08 Comparison of Liquid Chillers with Screw Compressor for Air-Conditioning Applications with Ammonia and R134a // Energetischer Vergleich von Schraubenverdichtern in Flüssigkühlsätzen

Liquid chillers with indirect cooling are standard products in air-conditioning technology. In
general, water or brine is the secondary refrigerant that flows through various components of
an air-conditioning system. This brief note presents the comparison between R134a and
ammonia.
In Germany the installation of ammonia refrigerating systems is allowed in public buildings if
there is an indirect cooling system and all the refrigerant-containing components are
installed in a machinery room or outdoors. In the past few years the refrigerant charge for
liquid chillers with ammonia have been drastically reduced by using plate heat exchangers or
enhanced surface shell-and-tube heat exchangers.
For water-cooled systems 0.060 kg/kW can serve as a guideline for dry evaporation and
0.040 to 0.100 kg/kW (depending on the type) for flooded evaporation. This allows liquid
chillers with less than 50 kg ammonia for a wide capacity range which may be installed in
publicly accessible rooms without a separate machinery room. The concept of water chillers
with ammonia does not differ from that of liquid chillers with other refrigerants (see Figure 1).
Like any other piece of machinery, liquid chillers also involve operating and capital costs.
The capital cost is comparatively less than the operating cost. For suitability of any system, it
is essential to project the running cost over a set period of time for the desired operating
conditions.


Comparison of Operating Costs
The efficiency of energy conversion of electric energy into “cold“ does not only depend on
the refrigerant but also on the operating parameters of the compressor and heat
exchangers. It is a proven fact that lowering evaporation temperature and raising
condensation temperature results in higher operating costs. In order to perform a realistic
energy analysis for any two systems with different refrigerants, it is important that the
terminal temperatures of chilled water and the cooling medium (water or air) be identical.


Comparison of Energy Costs for Screw Compressors in a Liquid Chiller with 500 kW

Refrigeration Capacity
Two different types of evaporators are assumed here: flooded and dry expansion (DX). The
condenser type determines the condensation temperature. The air cooled condenser has a
larger temperature differential than a water cooled condenser. Both condenser types are
included in this comparison.

COP Values and Energy Cost Evaluation
The performance parameters and operating costs apply for the two refrigerants at equal
evaporation and condensation temperatures. The COP values can be improved at constant
cold water temperatures if the medium logarithmic temperature differences at one or at both
of the heat exchangers are reduced. An elevation of the evaporating temperature by 1 K
increases the performance coefficient in the water-cooled liquid chiller by 2.8 % and 3.3 %
for ammonia and R134a, respectively. However, the size of the heat exchanger goes up and
leads therefore to higher production costs. It is a standard procedure to increase
performance coefficients this way so that the differences in efficiency between the
refrigerants ammonia and R134a as shown in Figure 4 and Figure 5 do not have to occur in
such a degree in actual liquid chillers.

An increase of the COP value can also be achieved by the reduction of the pressure drop
between the evaporator and the compressor suction side as well as by evaporator control in
dry expansion systems with the smallest possible superheat. Compressor control devices
with PI-control permit smaller superheat (3 K are possible) than the traditional expansion
valves driven by control deviation (superheating).

The decision for one or the other type of liquid chiller will depend on the purchase cost, the
actual conditions at the installation site and the expected number of annual operating hours.
Computation of the total cost taking capital cost and energy cost into consideration permits a
quantitative decision from an economic perspective.
Employing a piston compressor instead of a screw compressor does not cause a major
difference in the energy consumption as shown above.


Often the location of the machinery room does not allow the use of a water-cooled machine.
Hence, in such cases the increased energy demand of an air-cooled liquid chiller has to be
accepted. The decision for one or the other refrigerant can be made in an evaluation of the
total cost based on the operating hours if the purchase cost of the liquid chiller is added to
the pure operating costs. In any case this study shows that the energy cost is considerably
lower with the refrigerant ammonia than with the compared refrigerant R134a. The
requirement to give priority to environmentally benign technical solutions can be met by
taking the TEWI value into account for the decision process as the indirect portion of the
TEWI value is directly connected to the energy consumption.