Case studies

Letter Sorting Centre / Switzerland

Planning / Implementation / Main components: Johnson Controls Systems & Service GmbH
Refrigerant: Ammonia (NH3)
System start-up: 2007

The Swiss Post letter sorting centre in Mülligen near Zurich handles up to 4.5 million letters every day. The largest building in Switzerland needs lots of energy for heating and cooling. The energy supply system for Mülligen letter sorting centre was planned and installed by ewz (Zurich municipal utility company). The system is owned and run by ewz. Mülligen letter sorting centre is part of Energieverbund Schlieren (Schlieren energy consortium) and therefore provided with heat and refrigeration by ewz as energy contractor. On behalf of ewz, Johnson Controls has designed and produced one of Europe's largest ammonia heat pumps.


The Mülligen letter sorting centre was in need of an efficient and eco-friendly solution for heating and cooling.


The necessary energy is taken from the wastewater of a nearby sewage plant, constituting an energy source with a potential of 266 million kWh waste heat per year. The refrigerating capacity of the heat pumps is 4.3 MW, with a heating capacity of 5.6 MW at 62°C outlet temperature. Around 50% of the heat energy is taken from the treated wastewater from the sewage plant, with another 30% from the waste heat emitted by the building and the technical systems. If no room heat is needed, the waste heat from the refrigerating machine is cooled down again using the treated wastewater. This applies particularly in the summer, when the many automated letter sorting systems demand an increased cooling load of 4.9 MW.

The whole ammonia system is installed in a plant room of around 70 m². Three Sabroe reciprocating compressors are used for first-stage refrigeration at an evaporation temperature of 5°C and a condensation temperature of 30°C. In heat pump mode, five Sabroe high-pressure reciprocating compressors compress the ammonia and generate a temperature of 65°C. The high-pressure liquid is supercooled and expanded in two stages via the intermediate pressure vessel. The coefficient of performance (COP) in the heating phase was calculated at 4 without supercooler.      

Technical data:



Type of facility Heat pump
Refrigerant: NH3
Refrigerant volume 880 kg
Secondary refrigerant


Evaporating temperature


5 °C (first stage)
30 °C (heat pump mode)

Condensing temperature 30 °C (first stage)
65 °C (heat pump mode)
Refrigerant capacity 4.3 MW refrigerating capacity
5.6 MW heating capacity
Main components 3 Sabroe reciprocating compressors, 5 Sabroe high-pressure reciprocating compressors
Special features / Comments:

The system which was commissioned in 2007 reduces the consumption of fossil fuels by 85% with a reduction in carbon emissions of 3,400 t per year.


Johnson Controls Systems & Service GmbH
Ralf Zimmermann
Gottlieb-Daimler-Str. 6
D-68165 Mannheim

Phone: +49 (0)621 468 620
Fax: +49 (0)621 468 245
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