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 12C to 42C.  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.
Domestic hot water is not stored, removing the risks of Legionella within the water supply.
The store can be protected by the addition of corrosion inhibitors and anti-freeze.
Store water can be heated directly by a boiler, removing the need for a primary heating coil.
Mains hot water can typically operate at up to 10 bar pressure, without the need for pressure reducing valves or safety relief valves.
Store remains fully vented to atmosphere.

 

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.

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.

 

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.

 

 

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.