Climate Wizard Technical Description

Transcription

1 Climate Wizard Technical Description 1. Introduction Seeley International Pty Ltd is a manufacturer and marketer of direct evaporative cooler and gas heating appliances. Their product line in direct evaporative coolers comprises whole of house ducted coolers, small portable appliances and large industrial evaporative coolers. Traditional direct evaporative coolers offer a low energy consumption means of cooling in dry climates through the direct evaporation of water in evaporative media pads. As air is drawn through the evaporative pads, water is evaporated thereby cooling the air to temperatures which may approach the Wet Bulb temperature of the incoming air. However, while the temperature of the air is reduced, the water evaporated from the pads raises the humidity of the air, partially offsetting the improved comfort achieved by the reduced temperature. Technologies have existed for some time which utilise the advantages of evaporative cooling to reduce the temperature of air without the addition of moisture. These technologies invariably use some form of heat exchange media between the evaporation of water process and cooling of air. The low temperatures created by the evaporation of water provide a temperature differential to enable the cooling of air across a heat exchange barrier, which enables the transfer of heat from the air stream to be cooled without any direct contact with wet surfaces. The conditioned air is thereby cooled without the addition of moisture to the air stream. This technology is generally known as indirect evaporative cooling. A configuration for indirect evaporative cooling comprising counter-flow heat exchange in adjacent wet and dry air passages was proposed as early as 1976 by Maisotsenko et al in Soviet Patent No. SU lodged 9 September The configuration presented could demonstrate that temperatures of the incoming air could approach its Dew Point temperature, which is considerably lower than the Wet Bulb temperature which is the limit achieved with direct evaporative cooling. Practical models were built to demonstrate the principles and temperatures achievable, but the technology has never been developed due to its high cost and complexity. Seeley International has adapted the original principles demonstrated by Maisotsenko, to maximise the heat exchange process, and achieve super-cooling (i.e. cooling below the wet bulb temperature) of the delivered (dry) air. This occurs because the Wet Bulb temperature of the air drops in the wet channels and so the air can achieve temperatures well below those that could otherwise be achieved without super-cooling. As a result of this performance, Seeley International is pursuing a range of applications which include:

2 Page 2 Pre-cooling of the fresh air component for both existing and new refrigerated cooling systems (add-on cooling); Additional cooling for buildings with existing refrigerated cooling (supplementary cooling); Combination units for new installations but using smaller compressors (hybrid cooling); Independent cooling of buildings, including domestic dwellings (stand alone cooling). The Seeley construction uses a medium with the characteristics of high water retention and wickability as the wet channel, while the dry channel uses a moisture-impervious membrane. The construction allows for automation of the manufacturing process for the core heat exchanger, and allows for intense heat exchange and super-cooling via water evaporation, to take place within the core. The result is a construction which is effective as an indirect evaporative cooler and commercially viable as a competitor to alternative means of cooling air. The performance characteristic of indirect evaporative coolers in regards to their outlet temperatures approaching the dew point temperature of the incoming air suggests that they can produce air temperatures comparable with systems that use refrigeration.

3 Page 3 2. Cooling Process and Performance The process of cooling in a direct evaporative cooler is described as adiabatic cooling a process which takes place with no net energy exchange to the surroundings. In this process, sensible heat in both air and water becomes latent heat in the entrained vapour, and temperatures fall and equalise. This can be explained in a psychrometric chart, which displays the properties of moist air as determined by the temperature and humidity ratio as shown in Figure 1. Figure 1 - Psychrometric Chart, Direct Evaporative Cooling An important parameter for consideration of the performance of a direct evaporative cooler is the Wet Bulb temperature, defined as the temperature measured by a temperature sensor covered with a water moistened wick. When measured correctly, the Wet Bulb temperature approximates the temperature of adiabatic saturation and so is the lowest temperature that can be obtained by evaporating water into the air. A direct evaporative cooler can only ever produce air which approaches the Wet Bulb temperature. The extent to which a particular cooler can approach the Wet Bulb temperature is its effectiveness. A well made direct evaporative cooler will have an effectiveness of approximately 85%, and produce temperatures and humidity s represented by the red line in Figure 1. ASHRAE Standard No defines several categories of indirect evaporative cooler. While none of these categories directly defines the Climate Wizard indirect cooler subject of these tests, the Climate Wizard is essentially an integrated heat exchanger type of indirect cooler. An indirect evaporative cooler of this type reduces the temperature of the air through a heat exchange surface which is impervious to water. On one side of the heat exchange

