Waste water recovery


In today’s world, waste water recovery is one of the main challenges in the energy industry. Each used kilowatt-hour from waste heat has outstanding effects in reducing greenhouse gas emissions and primary energy saving. There are a variety of techniques which allow us to exploit the waste heat at different temperature levels and for a specific purpose. Sometimes we can accomplish this by using a simple heat exchanger, but sometimes substantial investments are required.

We introduce a unique heat engine – Stirling SPP – which can be used as a high temperature heat pump or as a device for the production of electricity from waste heat.

• Stirling cycle
When the gas is being compressed its temperature rises, and when it expends its temperature drops. This is a simple explanation of the Stirling cycle.

• SPP concept
By using the Stirling cycle in which environmentally friendly working gas helium flows and drives the electric engine, the energy is generated through heat pumps in the form of heat or cold as recycling of waste water in the industrial process.

• SPP uniqueness
It is the only heat pump on the global market that is capable of recycling low temperature heat and simultaneously returning the heat of 200°C back into the process.

A single phase process of SPP technology operates independently of boiling points and other restrictive factors present in traditional heat pumps.

• SPP usability
With a low temperature waste heat from an industrial process is usually mostly unused; we easily connect a SPP heat pump to the existing cooling and heating system or even waste water system.
High temperature heat pump and/or generating electricity from excess heat with the Stirling cycle, SPP HL and HP series. The machine is designed to produce either electricity from low or medium temperature level of the waste heat (from 70°C to 240°C) or as a heat pump for increasing the temperature level (from 70 °C to 200 °C).



SPP Stirling is a serial, four-cycle, Alpha type of Stirling engine where each cycle consists of two cylindric chambers, i.e. compression and expansion chambers which are connected with three sets of heat exchangers: compression heat exchanger, regenerator and expansion heat exchanger. This means that SPP Stirling is a 4-cylinder 90° V engine, with cross type double-acting pistons. The working gas in the Stirling is helium. The Stirling also has a separated system for lubrication and cooling, operation and control over the working gas, diagnostics and recording of alarm messages and errors.


HP  – waste heat power plant

The HP plant (heat engine) for the production of electricity from any waste heat with the following requirements:

• 1 MW of heat output for achieving the nominal plant output.
• Electric power depends on temperature differences between the waste heat (power source) and the temperature of the cooling water.
• For the optimum operation of the HP it is recommended that heat flow is over 120°C and the flow of the cooling water at 20°; operation is also possible at lower temperatures but this results in a lower power output and lower plant efficiency.
• Maximum temperature of the heat source is 200°C with serial-mounted seals and O-rings, while operation at 240°C is possible at a surcharge for replacement of the sealing material.
• The recommended and most effective solution for heat transfer is hot water with flow rate (10–40 kg/s) or steam with flow rate 0.5 kg/s in a closed circuit. Heat transfer is also possible by using thermal oil but with lower performance and reduced power output at the same temperature. For the use of other media such as heat exchangers it is necessary to conduct a preliminary verification.
• Cooling water is contained in a separate circuit (10–40 kg/s).
• Synchronisation of a power plant with the network is executed through a frequency converter. The power plant can be connected to an internal or external (distribution) LV three-phase AC network with 400 or 690V.

Electrical efficiency of HP follows the rate 0.5 x (1 – average cooling temperature/average heating temperature), by taking into account the average temperature at the entry and exit of each circuit in Kelvin.
Total HP electrical output depends on the operating conditions; it is recommended to range between 100 and 200kW.
HP is not sensitive to temperature fluctuations and available heat flow.
The control system constantly adjusts the device to the available heat and temperature changes.


HL  – high temperature heat pump

Our first two devices operate as high temperature heat pumps placed in the boiler room of a dairy and produce up to 430kW of heat at 120°C and up to 230KW of cold between 0 and 30°C.


The base of HP or HL must be fixed to a concrete foundation in the boiler or engine room. The total mass of the device is approx. 10,000kg. There must be at least 1m of room around the device for maintenance and operation services.


• Flanges for water connections as per ISO/DIN
• Cabinet for frequency regulator
• Access to Enthernet RJ45 for remote control and management
• 400V for control system and pumps
• Connection to work gas helium (R-704, balloon quality) from 50-litre gas cylinders



HP as well as HL are fully automated devices controlled through PLC with an interface for remote recording and transfer of data. Operating data are constantly remotely analysed through the Remote Control Centre.

Maintenance and repairs:

• Working gas helium: replacing the empty gas cylinders with full ones (devices constantly lose small quantities of helium which is harmless to human health and environment)
• Oil analysis every 8,000 operating hours, replacing oil filters every 8,000 operating hours, oil change 1x per year
• Normal inspection: inspection of the device according to the manufacturer’s protocol and replacement of piston rings every 8,000 operating hours
• Annual inspection: inspection of seals, sealing level of devices and key points
• Main inspection: replacement of seals every 2 years
• Replacement of bearings when necessary, usually every 2–4 years
• Expected service life is at least 25 years


– food industry (dairies, breweries, slaughterhouses, bakeries, sea food processing plants, etc.)
processing industry (chemical, construction, refineries, paper mills, etc.)
other (district heat, waste heat, etc.)


Devices are sold, installed, maintained and services by IMP Promont d.o.o., Ljubljana

Any additional explanations and questions are available through the following contacts:
Email: matevz.cokl@imp-pro-mont.si
MOBILE: 00 386 (0)51 380 558