07 Technical and energetic appraisal of ammonia refrigerating systems for industrial use // Technische und energetische Betrachtungen von Ammoniak-Kälteanlagen

0. General
The refrigerant ammonia (NH3) is the definitive operating fluid for industrial use. Throughout
its period of use as a refrigerant, NH3 has proven to be ideally suited to this purpose. Largescale
refrigeration plants, e.g. for the food product, beverages and chemicals industry, have
been and are being operated successfully with NH3.

Fig. 1: Refrigeration applications with NH3
These systems mostly used in dairies, breweries, abattoirs and large refrigeration plants,
and often filled with several tons of refrigerant, were installed for a large number of
operators, ran for decades and were then dismantled again and disposed of at the end of
their service lives. NH3 thus has the unmistakable advantage that many years of extensive
experience and know-how is available for dealing with this natural refrigerant, gathered
regularly from the above mentioned areas, but also in applications previously deemed to be
sensitive to NH3 (e.g. human air-conditioning systems).
Another important reason for the undisputed use of NH3 for decades in many areas of
industrial refrigeration is the economic efficiency resulting from its outstanding
thermodynamic properties. The efficiency, expressed in total costs, is one of the key criteria
for deciding for or against an NH3 system. Higher procurement costs resulting from the more
complicated technical work aspects of its production are offset by a lower refrigerant price
and less energy costs to operate the refrigerating system, so that the overall costs for an
NH3-refrigerating system are extremely favourable. The ratio of refrigerating capacity to
required power input (COP, Coefficient of Performance) for an NH3 system is often much
higher than that for traditional systems operating with synthetic refrigerants. This can offer a
significant reduction in the electrical energy costs, which will often pay back the additional
capital cost in a relatively short time, i.e. 1 to 3 years.
The number of annual operating hours of large-scale refrigeration plants used in the above
areas and in particular in the food product industry is very high. Priority must therefore
always be given to economic use of energy in fulfilling the refrigerating demands. Special
attention should be paid to fluctuations in the outside air temperatures (tA ≈ -15 to +35 °C)
and to the possibility of using such fluctuations to save energy in operating the refrigeration
plant. This is illustrated by the cyclic process for a refrigeration plant shown in Fig. 2:

The refrigeration plant transports heat from a low temperature to a higher temperature. The
compressor output required for this purpose depends on the heat quantity per time unit and
on the difference in temperature. This results in the following starting points for rational use
of energy:

· The room being cooled should have a minimum thermal load (refrigerating demand
from external heat, transmission, packaging, ventilator heat and similar)
· The temperatures in the room being cooled should only be as low as necessary
(regular controls, service cycles)
· Dissipation of the condensation heat must be brought to a low level with the available
media (air/water), paying attention to seasonal fluctuations in temperature.

The lower the temperature for heat absorption and the higher the temperature for heat
emissions, the more power is required.

1. Plant concept
Given the extensive range of applications for NH3 refrigeration plants in industrial
refrigeration, the evaporation temperature range extends from approx. -50 °C to +5 °C
(freeze-drying of coffee, through processing room refrigeration in meat and sausage
production to building air-conditioning application). NH3 refrigeration plants are designed in
one and two stages. The screw compressor with all its advantages has become the most
common solution in industrial refrigeration applications. It has a major advantage over the
reciprocating compressor of a wider single stage operational envelope. The reciprocating
compressor is not allowed to exceed a pressure ratio corresponding to -10 °C/+40 °C (+45
°C for water-cooled cylinders) because of the high discharge temperatures occurring here,
particularly with NH3. The resulting plant concepts are shown in table 1:


The plant concept defined in the planning phase with the corresponding operating
parameters essentially stipulates the economic efficiency of the future refrigeration plant’s
operation (COP, see section 0). Together with efficient rating of the compressors, the heat
exchangers used on both the heat absorption side (evaporator) and the heat emission side
(condenser) play a major role in the economic efficiency of the refrigeration plant. The
following Figs. 3 to 6 show diagrams of the various plant concepts.

A single-stage refrigeration plant with screw compressors can be fitted with an economizer
(operation with supercharging, Fig. 4) to enhance its capacity Q0 with a slight increase in the
required power input of the compressor motor Pe, e.g. D Q0 = +13 % at D Pe = +8 %.

