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ENERGY EFFICIENCY

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Introduction

Successive UK governments have been aware of the importance of energy efficiency and have introduced major initiatives to improve the acceptance of energy-saving technologies and techniques—in industry, commerce and the public and domestic sectors. Despite early successes of these initiatives, the relatively low cost of energy during the 1980’s resulted in energy efficiency receiving low priority within many organizations. Thus, at the end of the decade, numerous barriers to the adoption of energy-saving measures were still clearly apparent. In March 1989, the Department of the Environment’s Energy Efficiency Office (EEO) launched its Energy Efficiency Best Practice programme. This coincided with increased national and international concern about environmental damage associated with energy use and, in particular, awareness of the need to reduce fossil fuel consumption.

Best Practice Programme

The aim of the Best Practice programme is to stimulate adoption of energy-efficient, good practices in terms of technologies, techniques, processes and energy management. It seeks to be of value to all energy users, covering every aspect of energy efficiency through four interrelated programme elements:

  • Energy Consumption Guides;

  • Good Practice;

  • New Practice;

  • Future Practice.

Emphasis is placed not only on specific energy-efficient technologies, but also on design considerations, management techniques, operating practices, education and training, and staff motivation.

Benchmarking: Energy Consumption Guides

Energy Consumption Guides allow organizations reviewing their energy use for the first time to undertake a ‘benchmarking’ exercise—comparing their own consumption with that of other operators in the same industry or building-type—thus establishing where, and how much, potential saving is available to them. This series of guides allows comparisons to be made within a broad range of industries and building types: from iron foundry cupolas to the liquid milk sector of the dairy industry; from domestic housing to schools and offices. The information is gathered from a representative sample of organizations within the appropriate sector. Each guide also contains an Action Plan of practical, achievable energy-saving techniques. These may range from low-cost good housekeeping measures to capital projects or plant modifications.

Stimulating take-up of good practice

The Good Practice element of the programme is designed to provide energy users with detailed and up-to-date information about existing energy-efficient measures. It promotes examples of low-risk, proven techniques which have already achieved significant energy cost savings. An extensive series of case studies offers concise, specific examples of the application of these successful measures. Each case study project has the potential to stimulate national energy cost savings, whilst achieving individual project paybacks acceptable to the appropriate sector(s). Complementing the case studies are Good Practice Guides which detail the best practice currently associated with a particular industry, process or building type.

New practice: Support for novel energy efficiency measures

The role of the New Practice element is to stimulate confidence in novel energy efficiency measures. The programme seeks to offset some of the risks involved in the wider application of such measures by providing detailed and objective information on new or improved techniques, or on novel applications of existing technologies. Successful projects are promoted through New Practice reports and profiles, seminars and site visits.

A longer-term approach to energy efficiency

The Best Practice programme supports basic R & D into energy efficiency measures for industrial applications through its Future Practice element. The EEO works closely with the Department of the Environment’s Energy Related Environmental Issues (ENREI), which similarly supports buildings-related projects. Projects are again selected on the criteria of novelty, energy savings, environmental benefits and payback period. Findings are published through Future Practice profiles and reports.

The success of R & D projects can only really be measured when the developed technologies are adopted first as New Practice and ultimately as established Good Practice. Such acceptance has, for instance, been attained by compact heat exchangers developed under a number of Future Practice projects [see ETSU (1991), (1992) and (1993)]. Their adoption has already resulted in savings of around 1 million GJ/year in the UK. Furthermore, all the major UK heat exchanger manufacturers have now developed, or are in the process of developing, units of this type. Increasing availability will lead to their wider use in heat recovery and heat transfer applications in the future.

Compact heat exchangers

Compact heat exchangers are characterized by their high ‘area density,’ that is, high ratio of heat transfer surface area to heat exchanger volume. Table 1, cited from ETSU (1994) compares the area densities of a range of compact heat exchangers with that of a typical shell-and-tube exchanger.