4 Page 4 surface, the evaporation of water produces cool temperatures in water and air, allowing heat exchange to the incoming air on the other side of the heat exchange surface. The primary air, in the dry channel, is therefore sensibly cooled without changing its moisture content. It should be noted that the sensible cooling of air not only reduces the Dry Bulb temperature, but also its Wet Bulb temperature. Figure 2 - Indirect Cooler schematic In the configuration used to construct the Climate Wizard, a proportion of the air which has been heat exchanged to a lower temperature is returned along the wet channel as illustrated schematically in Figure 2. Since the air returned has a depressed Wet Bulb temperature, evaporation on the wet surfaces of the wet channel will produce temperatures approaching the now lower Wet Bulb temperature. This significantly lower temperature then further intensifies the heat exchange to the dry channel, further reducing the Wet Bulb temperature of the proportion returned to the wet channel. This process continues throughout the heat exchanger matrix continually intensifying the heat exchange and evaporation processes until the exit temperatures start to approach the Dew Point of the incoming air. The cooling process is represented on a psychrometric chart in Figure 3.

5 Page 5 Figure 3 - Psychrometric Chart, Indirect Evaporative Cooling In the example shown in Figure 3, air has entered an indirect cooler typical of the construction type used for Climate Wizard at the nominal design condition of 38 o C DB/21 o C WB corresponding to a humidity ratio of 8 g/kg. Cooling has taken place along a constant humidity ratio line with the final delivered air condition of 16 o C DB/13 o C WB and the humidity ratio still 8 g/kg. Note that the final delivered temperature is actually below the entering Wet Bulb temperature, resulting in a wet bulb effectiveness of greater than 100%. In the limit, with ideal heat exchange and evaporation, the temperature delivered can approach the Dew Point of the incoming air. 3. Cooler Description The Seeley Climate Wizard Cooler is shown schematically in Figure 4 with its major components. Outside air is drawn in to a backward curved centrifugal fan, which pressurises the air ahead of the indirect cooling core. The pressurised air passes through alternate passages within the core created from a moisture impervious membrane. These passages are the dry passages. Air emerges from the core into a plenum space, with flow to the delivery restricted by an orifice plate. The restriction provided by the orifice plate creates a static pressure in the plenum space, which drives a proportion of the air emerging from the dry passages back through the passages separating the dry passages. These passages have a moisture absorbent surface kept wet by a water distribution system, and are known as the wet passages. Air in the wet passages passes through these passages until it emerges from the exhaust, and is then discarded.

6 Page 6 Figure 4 - Climate Wizard Indirect Cooler schematic In operation, air passing through the wet passages evaporates moisture from the surfaces of the passage, thereby cooling the air as it passes through. Cooling of the air in the wet passage crates a temperature differential between the wet and dry passages, thereby cooling the air flowing through the dry passage by heat exchange. As the dry air is cooled, the proportion turned back through the wet passage is capable of further cooling the air in the wet passage, thereby intensifying the heat transfer. The core arrangement is capable of cooling air to temperatures approaching the Dew Point of the incoming air, without the addition of moisture to the air. 4. Performance Parameters The cooling capacity of the Cooler is defined as: Cooling Capacity (kw) = Airflow x Density x SH x (T db,in T db,out ) (Equ. 1) where Airflow = air flow rate delivered by cooler (excluding exhaust) m 3 /s Density = air density kg/m 3 SH = Specific Heat of air = kj/kgk T db,in = Dry bulb temperature into the Cooler T db,out = Dry bulb temperature delivered by the Cooler

7 Page 7 With refrigeration coolers the Coefficient of Performance (COP) for cooling is determined from the cooling capacity produced and input electrical power (see equation below). The same can be applied to indirect evaporative coolers using the equation above to calculate the cooling capacity and the total electrical energy consumed. Coefficient of Performance = Cooling Capacity / Motor Input Power in units of kw/kw. It is common practice in the USA to report the above in units of Btu/Wh, which is known as the Energy Efficiency Ratio (EER). The EER is 3.41 times the dimensionless COP. For comparison with similar tests conducted on Indirect Coolers in the USA, the Wet Bulb Effectiveness can be defined as follows: Wet Bulb Effectiveness = (T db, in T db, out ) / (T db, in T wb, in ) % (Equ. 2) Where T db, in = Dry Bulb temperature entering cooler T wb, in = Wet Bulb temperature entering cooler T db, out = Dry bulb temperature delivered by Cooler It should be noted that an Indirect Evaporative Cooler can cool to temperatures below the incoming Wet Bulb temperature, and can therefore have Wet Bulb Effectiveness greater than 100%. Extract from: University of South Australia Report On-site Testing of Climate Wizard Cooler at Roxby Downs, by Dr. Frank Bruno, Senior Research Fellow, Sustainable Energy Centre, 17 June 2009