Humidification, filtration and sound attenuation benefits of rigid media direct evaporative cooling systems while providing energy savings.
Evaporative cooling is not a new technology. The beneficial use of evaporation technology for cooling has been utilized dating as far back as 3,000 BC. Historical evidence shows that the Egyptians used evaporation for comfort and for cooling drinking water. Even today, in third world countries like India the rural communities that do not have access to electricity still use clay pots for storing and cooling water. The tiny pores in the pot allow the water to percolate and evaporate resulting in the cooling of the stored water. In more recent years, science has made several advancements in using evaporative cooling and our understanding of the technology has vastly improved with numerous test data. Although it may not be realized or noticed in day-to-day activity, evaporative cooling is used in a variety of ways. From HVAC applications for comfort cooling to electrical power generation, there is a lot of dependency on evaporative cooling.
Along with temperature control, proper humidity levels are critical for human comfort and health. If the relative humidity is too low in the conditioned space, it causes dryness of the skin and mucuous membrane. If it is too high, the evaporative cooling effectiveness of the body is reduced and in high temperatures it could cause heat exhaustion. Another important effect of humidity levels is the rate of growth of biological pathogens and the interactions of noxious chemicals. These can cause severe discomfort or illness to the occupant. As shown in Fig-1a, maintaining the relative humidity in the range between 30% and 60% significantly reduces the growth rate of biological organisms and reduces the speed of the chemical reactions. Relative humidity control is also important for electronic equipment like the servers in a data center. If the relative humidity is too high, it may cause condensation; if it is too, low there is a possibility of static electricity. High humidity levels could be detrimental to building structures as well. There are several instances of destruction of buildings with indoor pools without proper humidity control. Suffice it to say, proper humidity control is a very important factor that cannot be neglected. When the relative humidity is lower than the desired levels, water vapor needs to be added to the airstream. In HVAC applications, the moisture can be added to the supply airstream through Isothermal or Adiabatic process.
[FIGURE 1 OMITTED]
Isothermal and Adiabatic Process
In an isothermal process, the evaporation process takes place outside the supply airstream where steam is generated through an external heat source. This steam is introduced into the supply airstream and the humidification process takes place at near constant temperature.
In an adiabatic humidification process, the water and the supply airstream come in direct contact with each other. The water picks up the heat from the airstream to evaporate. Since this is an adiabatic process, there is no net change in the enthalpy of the airstream. The process follows the constant enthalpy line in the psychrometric chart. The air temperature is lowered while its humidity level is raised. In addition to providing proper humidity control, the adiabatic humidification process also provides beneficial cooling. As a result, this process is also known as the evaporative cooling process. There are several ways to achieve the evaporative cooling process. This paper will focus on the Rigid Media Direct Evaporative Coolers.
RIGID MEDIA DIRECT EVAPORATIVE COOLERS
The Rigid Media is made out of a special cellulose paper with a porous structure for improved interaction between water and air. This media is usually treated with a chemical for resisting bacterial growth. The media structure design (Figure-1b) has crossflutes with unequal angles which provides turbulent mixing of air and water and enhances the humidification process. The steep angle also directs the water towards the entering air and reduces the chances of any drift. The media is modular which allows multiple sections to be assembled together to accommodate the desired airflow. The cooling effectiveness can be varied by increasing or decreasing the depth of the media, air velocity, and water flow rate.
The typical installation for the media (Figure-2a) has its own water reservoir with a recirculating water pump. This allows minimal wastage of water and also reduces the pump energy, compared to a system with a remote reservoir. This device is typically placed in the air handler plenum or it can be a unitary system, ducted and with its own casing. The sump is equipped with a bleed-off valve to keep the mineral concentration in check and to prevent the formation of scale. Ideally, a conductivity controller and an automatic bleed-off valve are utilized to minimize water wastage. The pump is sized to provide an optimal flow rate for proper wetting of the media and to prevent any dry spots which may lead to scaling. The Rigid Media with proper wetting provides uniform distribution of water and increases the contact area for the water and air. This plays an important role in increasing the heat transfer process and makes it more efficient than spraying mist into an airstream. The Rigid Media does not require high pressure nozzles to create mist. One key feature of the Rigid Media is that it does not produce water droplets, and all moisture added to the airstream is in the form of water vapor. Water droplets are the main means of transportation for the Legionella bacteria.