Table 1. Area densities of compact and shell-&-tube heat exchanger

Compact heat exchanger typeArea density (m2/m3)
Liquid-liquid compact heat exchanger> 300
Gas-liquid compact heat exchanger > 700
Laminar flow heat exchanger > 3,000
Micro heat exchanger > 10,000
Conventional shell & tube exchanger (19 mm dia tubes) 100 (typical)

The resultant small overall size of compact heat exchangers has meant that their initial development and use has been in the aerospace, road transport and marine sectors. More recently, there has been increasing interest in the concept of process intensification, stimulated by the availability of compact units and by environmental and other constraints. Printed circuit heat exchangers (PCHEs), welded plate and plate-fin exchangers (PFHEs) have established good markets and are now challenging conventional types of heat exchanger. Their value is now being explored in the traditional process industries, particularly in the chemical, pharmaceutical and food and drink sectors.

Printed circuit heat exchanger showing gas and liquid paths through the core.

Figure 1. Printed circuit heat exchanger showing gas and liquid paths through the core.

Advantages of compact heat exchangers

Improved effectiveness

A major advantage of most compact designs is their greater efficiency or effectiveness. Effectiveness is expressed as the ratio of actual heat transfer to maximum possible heat transfer. It is a function of the heat capacity of fluid streams, the overall heat transfer coefficient and the area of the heat transfer surface.

Higher effectiveness allows the use of closer approach temperature differences between fluid streams. This can lead to significant energy cost savings in process heating and cooling duties since power requirements, particularly of plants such as refrigeration compressors, can be reduced.

Smaller volume

The smaller volume of compact units for a given heat transfer duty (compared with most shell-and-tube exchangers) provides benefits that extend well beyond the heat exchanger itself. Support structures and piping needs are reduced; the exchanger may be installed in a more convenient location (particularly important for ‘green field’ sites); less maintenance space is required for removal of the heat exchanger core.

Lower capital cost

When the total installed cost is considered, compact heat exchangers tend to be significantly cheaper than their conventional counterparts. This is particularly true when process requirements demand that the exchanger is made from an expensive material, such as nickel or titanium. Here, the cost per kg of raw material dominates the cost of the exchanger, and often the ancillaries.

Conclusions

Considerable advances have thus been made in the development and use of compact heat exchangers—incorporating new materials, construction methods and process integration techniques. Much of the work of overcoming the barriers to their acceptance has been supported by the Best Practice programme. This and other work of the Department of the Environment’s EEO is managed by the Energy Technology Support Unit (ETSU) at Harwell, Oxfordshire (for industrial projects) and the Building Research Energy Conservation Support Unit (BRECSU) at Watford (for projects relating to buildings).

Table 2. Compact heat exchanger types

TypeNature of fluids
Gasketed plate and frame Liquids; two-phase1
Brazed plate Liquids; two-phase
Welded plate and frame Liquids; two-phase2
Plate-fin Gases; liquids; two-phase
Printed circuit Gases; liquids, two-phase
Welded stacked plate Gases; liquids; two-phase
Laminar flow Liquids: two-phase3
Compact shell and tube Liquids; two-phase

1Two-phase includes boiling and condensation.

2One flow-path may have welded plates; the other relating gaskets.

3This and other types may be constructed using polymers.

Flow distribution in a polymer film heat exchanger matrix.

Figure 2. Flow distribution in a polymer film heat exchanger matrix.

Schematic of new porous matrix exchanger surface.

Figure 3. Schematic of new porous matrix exchanger surface.

REFERENCES

FPF 12. Testing of Printed Circuit Heal Exchangers. ETSU 1991.

FPF 29. Investigation of a Novel Compact Heat Exchanger Surface. ETSU 1992.

FPF 40. Design Data for Compact Polymer Film Heat Exchangers. ETSU 1993.

GPG 89. Guide to Compact Heat Exchangers. ETSU 1994.

References

  1. FPF 12. Testing of Printed Circuit Heal Exchangers. ETSU 1991.
  2. FPF 29. Investigation of a Novel Compact Heat Exchanger Surface. ETSU 1992.
  3. FPF 40. Design Data for Compact Polymer Film Heat Exchangers. ETSU 1993.
  4. GPG 89. Guide to Compact Heat Exchangers. ETSU 1994.

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