2. Refrigeration plant / high-pressure side (heat emission side)
The energy demand and thus the economic efficiency of a refrigeration plant depends
essentially on effective dissipation of the condensation heat. Heat dissipation takes place
with air (condenser; axial), with air/water (evaporation condenser) or with water (pipe bundle
or plate heat exchanger). Heat dissipation with water (surface water, river or seawater) is
only rarely used in Europe for land refrigeration plants, particularly in food product
refrigeration and storage. The chemicals industry mainly uses water from rivers and wells for
cooling. In the case of marine refrigeration systems, generally it is normal to use seawater
for heat dissipation. Land refrigeration plants make equal use of dry (axial condenser) or wet
systems (evaporation condenser). Various different factors (e.g. use of night operation,
installation requirements, erection weight, plant size, possibility of storing cold water, water
costs) affect the choice of the specific configuration. But the possibility of combining dry and
wet systems is also used in practice.

The choice of condenser design should certainly give due consideration to fluctuations in the
outside air temperature tA and thus to the possibility of always condensing as close as
possible to the minimum permissible condensation temperature tCmin. Selection of the
condenser design along these lines will result in minimum operating costs for energy and
water. The following diagram (Fig. 7) clearly shows how energy and water consumption in
refrigeration plants can be influenced by keeping close to the minimum permissible
condensation temperature tCmin.

The use of screw compressors with their integrated oil circuits permits discharge pressures
of up to the equivalent of +50 °C (compared to reci procating compressors with max. +40 °C).
The heat generated during compression is dissipated partly by the oil cooler in the oil circuit,
thus relieving the condenser. The dissipated oil cooler heat can be used for thermal recovery
without special apparatus. The high temperature level of this procedure (up to approx. +90
°C/+65 °C oil temperature) results in a large appli cation area for this waste heat.

3. Refrigeration plant / low-pressure side (heat absorption side)
On the low-pressure side of the refrigeration system, heat is absorbed from the room being
cooled or from the product in it via the evaporator. The evaporator can be part of the
following cooling systems:

In selecting the operating mode of the refrigeration plant, it must be borne in mind that the
COP and thus the economic efficiency of the refrigeration plant is influenced to a certain
extent by every heat exchanger contained in the plant i.e. also the evaporator. In terms of
saving energy, the aim must be to select the smallest possible difference in temperature
between the evaporating refrigerant and the air being cooled.

Dry evaporation (DX mode) with thermostatic expansion always demands an overheating
section in the evaporator so that the thermostatic expansion valve can work correctly. The
low mass flow density of the refrigerant NH3 (the density of NH3 is only about half that of
conventional HFC refrigerants) means that rating the evaporators is not without problems
eurammon-Information No. 7 / November 2007
and demands plenty of experience. Consideration of the following special aspects of
refrigerant NH3 compared to HFCs

· high evaporation heat, e.g. compared to R404A 1.328/181.5 kJ/kg at -20 °C
· low expansion vapour share
· lower refrigerant mass flow, e.g. compared to R404A ≈ 1/4

and the structural design of the evaporator including the chosen expansion permit such low
differences in temperature between the evaporating refrigerant and the air being cooled as
can be virtually achieved with evaporators in flooded mode. The advantages of the DX mode
consist in a lower quantity of refrigerant required to fill the refrigeration plant, thus saving on
refrigerant (environment and safety aspects).

Industrial applications, which are the main use for NH3 refrigeration plants, normally use the
flooded mode for medium-sized and large capacities. Refrigeration plants with evaporators
working with flooded evaporation (see section 1, Fig. 5) must be provided with a liquid
separator. The liquid separator sends liquid refrigerant to the evaporators which in this case
work in flooded state. The size of the liquid separator is to be rated on the one hand to
prevent any intake of liquid refrigerant into the compressor, and on the other hand to ensure
that the evaporator is supplied with liquid refrigerant. In industrial refrigeration plants,
refrigerant pumps are used for supplying refrigerant to the evaporators in large distributed
systems, such as food factories. Packaged chillers for process and air-conditioning
applications operate without pumps, usually using gravity fed, i.e. thermosyphon, refrigerant.
There are also examples of pumpless refrigeration systems supplying refrigerant to
evaporators in cold and chill store applications.
The functional principle for evaporators with pump forced circulation is necessary in

· if many consumers have to be supplied with refrigerant which are possibly far apart,
· if there are large flow resistances through the evaporators including delivery and feed
pipes, and
· if particularly constant evaporation is required (same flow form and thus same heat
transmission coefficients in the two-phase flow occurring in the pipe taking the flow,
during evaporation), particularly during part load operation or with several rows of
pipes in the depth.

This warrants very reliable refrigerant supply to the evaporators; slight differences in
temperature can also be expected between the evaporating refrigerant and the air being
eurammon-Information No. 7 / November 2007
cooled (approx. 3 K). It is thus possible to increase the evaporation temperature of the
compressor and thus improve the COP and the energy efficiency of the refrigeration plant
(see section 0, Fig. 2).

