Chilling Systems - Low Maintenance and Eco-Friendly

23.09.2019

The optimum temperature in the production process is often a decisive factor for the quality of a product. Steam jet chillers are environmentally friendly because they only require water or aqueous solutions instead of chemical cooling agents. Instead of high-value electricity, low-pressure steam (waste steam) can be used for the operation. Steam jet chillers operate under vacuum, corresponding to the steam pressure of the water at cooling temperature. By means of steam jet compressors, the evaporated amount of water (about 1.5 kg per kW refrigerating capacity) is compressed to a higher temperature level and is condensed together with the motive steam. Low-pressure steam (waste steam) can be used as motive media for the steam jet compressors. The inert gases created by degassing of the chilled and cooling water or by leaks are drawn off by means of a vacuum pump, preferably a steam jet vacuum pump. Steam jet chillers are used to cool water or aqueous solutions in the higher temperature range almost down to ice build-up. The capacity range reaches from approx. 100 kW up to more than 20,000 kW. Examples of application are process cooling in the chemical industry, the food industry, the plastic and pulp industry, direct product cooling, e. g. of moulding sand, paints and crystal mash as well as the generation of chilled water for use in the air conditioning technology.

Operating Principle of Steam Jet Chillers

Steam jet chillers operate under vacuum according to the principle of flash cooling. 1 Motive steam 2 Medium to be cooled 3 Chilled medium 4 Cooling water 5 Warmed cooling water A Flash vessel B Steam jet compressor C Condenser

Figure 1: shows the principle of a steam jet chiller

Thanks to it is high evaporation enthalpy, water is well suited for this purpose. The cooling effect is all the greater the lower the pressures are maintained in the system. If very low temperatures are required and the condensation temperature of the flash vapor is below the temperature of the cooling medium, the vapor must be compressed so that it can be condensed at a higher pressure and temperature level. If steam jet compressors are used for this purpose, the systems are called steam jet cooling systems or steam jet chillers.

Design of Steam Jet Chillers Steam jet chillers are designed as single-stage units for a very small refrigerating capacity or low cooling. For a higher refrigerating capacity and cooling at higher temperature differences, several flash stages are arranged in series. Figure 2 shows a schematic view of a 3-stage steam jet chiller in bridge design. 1 Flash vessel 2 Condenser 3 Jet pumps 4 Deaeration vacuum pump 5 Collection tank 6 Re-cooling plant 7 Circulation pumps

Figure 2: 3-stage steam jet chiller in bridge design.

Steam jet chillers essentially consist of simple parts such as flash vessels, mixing or surface condenser, pipelines and steam jet compressors. They are, therefore, easy to operate and always ready for use. As they only have a few moving parts, they require little maintenance. The freezing point of water determines the limit of refrigeration that can be obtained, unless a drop in the freezing point, due to the presence of dissolved matter, makes it possible to obtain lower temperatures. The energy consumption values of multi-stage systems are low. The motive steam and vapor condensate can be fed back again into the process. The condensation heat is either discharged via a cooling tower or similar or it can be conveyed to another suitable process or heat consumer. If an aggressive medium is to be cooled, it can be done with a steam jet chiller without any difficulty. Because of their simple construction and the absence of moving parts, steam jet compressors can be manufactured from any material used in the engineering industry. Basically, there are three different designs of steam jet chillers: • Compact design for small capacities, • Column design for medium capacity and large capacities where there is little floor area, • Bridge design for high capacities. [gallery size="medium" ids="eyJ1cmwiOiJodHRwczpcL1wvd3d3LnR1cmtjaGVtLm5ldFwvd3AtY29udGVudFwvdXBsb2Fkc1wvMjAxOVwvMDlcL2dlYV8uanBnIiwidGl0bGUiOiJnZWFfIiwiY2FwdGlvbiI6IjxiPkZpZ3VyZSAzOiAyLXN0YWdlIHN0ZWFtIGpldCBjaGlsbGVyIGluIGNvbXBhY3QgZGVzaWduLCA8YnI+Y29vbGluZyBvZiA0NCBtXHUwMGIzXC9oIG9mIHdhdGVyIGZyb20gMzAgdG8gMTBcdTAwYjBDLiA8YnI+Q2hpbGxpbmcgY2FwYWNpdHk6IDEwMjUga1c8XC9iPiIsImFsdCI6IiIsImRlc2NyaXB0aW9uIjoiIn0=,eyJ1cmwiOiJodHRwczpcL1wvd3d3LnR1cmtjaGVtLm5ldFwvd3AtY29udGVudFwvdXBsb2Fkc1wvMjAxOVwvMDlcL2dlYV8xLmpwZyIsInRpdGxlIjoiU29cdTAxMWZ1dG1hIFNpc3RlbWxlcmkiLCJjYXB0aW9uIjoiIiwiYWx0IjoiU29cdTAxMWZ1dG1hIFNpc3RlbWxlcmkiLCJkZXNjcmlwdGlvbiI6IiJ9,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"]

Areas of Application

Steam jet chillers provide an excellent way to cool water used as a cooling medium down to temperatures of approx.. 0°C. It is particularly easy to install steam jet chillers where: • Steam and cooling water networks are already available, • Hot water or steam (waste steam, vacuum or wet steam) are available at a reasonable price. In addition to applications with a relatively constant average annual cooling requirement, steam jet chillers are ideally suited when fluctuating cooling capacities are required or there is a seasonal need. Steam jet chillers are highly suitable for the temperature level of 12°C / 6°C that is generally needed in air conditioning units. They can react immediately to changes in the cooling requirement and are also ideal for use in many discontinuous processes. The required motive steam can be generated from biogas, motor heat and other sources.

Steam jet chillers are characterized by:

• Low investment costs, • Simple and robust construction, • Immediate response to changes in the required cooling capacity, • Very low maintenance and repair costs (few moving parts, therefore hardly any wear and tear), • Reliable and safe to use, • Direct and indirect cooling is possible; with indirect cooling, the cooling medium is almost always water and therefore easy to handle, • Very low chilled-medium hold-up volume, • Frequent or fast change of load possible, • Minimum electrical energy required, • Waste heat and flash steam used as the motive media, • Used very efficiently during seasonal and/or batch operation.

Steam Jet Compressors

Steam jet compressors are jet pumps, also referred to as ejectors, driven by a motive medium (Ṁ1). GEA has long-standing experience in the design and construction of ejectors and ejector systems.

1 Head 2 Motive nozzle mixing nozzle 3 Inlet cone 4 Throat 5 Diffusor p₁ Motive steam pressure p₀ Suction pressure p Discharge pressure ps Pressure at sonic velocity Δp_v Shock wave Ṁ1 Motive steam flow Ṁ0 Suction flow Ṁ Mixing steam flow

Figure 6: Working principle of a steam jet compressor and the pressure differences over the flow.

In the motive nozzle (2) an adiabatic expansion of the motive medium (usually steam) takes place, whereby a steam jet many times the speed of sound is created. The steam jet is expanded, after passing through the motive nozzle, where the pressure energy is transformed into kinetic energy. This energy stream draws the evaporated vapors (suction flow Ṁ0) from the medium to be cooled, accelerating them into the inlet cone (3) of the mixing nozzle. After passing through the throat of the mixing nozzle (4), into the diffusor (5), the kinetic energy of the mixed vapor stream is gradually converted back into potential energy, i. e. the medium is compressed to a higher discharge pressure. Steam jet compressors convey at high velocity and can therefore cope with the large volumes generated by steam under vacuum.