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SPC - Run-Around Coil Systems for Heat Recovery

Heat Recovery - Run-around Coil - Extract Air Flow - Pipework - Pressurisation - Frost Protection

New building stock is invariably designed with heat recovery equipment in the ventilation system; this is required by legislation.

Older, existing buildings incur high energy bills and have a correspondingly high carbon footprint, largely as a result of the fossil fuel burned in order to heat cold outside air due to the lack of a reliable heat recovery system.

Air to air heat recovery can be accomplished using a number of technologies; heat pipes, cross-flow or counter-flow plate heat exchangers and thermal wheels being the most obvious.

While these devices provide high effectiveness heat transfer between the return air and outside air, they all suffer from the same limitation i.e. the outside air and return air must be arranged to run adjacent to one another.

While new construction can incorporate duct runs and/or double deck air handling units to take advantage of the above technology, the juxtaposition of supply and extract airstreams on existing plant can prohibit their use.

Similarly, the need to displace extract outlets away from outside air inlet louvers on new ‘air quality critical’ projects can mean that run-around coil systems are the only alternative as they represent the only available technology which is suited to distant airstreams.

The savings available to the client as a result of the incorporation of run-around coil heat recovery vary with the throughput of air, but a system recently renovated at Glasgow Royal Infirmary showed annual energy savings based on year round operation of over 6,000,000 kWh.

At a gross cost of £0.05/kWh to take into account the efficiency of the existing fossil fuel boiler and distribution system, this represents an annual cost saving of over £300,000 and a reduction in CO2 of over 1200 tonnes. All this was achieved with a payback of less than 2 years.

The example given above relies on a total ventilation for the healthcare unit of just over 100m³/s, but savings on a smaller scale will provide identical benefits to commercial properties.

Run-around coil effectiveness (efficiency) values cannot always match those of competing technology, but with careful equipment selection they can approach the above and allow the incorporation of meaningful heat recovery where it would otherwise be unfeasible.

Steps to take when considering a run-around coil installation:

  • A good starting point is to aim at a system effectiveness of 50 to 55%. Experience shows that this will give the best result in terms of net heat recovery (gross recovery less additional fan/pumping power required).
  • Higher effectiveness values of up to 65% are possible but this would require coils of a significant depth and cross section to reduce air velocities and maximise contact times. Space is often at a premium in retro-fit applications and low air velocities will not be practical.
  • Get the expert coil manufacturer involved at an early stage – SPC will advise on the correct selection and will assist with any site surveys required.
  • Determine the available airflow rate for both the outside air and extract air. Note that outside and extract air flow rates need not be equal.
  • Assess the existing plant equipment and available space for the additional coils. Select suitable coils and determine the airside resistance, water side resistance and water flow rate requirements.
  • Determine whether amendments need to be made to existing blowers in the system to overcome the resistance of the additional fin blocks.
  • Plot the required pipework runs between supply and extract coils. Pipework would normally be only small/medium bore and would require standard pipe insulation.
  • Determine where the circulating pump is to be installed and select a suitable model to match the flow rate required and the pressure drop of the coils plus an allowance for adjoining pipework.
  • The run-around loop is a closed system and will require an expansion vessel to be incorporated to absorb volume fluctuations. Consideration should also be given to the incorporation of a pressurisation system to ensure that system pressures are automatically maintained.
  • SPC offers a range of pump sets designed specifically for run-around coil applications and these incorporate circulating pumps, expansion tanks and pressurisation sets. The circulating pumps include contacts for remote switching via BMS and electronically controlled continuously variable speed circulating pumps. This allows simple system control without the need to incorporate control valves.
  • Consider frost protection of the system. Supply coils can be subject to sub-zero temperatures and are at risk of freezing if the system is not operating.
  • If no other frost protection is available then the system should use an antifreeze solution.

For more information and help designing your run-around systems for heat recovery, please contact SPC on 0116 249 0044.

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Heat Recovery

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SPC (S & P Coil Products Ltd)
SPC House
Evington Valley Road
Leicester
LE5 5LU

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Tel: 01162 490044
Fax: 01162 490033

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