Under pressure

Changing water volumes in sealed water systems due to temperature and pressure variations require expansion control to safeguard equipment. Jim Martin of Flamco Middle East outlines the issues and possible solutions.

For many years closed systems such as those used in district cooling plants have been the preference of hvac designers for heated or chilled water used for climate control.

Water is either heated or chilled and circulated throughout a pipe network and heat exchangers to provide heating or cooling as necessary.

These systems are in theory completely sealed from the outside environment, therefore the volume in which the water can circulate is finite.

However, as water is incompressible any change in either temperature or pressure will result in a change of water density and consequently the volume that it occupies will have to alter.

Water lost through leakage must also be replaced from an external source to maintain the initial volume.

Adequate expansion control is therefore imperative to cope with such volume changes and avoid damage to the system during normal operation or when a chiller plant is shut down for maintenance, power is interrupted or breaks down.

During such periods the water temperature could rise to ambient temperature, say 46°C in summer.

Relying on a simple pressure relief valve alone is foolhardy and costly as water will be ejected from the system, necessitating make-up from the external supply.

In the past, a simple compression tank was used as a solution.

Compressed air and water were contained in this to provide a buffer, however this method brought its own problems.

As there was no interface between the water and compressed air it was argued that air could be forced into the solution, hence the system, only to be removed by air separators or vents in the system as it was circulated.

Once removed, more water would have to be introduced to maintain the volume that had been reduced by the removal of the air.

Eventually the compression tank would become waterlogged because the proportion of air originally in the tank had been absorbed and subsequently removed.

This means that the tank would have to be isolated, drained, recharged with compressed air and filled with water according to the design specifications.

Some 50 years ago in Europe Flamco introduced the pre-pressurised bladder type expansion vessel as an alternative to the compression tank.

Such products offered operational and practical advantages.

Firstly, the expansion water is contained within a butyl rubber bladder within the pressure vessel, which separates it from the compressed nitrogen charge on the other side of the bladder.

The nitrogen is pressurised to approximately the same pressure exerted by the static head of the system.

Therefore, when the system receives its initial fill, water will enter the vessel until the pre-charge pressure balances against the effect of static head.

The bladder ensures that there is no chance of air entering the system water, effectively isolating the installation from the ingress of additional air.

As temperatures and pressures in the system increase, the extra volume of water is forced into the expansion vessel.

Similarly when the temperature and pressure reduces, water is forced back into the system.

This method of expansion control is excellent and is still widely used today, being a more cost-effective, space efficient and virtually maintenance-free solution, especially when compared to traditional compression tanks.

One disadvantage is that a proportion of the total volume of the vessel is taken up during the initial fill.

This means that only the remaining volume is available to accommodate the expansion water. This factor is termed ‘acceptance�.

In general the best acceptance would be approximately 65%.

In systems where higher pressures are involved the acceptance value can drop as low as 20%, meaning a 1000 litre vessel will only accommodate 200 litres of expansion water.

To overcome these issues Flamco developed the Automat, pressure-less vessel technology.

These products have been used in European heating and cooling applications for several years and have been offered in the Middle East for around one year.

Pressure-less vessel technology results in an acceptance of better than 85% regardless of system pressure.

This means that plantrooms can be smaller and costs and the quantity of vessels can be reduced, particularly in high volume, high pressure installations.

The additional benefit is that with microprocessor control the system pressure can be controlled within very tight margins, as can make-up water.

Cost reductions can also be achieved by using thinner walled vessels as they do not need to withstand the system pressure.

Designers and engineers in the Gulf are acknowledging the advantages of cost, size and performance that products such as Automats offer and an increasing number of projects are specifying their use over pre-pressurised vessels.

QIPCO Towers, QIC Tower and the Ramada Hotel Extension in Doha are among those where the product has already been installed for chilled water systems, while Hadath University in Beirut has a 30,000 litre Automat for its hot water system.

The product comprises a pressureless expansion vessel, pump system and controller.

Installed in a chilled water system in the same way as a pre-pressurised vessel, the controller will constantly measure system pressure; should it increase, a solenoid operated valve will allow water to exit the system into the vessel until it returns to a pre-set value.

When pressure decreases, water is pumped back into the system until it stabilises.

Flow from the system into the vessel is regulated through reducing valves to maintain the pressure-less zone.

A sensor attached to the main vessel will identify a low water level condition and automatically accept make-up water until the pre-set minimum level is achieved.

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