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Thermal stores: How they work |
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| Heat Banks or thermal stores are advanced hot water storage units,
providing high flow rates of mains pressure drinkable hot water. Richard Hanson-Graville,
Technical director at DPS Heatbanks, a leading manufacturer and supplier of water
heating systems, explains how thermal stores work. |
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The Heat Bank stores Heat Energy which can be utilised to provide
domestic hot water or even central heating if required. This heat energy is transferred
to mains water by a plate heat exchanger. No domestic water is stored, hot or
cold. |
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| The standard range of Heat Banks are unpressurised and fully vented
to atmosphere, with the option to have an Integral Feed & Expansion (F&E)
Tank or to connect to a remote F&E Tank. The water within the Heat Bank (stored
water) can be heated by various methods, including Gas/Oil Boilers, either directly
or indirectly (using a coil), or by Electrical Immersion Heaters utilising various
tariffs.
The mains hot water is not stored and is therefore drinkable, and removes the
need for a cold water storage tank. There is no chance of the introduction of
Legionnaires' Bacteria into the domestic supply. |
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When a hot water outlet is opened the resulting flow of water
is sensed by the Flow Switch, which in turn switches on the Heat Bank Pump.
The Pump draws hot stored water from the top of the Heat Bank and passes it
through the Heat Exchanger and back into the bottom of the Heat Bank below a diffuser
plate. This plate prevents turbulence within the unit and keeps the stored water
stratified, thereby increasing efficiency.
As the stored water passes through the Heat Exchanger its heat is rapidly
transferred to the Mains Water also running through the Heat Exchanger but separated
from the stored water by numerous plates.
When the hot water outlet is closed, the Flow Switch will turn off the Pump,
thereby using energy only when required. |
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| This method of heat transfer, referred to as forced convection,
affords heat transfer rates of up to 100kW (340,000 Btu/h), thereby providing
mains pressure hot water at up to 45 litres per minute, raised through 35°C
(see output graph below). In addition, more than one heat exchanger may be fitted
to double, or even triple, the mains water output. |
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The plate heat exchanger
The plate heat exchanger separates the pressurised mains water from the stored
water. While the mains water pressure can be anything from 0.5 to 6 bar, the stored
water is not pressurised, allowing the Heat Bank to be vented to atmosphere. The
compact brazed plate heat exchanger, shown here in cutaway, is fitted internally
within the Heat Bank and is manufactured from corrosion resistant steel and tested
to 15 bar pressure. |
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All Heat Banks are fitted with internal diffuser plates to prevent
turbulence caused by water returning from the heat exchanger. This helps to keep
the stored water well stratified, increasing the volume of available hot water,
and thereby increasing the total supply of hot water that can be drawn from the
Heat Bank
The stored water never changes. The build up of scale within the cylinder is
therefore eliminated and extra system protection is achieved by the addition of
corrosion / scale inhibitors. If required, anti-freeze may also be added. |
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| The stored water can be heated to a maximum of 90°C thereby
increasing the amount of heat energy available for use, while the use of CFC Free
Foam Insulation (to British Standards 699, 1566 & 4735) contains the heat
within the stored water.
Heat bank units are available in two basic types:
Everest Heat Banks are fitted with an integral Feed & Expansion Tank. Units
are fitted with Cold Feed and Vent pipes, as well as a Ball Valve (unless the
Solenoid Feed Option is requested), an Overflow Connection and an Aluminium Lid.
Xcel Heat Banks are to be used on systems B,C & F where additional head
is required for the Primary System, via connection to a separate Feed & Expansion
Tank - F&E Tanks can be supplied fitted with a Solenoid Feed if required.
This tank must be large enough to accept the expansion (4% by volume) from the
Heat Bank unit, as well as the Primary System. Xcel Heat Banks are fitted with
Cold Feed & Vent connections. |
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One of biggest advantages of Heat Banks is the fact that they
are vented, and the stored water is not at high pressure. This has a number of
benefits: |
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| The stores can be made as square or rectangular tanks, allowing
more water to be stored in the same space than is possible using a pressurised
(and therefore circular) storage cylinder. |
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| There is not requirement for multiple safety relief valves and
thermal cutouts. |
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| The units can be designed and made to suit a particular installation
without the need to submit the unit for the strict (and costly) safety tests that
are required of all unvented designs. |
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| Heat Bank units can be installed by any competent installer whereas
additional training is legally required to install unvented cylinders. |
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A good innovative example of a thermal store unit is the Stowaway
by DPS Heatbanks. The design of the stowaway has enabled the removal of the overflow
warning pipe, leaving only two connections - a cold in and a hot out.
By not having to fit an overflow from the hot water system removes the need
to run pipework outside the property. In turn units can be sited anywhere within
the dwelling you can get a mains water supply to. |
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 More
details and advantages on the 'DPS thermal store Stowaway' |
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