Heat Exchanger, where some desorption of the
ammonia and water vapour occurs. It is divided into two
sections each containing a fill material to aid the
distillation process. The rectifying section is above the
feed point and the stripping section is below the feed.
Liquid ammonia from the receiver is introduced as reflux
into the top of the column to allow the ammonia vapour
to be purified with a residual water content of 100 ppm or
less. Reflux adds to the quantity of heat required for the
column and must be kept to a minimum, consistent with
maintaining an acceptable vapour quality at the outlet.
All liquid ammonia introduced as reflux must be
evaporated by heat from the reboiler. To offset the
amount of heat for the reboiler a cold reflux from the
absorbate pump can be used. This improves the heat ratio
of the plant.
In the stripping section, ammonia is stripped from the
ammonia-water solution, as it contacts ammonia and
water vapour rising from the base of the column. The
stripped solution is not allowed to mix with the weak
ammonia-water solution in the base of the column, as a
mixture would necessitate a higher column temperature.
Instead the stripped solution passes to the reboiler
(desorber) where heat is added to drive the process.
A considerable difference in temperature occurs over the
column length with the highest temperature at the base,
falling to condensing temperature at the top of the
column. This fact allows heat to be introduced into the
column at various levels, if desired.
THE OFFICIAL JOURNAL OF AIRAH - august 2002
27

FORUM
At the base of the column a weak ammonia-water solution
is collected before passing to the Aqua-Heat Exchanger
and absorber for re-use.
j. The reboiler accepts the stripped ammonia-water solution
from the column and adds heat to drive the ammonia from
the water.
It is possible to pump the ammonia-water solution
through the reboiler, but this only adds unnecessary
complications to the circuit.
During the heating process, some water vapour is driven
off with the ammonia and this is why it is important for
the fractionator column to remove as much moisture as
possible using well designed reflux for the column.
The mechanical vapour compression system is shown below
to illustrate the differences in complexity of the two
systems.
Mechanical Vapour Compression
CO-EFFICIENT OF PERFORMANCE
Co-efficient of performance (COP) is defined as the ratio of
refrigeration effect, divided by the heat or power input. For
an Absorption system this is generally below unity, although
it is theoretically possible to obtain figures well above unity.
Losses in the system and thermodynamic irreversibilities will
generally prevent a COP greater than one for refrigeration
cycles.
The co-efficient of performance is also known as the Heat
Ratio in absorption systems and can be shown as follows:
Coefficient of Performance = Refrig Effect
(Heat Ratio) Heat Input
Theoretical Carnot COP = (Tevap ) (Tgen - Tcond)
(Absorption Plant) (Tcond - Tevap ) ( Tgen )
Theoretical Carnot COP = Tevap
(Mechanical Plant) (Tcond-Tevap)
Tevap = Absolute evaporating temperature (k)
Tcond = Absolute condensing temperature (k)
28 THE OFFICIAL JOURNAL OF AIRAH - august 2002
Tgen = Absolute generator temperature (k)
It is important to note that the COP for an absorption plant
is the same as for a mechanical plant but is modified by the
COP of the heat engine to drive the plant. If the COP of the
power generating station together with transmission losses
were added to the mechanical system then the overall COP of
both systems become much closer.
Typical COP
Calculation of COP that can be expected from an absorption
plant and a mechanical vapour compressor system is given
below.
Operating Conditions
Tevap = 253 K (-20ºC)
Tcond = 308 K (+35ºC)
Tabs = 308 K (+35ºC)
Tgen = 423 K (150ºC)
Carnot COPabs = 1.251 Practical COPabs = 0.6
Carnot COPmvc = 4.6 Practical COPmvc = 2.75
Tabs = Absolute absorber temperature (k)
Comparison of COP
Co-efficient performance is affected by evaporating
temperature and from the above figure it is quite evident that
the co-efficient of performance of the two systems are vastly
different. The COP for the absorption plant is much less
affected by a drop in evaporating temperature and this is a
significant advantage in overall economy.
A number of other factors also affect COP in an absorption
refrigeration plant and these are:
• Column temperature
• Ammonia purity (reflux ratio)
• Condensing temperature
• Cold reflux ratio