Although the primary function of the Rigid Media is to provide evaporative cooling and humidification, there are some additional benefits of using them. Test results have shown that the Rigid Media acts as a good air filtration device and also provides benficial sound attenuation. These are discussed below.
Figure 2: (a) Rigid Media DEC setup with pump and sump. (b) Net Insertion loss in db for a 12" Rigid Media. Air Static Octave Band Center Velocity Pressure Frequency-Hz FPM "H20 63 125 250 500 0 0 2 0 2 S 12" thick dry 400 0.109 2 1 3 5 forward flow 550 0.206 2 1 2 S 750 0.382 2 1 3 5 0 0 4 0 2 4 12" thick dry 400 0.109 4 1 2 4 reverse flow 550 0.206 4 1 2 4 750 0.382 4 1 3 4 0 0 1 0 3 4 12" thick wet 4Q0 0.122 1 1 3 3 forward flow 550 0.232 1 0 3 3 750 0.430 3 1 3 3 0 0 2 0 3 3 12" thick wet 400 0.122 3 0 3 3 reverse flow 550 0.232 3 1 3 3 750 0.430 3 1 3 3 Air Static Octave Band Center Velocity Pressure Frequency-Hz FPM "H20 1000 2000 4000 3000 0 0 4 S ID 14 12" thick dry 400 0.109 4 5 10 14 forward flow 550 0.206 4 S 10 14 750 0.382 4 5 10 14 0 0 4 4 10 13 12" thick dry 400 0.109 5 4 10 13 reverse flow 550 0.206 S 4 9 13 750 0.382 5 4 10 13 0 0 3 4 8 13 12"thick wet 4Q0 0.122 3 4 5 9 forward flow 550 0.232 3 4 6 9 750 0.430 3 4 6 10 0 0 4 4 5 8 12" thick wet 400 0.122 3 4 4 8 reverse flow 550 0.232 3 3 4 8 750 0.430 4 3 4 8
It is quite obvious that the presence of different components in the path of airflow in an air handler plenum or duct will provide some sound attenuation. However, it has been found that the Rigid Media DEC, in particular, is quite effective in reducing sound pressure levels. Test data has shown that the Rigid Media has very good sound attenuation properties. These tests were done in accordance with ASTM E477-90: "Standard Method of Testing Duct Liner Materials and Prefabricated Silencers for Acoustical and Airflow Performance". During the independent testing, two measurements of insertion losses were made: one with the airflow in the direction of sound and the other with a reverse air flow. Measurements were also made at different velocitites and for different media depths. Figure-2b shows the results of the insertion loss measurements for a 12" deep media. It is known that the reduction of noise levels in the lower frequencies is usually the hardest. From the insertion loss table, it shows that a Rigid Media DEC is quite effective in reducing low frequency noise levels. This paper does not advocate the use of Rigid Media as a sound attenuator but merely points out the additional benefits attained while using them.
The Rigid Media DEC helps improve the IAQ by bringing in 100% outside air without incurring any huge energy penalties. One other notable way it improves the IAQ is by removing particulates from the air stream. The particulate removal efficiency of the Rigid Media was obtained from tests conducted in accordance with ASHRAE Standard 52-76: "Dust Spot Efficiency Test". During the test, the particle size and count were measured using a laser counter upstream and downstream of the media. The tests were done for 6" and 12" depths of Rigid Media for two different velocities and water flow rates. Figure-3a shows the particulate removal efficiency of the 12" deep media for various particle sizes present in the airstream at an air velocity of 500 fpm and water flow rate of 1.5 gpm/sq.ft. The corresponding ASHRAE Standard 52-76 "Dust Spot Efficiency" was 16%. The analyses of the test results indicate that the 'Dust Spot Efficiency" increases with the growth in media depth, face velocity, and water flow rate.
[FIGURE 3 OMITTED]
The analysis of the test results also indicate that the most dominant particle removal method for the media is through the inertial impact. This indicates that the media will be very effective in removing large particles of size 10 microns and above. In this particular test, the size of the largest percentage of particles was less than 0.5 microns. It is safe to assume that if the tests were conducted with a different particulate distribution having a larger percentage of particles in the 5-10 microns, the dust spot efficiency would be significantly greater.
EVAPORATIVE COOLING BENEFITS
Due to the nature of the adiabatic humidification process where beneficial cooling is provided while humidifying the air, DEC can be effectively utilized as an air cooling device for HVAC applications in dry climates. The effectiveness of an evaporative cooling process depends on the "Wet bulb Depression", which is the difference between the dry bulb and wet bulb temperatures of the airstream. The efficiency of the evaporative cooling process is defined by the term "Saturation Efficiency" or "Wet bulb Depression Efficiency".
