New and improved

It seems that it's not just 80s bands (thank you, Duran Duran, T'Pau and Rick Astley for your comeback tours) that are back in fashion - the revival stretches to engineering, too.

In the early 80s, a design was put forward that promised to tackle one of the major shortcomings associated with evaporative condensers. The idea was straightforward: by reworking the way water was used to cool the condenser tubes, its potential as a source of legionella could be eliminated. This had a number of benefits - the condenser would no longer require a treated water supply or regular inspection and cleaning to check for the presence of bacteria, saving time and money.

Three units were manufactured using this design, but the idea wasn't pursued any further - partly due to technical difficulties but also because energy efficiency wasn't high on the agenda and the legionnaires' disease crisis had yet to erupt.

Twenty-five years on, things are much different and the idea has resurfaced; this time pioneered by Star Refrigeration, which has patented a new design and is currently running tests on a prototype.

Efficient solution

The design makes use of the fact that when it comes to rejecting heat from a refrigerating system, evaporative condensers offer one of the most efficient and compact solutions. Unlike air-cooled condensers, which reject heat close to the dry bulb temperature of ambient air, evaporative condensers reject heat close to the lower, wet bulb temperature so they consume less energy. Even more important, the heat transfer coefficient is vastly increased by wetting the tubes and the mass flow of air is reduced to around half.

However, like cooling towers, evaporative condensers are seen as a potential source of legionnaires' disease and so it is mandatory to inspect and clean units at regular intervals and to check for bacterial presence. This greatly increases the operating cost and businesses need to consider the potential impact of a refrigeration system being shut down for an indefinite period.

Star Refrigeration's patented design achieves some of the advantages of an adiabatic type air-cooled condenser where water is sprayed onto a fine mesh screen to cool the air before it enters the condenser. Such condensers do not contain a tank for re-circulating the water, and as such are exempt from inspection for legionella because they are considered to be a once-through system.

In the ‘once-through' design put forward in the ‘80s, the condenser consisted of a tube bundle mounted above a perforated plate. A mixed flow fan drew air up through perforations in the plate, through the tube bundle, through an eliminator and through the fan to atmosphere. A header tank with a ball valve was positioned so that the water level in the tank was slightly below the perforated plate (see figure 1). When the fan was switched on, water was drawn into the condenser and levitated among the tubes.

Although heat transfer for the system was good, pressure drop and fan noise were high - there wasn't sufficient fan power reduction compared to a conventional evaporative condenser for the units to be attractive.

Star Refrigeration has moved this original design on, doing away with the header tank as it was thought this would no longer be acceptable to the HSE.

The new design

With the new design, water is sprayed into the condenser, either via the air intake or into the space above the tubes. The perforated plate is replaced by a series of slots running parallel to the tubes. The tubes in line with the slots will be omitted so that high velocity air can be drawn up through the gaps with sufficient speed to entrain drops of water. An eliminator above the tubes captures the entrained water, which then drips back down onto the tubes to keep them continually wetted.

One of the main technical challenges is to determine the air velocity required to lift the water droplets through the condenser. In Star's ‘once-through' design, the water droplets are lifted from the edge of a downward-sloping plate. The velocity of air flowing over the edge must be sufficiently high to overcome the effects of gravity and surface tension to detach it from the plate. Naturally occurring water droplets are about 5 mm diameter and to help encourage the formation of small droplets, the edge of the plate will be made as sharp as possible. Based on this, it was decided that the air velocity through the slot should be greater than 12 m/s, which is just above the terminal velocity of a 5 mm diameter water droplet.

Prototype in action

Star Refrigeration is now studying the air and water flow rates on a prototype built by Coolers and Condensers. The rig (see above) comprises a square box, on top of which is mounted an 800 mm diameter Woods aerofoil fan, driven through an inverter so that the speed can be varied. The box contains a Knitmesh droplet eliminator above a nest of 16 mm diameter stainless steel tubes. Water is sprayed into the plenum chamber between the tubes and eliminator and flows over the tubes in the centre of each stack. It is then lifted up over the tubes of the stack by the air stream that is pulled up through the slots at the base of the unit.

Initial testing indicates that water droplets can be lifted from the slot edge to within the tube nest. Whether the water film is of sufficient thickness and uniformity to cope with heat rejection remains to be seen.

As the theory indicated, the sharpness of the slot edge appears to have significant influence on the droplet size produced. With this in mind, the effect of using a surfactant will also be tested. At 20ºC, water viscosity is 72.8 dyne/cm. Addition of a surfactant can reduce the effective surface tension to about 25 dyne/cm, reducing the diameter of the water droplet to around 3.5 mm. For a condenser rejecting 150 kW of heat evaporating 250 litres of water per hour surfactant would need adding at no more than 1.25 cc per hour, assuming that all the surfactant was lost with the evaporating water.

The next stage of testing is to determine the optimum tube and slot arrangement and the maximum number of rows through which water droplets can be raised. When the design has been optimised, the rig will be converted to an evaporative cooler by welding on headers and bends.

Conditions are now more favourable for the introduction of a ‘once-through' evaporative condenser as legionnaires' disease has proved to be a continuing threat. In addition, there is the ever-increasing need to save energy through the use of more efficient technologies - such as evaporative cooling. If the results prove promising this could well become the system of choice in the future.