1 GENERAL BAT CONCEPT FOR INDUSTRIAL COOLING SYSTEMS SYSTEMS
1.5 Selecting a cooling technique in order to meet environmental requirements environmental requirements
1.5.3 Options for a technological change of existing systems
For a new cooling system, there will be more flexibility to select between complete systems and to assess the alternative options, whereas for an existing installation a change of technology often is a drastic solution. Sometimes in specific cases, it is possible to change the technology, but the number of options to reduce emissions via technological solutions is limited for existing installations. As the BAT “approach” considers that prevention of emissions prevails, taking into account also the economical aspects, change of technology is an option that should be considered before the optimisation of operating a cooling system is to be further assessed. In the
following paragraphs observations and experiences by suppliers are presented to give examples of possible optimisation steps in the BAT “approach” (See also Annex XI).
1.5.3.1 Retrofit – reasons and considerations
Retrofitting existing installations can be considered for the following reasons:
1. replace existing technology by a different technology with lower operating demands, 2. replace outdated technology equipment by modern equipment with higher efficiency, and 3. modify existing equipment to improve performance or to meet additional demands.
Different from the selection of a new installation, where the site parameters can be more or less defined, in retrofit scenarios usually the following number of parameters is fixed:
• space - the retrofit installation must fit into the existing space,
• the availability of operating resources – the new installation should not exceed the operating resources, which were needed for the old one, new infrastructure would result in an increase in costs, and
• legislative restrictions – environmental impacts, like sound criteria, usually have to be at the same level or below the ones of the old installation.
Space is often an important reason for retrofitting itself. If a plant or building will be built new on an existing space-restricted site, it could be a solution to select a new type of cooling system, which can be placed on the roof of a building or which needs less space than the old one.
The preferred solution would be a new installation with lower operation needs, so that the retrofit is also associated with lower operating costs. Lower operating cost will be one of the main reasons for retrofitting. It is preferred, however, to consider a retrofit scenario, which reduces the emissions as well as the consumption of operating resources. In general this will require higher investment cost. Considering the operating cost savings and any potential reduction in emissions, larger investment costs can pay off in short periods of time.
All retrofit scenarios have to consider both the cooling technology and the process to be cooled.
Both have to be seen as one system. Changes in the cooling system may have effects on the process and vice versa. The first aim of any retrofit must be to maintain, or if possible improve, the efficiency of the process to be cooled. On the other hand, changes in the process to be cooled will also result in different demands on the cooling system. This could be another important reason for retrofitting.
Changes in the process to be cooled can result in a change of demands on the cooling system.
• Due to new technology less waste heat is generated by the process, less cooling capacity is needed (example: computer terminals, processes with friction).
• The temperature level of the waste heat has changed, both to higher or lower temperatures (example: incineration processes).
• Larger parts of the generated heat of the process are recuperated, so less waste heat has to be removed to the environment.
• The temperature sensitivity of the process is increased, a more efficient cooling system is needed.
Table 1.8 summarises the options for technological upgrading that, according to suppliers information, can be considered to be technically easy (E), possible (P), difficult (D), not possible (NP), or does not apply (NA). Generally, each system has a varying number of options for retrofit. NP-E is an indication that the application of an option is largely dependent on the specific situation, in which the cooling system operates. (See also Chapter 3 and Annexes).
Table 1.8: Technological upgrading options for existing systems (pers. comm.)
Industrial cooling systems1 Option
OTCS OWCT OWDCT CCWCT CCDCT CCWDCT
General E E E E E E
Improve capacity E E D D D D
Reduce kWe D E D E D D
Reduce water-use NA NP-E D NP-E NA D
Reduce plume NA NP-E NA E NA NA
Reduce noise NA E D E D E
Reduce drift NA E E E NA E
Notes:
1System code (see also Chapter 2):
OTCS – once-through cooling system OWCT – open wet cooling tower OWDCT – open wet/dry cooling tower CCWCT – closed circuit wet cooling tower CCDCT – closed circuit dry cooling tower CCWDCT – closed circuit wet dry cooling tower
There are many possible ways to retrofit a cooling process and some typical scenarios along with their relevant considerations are listed in the following paragraphs.
1.5.3.2 Change of heat transfer technology
Usually, lower operational costs associated with a new technology or legislative restrictions are major reasons for the replacement of one heat transfer technology by another technology.
A typical example is the replacement of a once-through system by a recirculating system, saving on operating costs (water and sewage) and following restrictions on heat emissions to a surface water. The economic performance of the recirculating system depends on the specific costs for water, sewage and electrical energy. Assuming average water and sewage costs of 1 [€/m³] and electrical energy costs of 0.1 [€/kWh], the operating costs in this example are 38800 € for the once-through system and 48000 € (2100 € for water and 27000 € for energy) for the recirculating system. The annual saving is 34000 €, which is higher than the investment costs of 21000 €. If the balance favours the environment in the first place and investment costs will be much larger than annual costs, the investment recovery period will become an important factor.
In this example both the environment with respect to water requirements and the company benefit from a change in technology at the same time. The environmental costs however are due to additional energy requirements for extra fan and pump energy. Water use in this example is by large affected by the evaporation loss which has been calculated by assuming that they amount to 1.8% of the circulation per 10K of cooling (see Annex V.3).
This example merely shows how to approach changes in technology. With different price levels the outcome will be quite different and may favour the once-through system. For example, in Italy, where electricity cost is about 0.05 [€/kWh] and water cost for an open circuit 0.01 [€/m3] against 0.1-0.2 [€/m3] for a closed circuit, the once-through systems would be more favourable from an economic point of view.
