Methods of Pressurisation
Pressurisation by Expansion
This, at the simplest domestic level, involves little more than the addition of an unvented expansion vessel to a heating system which is then charged with water and sealed.
The function of the vessel is to take up the increased volume of water as it is heated and by so doing, apply additional pressure.
In practice vessels are used which incorporate a flexible rubber diaphragm which separates the water on one side from a factory applied charge of nitrogen on the other.
The size of the vessel is a function of the initial and final pressures and the water capacity of the system.
Application of a Simple Expansion Vessel
In addition to the vessel, other fittings necessary are a safety valve fitted to the boiler, a fill/non-return valve which will accept a temporary hose connection from a water supply and an automatic air release valve, possibly associated with a centrifugal air separator.
For larger systems operating at low temperature, the principles of operation remain the same.
The expansion vessel will increase in size and may even be duplicated.
A small filling unit is usually provided for 'topping up' purposes, consisting of a cistern with a ball valve for water supply and an electrically driven pump as shown below.
A low pressure switch is fitted to or near the boiler to control the operation of the pump.
This ensures that a minimum water pressure exists and a parallel high pressure switch may be incorporated to stop firing of the boiler in the event of over pressure, at a level below safety valve operation.
The various components of a system of this type may be built up into a complete packaged unit.
Pressurisation by Pump
For medium temperature hot water systems, operating at temperatures up to about 110oC and at pressures of about 4 bar (400 kPa), an alternative approach to pressurisation has led to the introduction of pressurising with a pump as shown below.
This system relies on pressure-pump operation in conjunction with a spill valve and a cistern fitted at low level to take up the water of expansion.
Some units of this type are factory assembled and are provided with duplicate pumps and controls.
When starting a system from cold, the spill valve allows water of expansion to escape into the cistern once a pre-set pressure is reached.
While the system remains at the design working temperature, and constant pressure, the spill valve remains closed and the pump is idle.
When the system temperature and pressure fall as a result of decreasing demand or boiler cycling, the pump will start and the pre-set pressure will be restored.
Since the rate of fall in pressure may vary considerably, the pump does not run continuously on demand but is arranged to cycle 'on/off' by means of timed interruption of the electrical supply.
This gives the equivalent of variable pump output.
A back up spill valve may also be incorporated into the system in case the primary spill valve fails to function.
It is advisable to fit a water meter to the cold water supply to the spill tank to determine if leaks in the system have occurred.
The function of the buffer vessel is to reduce surging of water from the pumps into the heating system and to minimise hot water from the heating system entering the expansion vessel and spill tank.
Pressurisation by Gas
This was, in chronological sequence, probably the first independent method of pressurisation to be introduced.
It was conceived as a result of the difficulties experienced in using steam to pressurise for high temperature hot water systems.
A pressure cylinder is connected to the heating system usually in the main return near the boiler where the water is coolest.
This cylinder is filled partly by water and partly by air or an inert gas (such as nitrogen).
The initial supply of gas is from a small air compressor or gas bottle.
An initial pressure can therefore be applied to the system at a pressure level well above the boiling point of the water content.
The water of expansion is discharged from the system by a spill valve to a cistern which is open to atmosphere and, as the system cools and contracts, a pressure pump draws water from the spill cistern and returns it to the system, as shown below.
The pressure controller regulates the admission of water from the pump or its expulsion through the spill valve.
The use of an inert gas, such as nitrogen, for the cushion, instead of air, offers the advantage that it is less soluble in water.
The more sophisticated units very lightly pressurise the spill cistern also with nitrogen.
In most units two pumps are used to run in parallel meeting unusual demands and the water of expansion passes through a heat exchanger to lower the temperature and, if possible, prevent flash.