Plate Heat
Exchangers
by Richard Hanson-Graville FIOP,
Managing Director of Dedicated Pressure Systems Ltd.
The use of plate heat exchangers in the domestic heating
industry has grown rapidly in recent years, mainly in the manufacture of
combination boilers. However, their operation and application
generally remains unknown to most plumbers, even though there are numerous
instances where their use would be highly beneficial.
To start from the beginning, what is a plate heat
exchanger ? Quite simply, it is a device use to transfer heat from
one liquid (or gas) to another. Most plumbers will understand the
principal of a coiled heat exchanger, as used in standard domestic
indirect hot water cylinders to transfer heat from primary water to the
stored domestic water. Hot water from the boiler flows through the coil, transferring
heat through the wall of the coil to the stationary water within the
cylinder.
In contrast, a plate heat exchanger transfers
heat from hot water flowing in one direction (primary) through the heat exchanger to
cold water flowing the opposite direction (secondary). The two sets of water are
kept separate by numerous stainless steel plates through which the heat is
conducted. Each waterway can operate typically up to 10 bar
pressure, although models are available to take far higher pressures.
The rate at which heat is transferred increases with
the temperature difference between the two water flows, and also with the
flow rates. A typical plate heat exchanger measuring only 20 x 7 x
12cm can transfer heat at over 100kW - enough to heat 45 litres per minute
of hot water from 12°C to 42°C. It is this combination of high
efficiency and small size that makes the plate heat exchanger perfect for
a wide range of applications.
Pressure Isolation
The ability of plate heat exchangers to transfer heat from a high
pressure circuit to a low pressure circuit makes them the perfect choice
for applications where the operating pressures vary through a
system. A typical example would be the use of a vented boiler system
to heat a sealed heating system. The heating system may require high
pressures to supply radiators located at high level, such as in a
loft. A plate heat exchanger allows the two systems to be fed and
operate separately, while providing sufficient heat transfer for correct
operation. Similarly, combination boilers use plate heat exchangers to
transfer heat from primary water to mains water at high pressure.
Thermal
Stores
A growing area for the use of plate heat
exchanger is in thermal storage. Thermal stores take a different
approach to providing hot water that would a standard vented system or an
unvented cylinder. Instead of storing domestic water, the store
contains primary water, fed and vented using a feed & expansion tank,
just like a typical primary boiler system. The stored water is heated
by a boiler or by immersion heaters, and a heat exchanger is used to
transfer the stored heat to domestic water as required. Such systems
offer numerous benefits over a standard storage system, including:
No limescale build up within the store, on primary heating coils or
on immersion heaters. |
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Domestic hot water is not stored, removing the risks of Legionella
within the water supply. |
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The store can be protected by the addition of corrosion inhibitors and
anti-freeze. |
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Store water can be heated directly by a boiler, removing the need
for a primary heating coil. |
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Mains hot water can typically operate at up to 10 bar pressure,
without the need for pressure reducing valves or safety relief
valves. |
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Store remains fully vented to atmosphere. |
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Basic types of thermal store use a coil heat exchanger to provide hot
water. Mains cold water will flow through the coil, which transfers
heat from the surrounding store water to the mains water. However, the amount
of stored heat that can be utilized is limited, and often results is
output temperatures steadily dropping as water is drawn off and the
average store temperature drops.
 More advanced thermal stores make us of a plate heat exchanger instead
of a coil, with hot water pumped from the store through one side of the
heat exchanger as mains water flows through the other side to
outlets.
A flow switch fitted into the cold mains inlet is typically
used to detect when hot water is being drawn, and in turn operates the
pump. The output temperature will remain constant until the entire
body of stored water has been circulated through the heat exchanger, after
which temperatures will drop. Depending upon the size of heat
exchanger, over 90% of stored heat can be extracted.
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Converting a standard boiler
into a combi-boiler Using a plate heat exchanger it becomes
possible to add combi-boiler function to any existing boiler, providing
continuous hot water without the need for stored hot water. This is
achieved by the use of a flow switch and diverting valve in conjunction
with the heat exchanger. The following schematic of the DPS ShowerMaster*
shows the operation.
 The flow
switch detects when there is a hot water outlet open, and operates a
motorised three-port valve, redirecting the boiler flow from the heating
and hot water cylinder to the heat exchanger. At the same time, the
flow switch operates a relay switch which in turn switches on the boiler
and primary pump, providing heat. The plate heat exchanger transfers heat
from the hot water in the boiler circuit flow to mains domestic water,
thereby providing hot water to the taps.
Demand from the central heating is bypassed allowing the full boiler output
to be utilised for hot water. When all hot outlets are closed, the three-port
valve redirects the boiler flow back to its usual route. In addition, the mains
hot water supply provided is independent of the existing hot water cylinder, and
will be relatively unaffected by the opening and closing of hot water outlets
fed from the cylinder. This makes the system perfectly suited to a dedicated
shower supply. Alternatively, the dedicated supply can be used to feed the
kitchen and utility room outlets, removing the need to heat up the entire hot
water cylinder just to supply the small volumes of water required.
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District Heating & Central Boiler Application
 Where a installation uses a
central boiler plant as the heat source for a number of dwellings, such as
district heating, a plate heat exchanger may be used to provide the hot water
for each dwelling. The following schematic of the DPS BoilerMaster* shows how
this can be achieved.
The heat exchanger extracts heat from the boiler circuit and
uses this to heat up mains cold water.
A flow switch detects when there is a hot
water outlet open, and immediately opens a solenoid valve, allowing hot
water from the boiler to flow through the heat exchanger, providing heat.
The plate heat exchanger transfers heat from the hot water in the
boiler circuit flow to mains water. When all hot water outlets are closed,
the solenoid valve closes preventing any unnecessary drain upon the boiler
supply.
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Swimming Pools
A growing application for plate heat exchangers is in heating swimming
pools. There are specialist swimming pool boilers on the market,
designed to work with the high flow rates and treated water. Large
cost savings can be obtained however if a standard domestic boiler is used
in conjunction with a plate heat exchanger.
The heat exchanger acts
to separate the primary boiler water from the swimming pool water,
transferring heat from one circuit to the other, as shown. The
boiler is generally set up as a sealed system, and will never come into
contact with the swimming pool water.
Limescale
A question I am
often asked is 'what about limescale build up within the plate heat
exchanger ?' The problems caused by limescale are often overcome by
a plate heat exchanger due to the highly turbulent flow within the
device. The plates within the heat exchanger are embossed with a
corrugated pattern, designed to maximise turbulence and heat
transfer. Providing flow rates are reasonable, the turbulent flow
prevents scale deposits from sticking to the plates. In addition,
the slight flexing of the plates during operation helps to break up any
deposits that do form.
Of course, in areas of particularly hard
water, additional protection against scale should be taken, and it may be
wise to provide connection points to allow the heat exchanger to be
flushed at some future time.
Richard Hanson-Graville
* Patent applied for.