Table 1.9: Example for conversion of a once-through system into a recirculating system [tm139, Eurovent, 1998]
Example:
air compressor 500 kW Once-through system Recirculating system
inlet temperature 15 °C 27 °C
outlet temperature 35 °C 35 °C
flow rate 6 l/s 15 l/s
annual operating hours 1800 h 1800 h
evaporation loss - 1400 m³/a
blow down - 700 m³/a
annual water use 38800 m³/a 2100 m³/a
extra fan and pump energy - 15kW
investment cost - € 21000
If a change of the cooling configuration is considered, the effects on the overall efficiency must be taken into account. If possible, the efficiency should be increased. For temperature sensitive processes, it needs checking whether a cooling technology can provide lower end temperatures at the same level of safety.
The example of replacing a water-cooled condenser with an open cooling tower by an evaporative condenser shows an effect on end temperature and system efficiency. Such a technological replacement can potentially reduce the condensing temperature by 4 – 6 K depending on actual conditions. The efficiency gain of such retrofit can be estimated in order of magnitude of 12 – 15 % of the power requirement of the refrigerant compressor [tm139, Eurovent, 1998]
For temperature sensitive applications in the medium temperature range, the introduction of hybrid systems could be favourable, where water use and/or water and sewage costs have to be reduced. Such a change, generally, does not increase electrical demand, but can reduce the annual water consumption considerably. Depending on actual conditions and required size, hybrid concepts may require additional space.
1.5.3.3 Replacement of outdated heat transfer technology by modern one
Often a change of cooling technology for different reasons is not suitable. However, also a modification of the existing technology could lead to better efficiency, better performance, less emissions and lower operating costs. Development of air moving systems and heat transfer surfaces, as well as the application of more durable construction materials, are main reasons for replacement scenarios.
As there is usually no change in process temperatures (same technology) the main focus in this scenario is to reduce operating resources and environmental impacts as well as to achieve an extension of equipment’s life. Equipment’s life extension of more than 10 years can be realised by the use of new durable materials. It is very likely that any equipment installed 15 or 20 years ago, can now be replaced by modern equipment with higher operating efficiency and better environmental and economic performance.
A typical example for improvement of once-through cooling systems is the application of the more efficient plate and frame heat exchangers. For evaporative cooling systems for example, major developments have taken place to improve the performance of fill packs and of air moving systems, resulting in a more compact design with higher energy efficiencies. For air-cooled systems, new technology to shape fins in various ways has achieved similar results. An example of what could be the effect on energy use if applying better efficiency is illustrated in
Table 1.10. In this case the investment costs need to be balanced with the yearly operation costs for energy use and maintenance of fill.
Table 1.10: Example for conversion of an outdated mechanical draught wet cooling tower into modern design
[tm139, Eurovent, 1998]
Example:
Mechanical draught wet cooling tower
Outdated design:
induced draught concept with low-efficiency fill and
fan system
Modern design:
induced draught concept with high-efficiency fill and
fan system
Capacity 1200 kW
Inlet temperature 38 °C
Outlet temperature 28 °C
Wet bulb temperature 21 °C
Water flow 28.7 l/s
Fan power requirement 7.5 kW 4 kW
Energy consumption for fans 9 MWh/yr 4.8 MWh/yr
Investment cost - € 14000
1.5.3.4 Upgrading existing heat transfer technology
Often it is not necessary to replace the whole cooling system. The performance of existing cooling systems can also be improved by upgrading. Major components or accessories of the system are replaced or repaired, while the existing installation remains in situ. Upgrading can increase system efficiency and reduce the environmental impact. Examples of upgrading are new and more efficient fill packs of cooling towers and the application of sound-attenuation.
The cases in Table 1.11 and Table 1.12 should be considered as simplified illustrations. For an integrated assessment of the environmental gain other factors should be considered as well. For example, with the replacement of cooling tower fill, the environmental costs for the old fill that has to be disposed of must be included also.
Table 1.11: Example for replacement of outdated fill of a mechanical draught wet cooling tower with modern high efficiency fill
[tm139, Eurovent, 1998]
Example: mechanical draught wet
cooling tower Outdated fill High efficient fill
Capacity 3600 kW
Inlet temperature 38 °C
Outlet temperature 28 °C
Wet bulb temperature 21 °C
Water flow 86.1 l/s
Existing cell floor space 26 m²
Fan power requirement 22.5 kW 13.5 kW
Energy consumption for fans 81 MWh/yr 48.6 MWh/yr
Investment cost - € 29000
That not all changes have only positive effects can be observed from Table 1.12 where a considerable reduction of the noise level has been achieved. However, noise abatement usually leads to fall of pressure, which must be compensated by a higher performance of the fans. This in its turn raises the direct energy use of the cooling system. It will be a matter of local
preference whether a lower energy use or a lower noise level prevails. Investment and maintenance costs should be compared with reduced costs for energy consumption.
Upgrading the operational strategy is another example of efficiency improvement. The on and of cycling of fans can be changed into modulating control with frequency converters. This can result in significant savings of electrical energy, which, depending on conditions, can be 70%
and more.
Investment costs for upgrading can differ greatly and depend on the type of upgrading and the age of the existing installation. The investment is accompanied by lower operating costs as a result of a higher efficiency. Investment costs for upgrading will generally be lower than those for technology changes or replacements of equipment.
Table 1.12: Example for the improvement of acoustic performance by addition of sound attenuation
[tm139, Eurovent, 1998]
Example: mechanical draught wet
cooling tower Existing wet cooling tower Upgrading with sound attenuation
Capacity 1200 kW
Inlet temperature 38 °C
Outlet temperature 28 °C
Wet bulb temperature 21 °C
Water flow 28.7 l/s
Fan power requirement 15 kW 18kW
SOUND POWER LEVEL 90 dB(A) 81 dB(A)
Investment cost - € 12000