In gravity circulation systems, the refrigerant flows from the liquid separator to the
evaporator through the difference in density in the liquid separator and evaporator and as a
result of the inlet height. This requires a precise arrangement of liquid separator and
evaporator. The height of the liquid separator must be stipulated so as to overcome the
pressure resistances in the pipe to the evaporator and in the evaporator itself. However, if
the liquid separator is positioned too high, the liquid column in the inlet pipe results in
inadmissible increases in pressure with superheating in the evaporator.
Together with the energy savings from increasing the evaporation temperature, the choice of
evaporator must also look at its functions in the cooling system. These include:

· Adapted air volume of energetically optimised ventilators (efficiency ventilator/motor,
size of the dynamic pressure share)
· Compliance with low temperature differences between evaporating refrigerant and
the air being cooled, with the following advantages:
o Minimum icing
o Longer defrosting intervals
o Less energy required for defrosting
· Adapted relative humidity of the air being cooled to preserve the quality of the
products being cooled
· Optimum air control in deep-freeze plants, making use of natural thermals according
to the cold air lake principle

4. Refrigeration plant with indirect cooling system
Refrigeration plants with an indirect cooling system (see section 1, Fig. 6) are used where a
direct cooling system is not suitable, for safety reasons, on account of demands to save
refrigerant, or for various requests expressed by the operator. In other words, in nontraditional
NH3 application areas where parts conveying refrigerant must not come into
contact with the substance being cooled, possibilities for using NH3 are created by indirect
cooling systems with double separation between refrigerant and the air being cooled. Indirect
cooling systems with NH3 as refrigerant are used both in industry and for air-conditioning
systems. The main areas of application here include:

· Industrial and human air-conditioning
· Food product industry
· Chemicals industry
· Automotive industry

NH3 air-conditioning systems in particular result in refrigerant charges of less than 0.1kg/kW
for the production of cold water (see eurammon-information No. 9). As well as fulfilling safety
requirements, industrial applications in particular offer significant refrigerant savings
compared to refrigeration plants with a direct cooling system, as the NH3 refrigeration system
is usually accommodated centrally in a special machine room. Processing rooms in
particular are mainly equipped with air coolers using secondary refrigerant (corresponding to
Fig. 6).

The secondary refrigerants mainly consist of blends of ethylene glycol or propylene glycol
(for food product applications) with water in various ratios and with a wide range of different
trade names. Another frequently used secondary refrigerant is Pekasol 2000, a combination
of organic salts which can be used for cooling down to -70 °C with still acceptable viscosities.

Brine as the traditional secondary refrigerant of former times is only rarely used today (note
the use of materials here). Brine consists of salt-and-water solutions with sodium chloride,
(NaCl), magnesium chloride (MgCl) or calcium chloride (CaCl).
A highly favourable secondary refrigerant in the temperature range below +15 °C is carbon
dioxide (CO2, freezing point -56.7 °C). In particular the possi bility offered by CO2 of working
with phase transformation brings significant improvements compared to conventional
secondary refrigerants in the rating parameters for refrigeration systems with an indirect
cooling system (see table 3).

When using the secondary refrigerant CO2 with phase transformation, savings are possible
in the following areas:

· Pipe dimensions: reduction by approx. two to three nominal widths
· Pumping capacity: volume flow is decreased approx. 5.5 fold, more favourable
viscosity values by approx. two to the power of ten
· The evaporation temperature in the refrigeration plant can be increased by approx.
3 K (corresponding to -25 °C to -22 °C)

One reference project for this kind of system is Europe’s most advanced fish processing
plant in Sassnitz on the island of Rügen, Germany. Here the secondary refrigerant CO2 is in
the secondary circuit of a cascade refrigerating system, with a two-stage NH3 refrigeration
plant as primary circuit (Fig. 8). The refrigeration plant is installed centrally in a refrigeration
machine room. In contrast to conventional secondary refrigerant circuits, the CO2 undergoes
phase transformation on absorbing heat in the air cooler: it boils into a gas. The CO2 gas
then enters the NH3/CO2 cascade heat exchanger where it condenses. The individual
sections of the overall plant are equipped with different secondary refrigerant circuits, as
shown in Fig. 8.

The use of the propylene glycol/water mixture as secondary refrigerant in the circuit for the
processing rooms results from the high pressure level (particularly for rating pipes and
fittings) which would occur when using CO2 under these conditions.