CCT Chamber - ACC32 Water Fog Humidity - how it works
CCT Chamber - ACC32 Water Fog Humidity
Some tests recommend high humidity conditions (95-100% RH) are created by spraying water as a fog, rather than by evaporation from a heated water bath. This option provides a second 100 Ltr/26 US Gal water reservoir, additional chamber mounted fog atomiser(s) and associated pumping equipment, to enable a water fog humidity environment to be created at user programmable times. This function works in much the same way that the standard salt spray mode of operation works, with many duplicate systems operating in similar ways, as described below.
The control system fitted to Ascott CCT chambers enables the user to program whether water fog humidity is conducted alone, or in sequence with other climates, to form a cyclic corrosion test, for automatic execution by the chamber.
Compressed air inlet
Compressed air from a separate compressed air supply is required for the chamber. The primary use of this is to atomise water into a humidity fog at the water fog atomiser, located inside the chamber. The compressed air supply should be clean, dry and oil free, pressurised to within the limits 1.4 to 6.0 bar (20 to 87 P.S.I.). Between these pressures the air supply should be capable of delivering a flow rate of at least 75 Litres (2.6 cubic feet) per minute, which equates to a free flow at atmospheric pressure of approximately 102 standard litres (3.6 standard cubic feet) per minute.
Where a local supply is either unavailable or unsuitable Ascott offer an independent oil free air compressor as an optional accessory (ref.: ACC04). This requires connection to its own single phase electrical supply.
During water fog humidity testing, the compressed air utilised to generate the fog is bubbled through the air saturator (also referred to as a bubble tower or humidifier) in order to raise its humidity to c.100%RH at the point that it leaves the atomiser. This ensures a 'wet' and 'dense' fog is created. The temperature of the air saturator is user adjustable up to +63°C/145°F and is usually set several degrees above the chamber test temperature, in order to compensate for the cooling effect as the compressed air expands to atmospheric pressure on leaving the atomiser. Ascott air saturators are configured to top themselves up with water (distilled or deionised) automatically, but where a suitably pressurised supply of such water is unavailable, an optional accessory (ACC66) is offered to enable topping up of the air saturator by hand.
An additional 100L (26 US Gal) free standing, water reservoir, is provided with this optional accessory. This stores the water to be sprayed into a humidity fog. It is mounted on castors for mobility and incorporates a transparent graduated sight glass for ease of monitoring the solution level inside.
Solution pump and flow meter
During water fog humidity testing, the salt solution pump positively draws water into the chamber from the separate water reservoir, by peristaltic action, so avoiding the need for a gravity fed system and the consequent difficulties in maintaining a constant 'head' of water to be sprayed. This water is delivered, via a graduated flowmeter, to the atomiser inside the chamber. The speed of the pump is fully adjustable by the user, to allow the quantity of water atomised and hence the fall-out rate, to be precisely controlled. Two pumps and flowmeters are fitted to 2000L size chambers.
Water fog atomiser
This is a specially designed atomiser, made from thermally stable, high performance polymer, to guarantee a uniform humidity fog and featuring an integral filter to minimise blockages. During water fog humidity testing, it is here that the compressed air, delivered via the air saturator, meets the water, delivered via the solution pump and flowmeter, to create a finely divided water fog. Two water fog atomisers are fitted to 2000L size chambers.
Forming the centre-piece of the ergonomically designed control panel is a state of the art, touch-screen, Human Machine Interface (HMI). It is here that the user controls and monitors the various chamber functions. The HMI incorporates alpha-numeric text messaging and digital displays of chamber variables such as temperature, humidity and time. Also included is a useful trend logging facility. This generates a graphical display of set values for temperature and humidity, over a 72 hour time period, and compares this to the actual temperature and humidity achieved inside the chamber, over the same period.
All chamber control panels incorporate an emergency stop and other safety facilities.
Chamber heating mats
Silicone rubber encapsulated electrical heating mats are strategically bonded to the outside surfaces of the bottom and sides of the inner working chamber. Here they radiate heat uniformly into the chamber, under the control of the chamber temperature controller. In the liquid immersion mode of operation, these heaters give the chamber a user adjustable temperature range from ambient to +50°C/122°F.
The chamber canopy is effortlessly opened/closed using pneumatic cylinders, which are activated at the touch of a button on the control panel. An automatic purge facility is incorporated to minimise the risk of corrosive salt spray escaping into the laboratory when the chamber is opened.
Strategically located sensors, mounted inside the chamber and air saturator, monitor the climate continuously and convey this information to the Human Machine Interface (HMI), where it is displayed digitally at the control panel. Temperatures are monitored by PT100 precision temperature sensors and displayed to a resolution of 0.1°C. CCT chambers are also equipped with a special design of corrosion resistant humidity sensor. This measures the relative humidity electronically inside the chamber and displays this at the HMI as %RH to a resolution of 1%RH.
Test samples are accommodated within the chamber generally by placing on, or by suspending beneath, sample racks. A full set of sample racks are provided as standard with each new chamber (see chamber data sheets for quantity) in a choice of styles. Additional sample racks are available as optional accessories, if required, as is a reinforced false floor (ref; ACC19) for supporting large/heavy test samples directly on the chambers internal base.
This vents excess water fog from inside the chamber to atmosphere. The chamber should therefore ideally be located adjacent to an outside wall, and a hole made through the wall to accommodate the exhaust pipe. The end of this pipe should be directed away from persons and property, and be shielded from wind to prevent any disturbance to the conditions inside the chamber.
A floor level drain is required to remove to waste the excess water fog condensate etc. which accumulates over the internal base of the chamber interior during testing.
If access to a floor level drain is not possible or inconvenient, Ascott offer an optional accessory (ref; ACC20) comprising of a waste water trough and pump, which locates on the floor behind the chamber. Condensate from the chamber drain outlet is directed into this. When the holding tank becomes full, the condensate will automatically be pumped out by a marine specification pump. This will discharge waste water to a remote drain or effluent tank located up to 10M/30ft away horizontally and up to 3M/10ft vertically. ACC20 is not suitable for chambers fitted with the SO2 gas dosing optional accessory (ref; ACC46) due to the possible existence of H2SO4 in the condensate.
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