• Absorber temperature
• Heat exchanger effectiveness
Source Temperatures
There is almost a direct relationship between the maximum
temperature of the fractionator column, the condensing
temperature and the evaporating temperature.
As the condensing temperature increases, the base
temperature of the column increases. Also as the evaporating
temperature decreases, then the base temperature of the
column increases and this is shown below.
For low temperature applications, high grade waste heat is
essential, but if this is not available, then economic
operation is quite possible using direct steam or gas heating.
It is also important to note the thermal gradient in the
fractionator column means that heat can be added at lower
temperatures at strategic points in the column. This reduces
the quantity of heat required at maximum temperature, but
may enable other heating sources to be utilised.
OPERATING ECONOMICS
Even though the thermal efficiency of the absorption plant is
quite low, the direct operating costs must be considered in
evaluating the plant.
Running Cost Comparisons (500 kW Plant)
FORUM
The above figure compares the expected running cost per
hour for a mechanical vapour compression system using power
at a cost of 8.5c/kWhr and also an ammonia absorption
refrigeration system using steam generated from a boiler at
costs of $10, $15 and $20 per tonne of steam produced.
The upper curve is for $20 tonne of steam produced and at
this level the AAR plant cannot compare with a conventional
plant. However at lower costs of $15 and $10 / tonne the
AAR plant does have lower running costs as the evaporating
temperatures become lower.
CAPITAL COST
Preliminary estimates indicate that the ammonia absorption
plant would be 20% to 30% more expensive than
conventional equipment in sizes above 350 kW capacity. This
only compares equipment costs, and does not include services
such as transformers, electrical starters, electrical mains and
buildings, which are all required for a conventional plant, but
are not required for the absorption system.
ADVANTAGES
There are a number of advantages for Ammonia Absorption
Refrigeration Plants which need consideration:
1. The equipment produces no noise other than pumps, which
are small low power units.
Noise is a major problem with plant rooms, and with
tighter specifications, conventional systems with screw
compressors are difficult and expensive to attenuate.
2. There is no oil the system and the plant requires no oil
management. In a new plant, oil will not contaminate
heat exchange surfaces, which, if kept clean, will always
deliver maximum performance.
Oil is always a problem in ammonia systems, with a
continuous loss to the system occurring. Recovery is
difficult and the oil is normally unsuitable for re-use in
the system.
3. If waste heat is available, running costs are very low, as
only condenser fans, aqua pumps and absorber fans need
to be powered.
In a conventional system, the compressor will absorb up
to 90% of the total power used. If waste heat is available
at the right temperature, then the power savings are very
significant.
4. High turn down ratios are possible, giving efficient part
load performance.
Generally speaking, all refrigeration plants are only sized
for peak load conditions. For the majority of the time,
plants run at part load, with low efficiencies. Screw
compressors at high compression ratios have particularly
poor part load characteristics, resulting in excessive power
consumption.
THE OFFICIAL JOURNAL OF AIRAH - august 2002
29

FORUM
5. The number of moving parts is restricted to fans and
pumps, which are easy to service and give long troublefree
life. Maintenance costs are low, as there are no
expensive compressors to maintain.
6. Ammonia Absorption Plants can be fitted to existing
systems to replace conventional compressors or
supplement existing capacity.
7. The fractionating column and condenser can be located
several hundred meters from the evaporator and absorber,
with small interconnecting lines, which need not be
insulated. This permits maximum flexibility in plant
layout, without compromising overall efficiency. This is
simply not practical with a conventional plant, due to
system pressure drops.
8. For AAR plants, plant rooms are not required, as the
absorber must be located outside and the fractionating
column can be conveniently located at the heat source.
Savings in building costs would be substantial.
9. As the demand for electric power is minimal, the need for
large transformers and electrical mains, normally
associated with a refrigeration plant, is eliminated. This
cost is often overlooked in the cost of conventional
refrigeration equipment and yet it is fundamental to the
operation of the plant.
10.Liquid carryover is not a problem with Absorption Plants,
as there are no moving parts to damage. In certain
instances, liquid carryover from evaporators can be
desirable, to prevent the build-up of water in the
evaporator.
30 THE OFFICIAL JOURNAL OF AIRAH - august 2002
Liquid carryover in conventional plants is a major reason
for compressor failures. Its causes are many, and even
well maintained plants can suffer, due to sudden load
changes or control malfunctions.
11.No upper limit for the size of the plant.
12.Waste heat can be conveniently converted to refrigeration,
without the need for conversion to electrical energy.
DISADVANTAGES
There are some disadvantages with an ammonia absorption
plant and these are detailed below.
• Capital cost higher than mechanical plant
• More complex refrigeration system
• High grade heat required
• More space required
• Perception that it is outdated technology
CONCLUSION
The Ammonia Absorption Refrigeration Plant offers a number
of advantages at temperatures below 0ºC and where waste
heat or cheap steam is available, significant running costs
savings can be made.
With flexibility in operation, absence of compressor noise,
very low maintenance and high reliability, industrial Ammonia
Absorption Plants, should be considered as a viable
alternative to mechanical vapour compression plants.
AIR-FLOW MEASURING INSTRUMENTATION
•
sales repair calibration
•
•
• ANEMOMETERS • AIR-FLOW HOODS
• MANOMETERS • PITOT TUBES
• VELOMETERS • AIR-FLOW MONITORS
Australian distributors for Alnor Instruments
CALL FOR A FREE PRODUCT CATALOGUE
Full service and calibration facilities available
for Alnor and other brand name instruments
Unit 1/20 Kareena Road North
MIRANDA, NSW 2228 AUSTRALIA
Tel: (02) 9540 4626
Fax: (02) 9540 2351
e-mail: info@flowcal.com.au
Web site: www.flowcal.com.au