Percent Saturation Efficiency = [[t.sub.1] - [t.sub.2]]/[[t.sub.1] - [t.sub.3]] x 100 (1)
[t.sub.1] = Entering air dry bulb temperature in degrees Farenheit
[t.sub.2] = Leaving air dry bulb temperature in degrees Farenheit
[t.sub.3] = Entering air wet bulb temperature degrees Farenheit
According to this equation, as the wetbulb depression increases, the effect of the evaporative cooling process increases. Test results have shown that a 12" deep media has a saturation efficiency of 90% when the airflow velocity is 500 FPM. This efficiency is achieved with a very minimal pressure drop of 0.22" W.C. Such a high efficiency with very low parasitic loss allows the system to achieve a sensible cooling EER close to 100. For some refrigeration systems the comparable sensible EER would be between 10 and 15 for the same amount of cooling.
Combining Rigid Media Evaporative Cooling with Indirect Evaporative Cooling
To get the most benefit out of the Rigid Media Direct Evaporative Cooler (DEC), it is often combined with an Indirect Evaporative Cooler (IEC). In an IEC the supply air stream does not come in contact with water. It typically uses an Air-to-Air heat exchanger or a water cooling coil to separate the supply airstream from water. There is an additional secondary airstream on the exhaust side of the heat exchanger to pick up the heat. As a result, the primary supply air gets sensible cooling without any increase in moisture levels. The commonly used methods for IEC are: 1. Directly spraying on the heat exchanger on the secondary airstream side; 2. Using a Rigid Media DEC upstream on the secondary airstream side and evaporatively cool the air; 3. Using Rigid Media DEC as a cooling tower to evaporatively cool the water and provide Indirect Evaporative Cooling using a coil in the supply airtream. The cooling tower to coil approach has one significant drawback as it cannot provide winter heat recovery.
Test results have shown that a direct spray IEC is more efficient than using a DEC upstream of the heat exchanger on the exhaust side. Since this paper focuses on the Rigid media DEC, we will consider the Rigid media DEC upstream of the Air to Air heat exchanger method (see Figure-3b) for Indirect Evaporative Cooling in the following energy saving analysis.
ANNUAL OPERATING SAVINGS ANALYSIS
The annual energy consumption analysis of a 2-stage Indirect/Direct Evaporative Cooling (IDEC) system using Rigid Media when compared to a conventional refrigeration system shows significant savings in various aspects as well as improving the IAQ. The analysis is based on the comparison of a 100% outside air IDEC VAV system to a 30% outside air economiser VAV system providing 10,000 cfm supply air. A 365/24/7 operation cycle is considered for the analysis. The TMY3 weather data used is for Denver, Colorado and the building return air is assumed to be 75[degrees]F/50% RH. Both VAV systems have a minimum turndown of 50%. The Air-to-Air heat exchanger effectiveness is assumed to be 65% and DEC media saturation efficiency is assumed to be 90%. The desired supply condition is assumed to be 55[degrees]F.
[FIGURE 4 OMITTED]
The 100% O/A IDEC VAV system will be set up as shown in Figure-3b. The supply and the return air streams are separated by an insulated double wall. The heat exchanger is provided, on the supply side, with face and bypass dampers for temperature control and to reduce parasitic losses during the economiser mode operation. The Rigid Media DEC upstream of the heat exchanger on the return airstream acts as the IEC. This will act as the first stage of cooling. The DEC and cooling coil downstream of the heat exchanger on the supply side act as the second and third stage of cooling. A heating coil at the end in the supply side provides the second stage of heating in winter after free air-to-air heat recovery through the heat exchanger. The supply fan is in a blowthrough position to add the fan heat prior to the IEC to increase its cooling capacity. The return/exhaust fan is in a drawthrough position to avoid adding heat prior to the heat exchanger. This blowthrough supply and drawthrough exhaust arrangement also prevents any possibility of cross contamination and reduces noise levels at the duct connections to the buildings.
Using the TMY3 weather data for Denver, the annual hours can be divided into multiple zones (Figure-4b) based on how the 100% outside air IDEC VAV system will be operating to handle the heating, cooling or dehumidification loads. When the ambient temperature is above 55[degrees]F, the unit will be in cooling or dehumidification zone depending on the ambient dewpoint. The system will be in cooling/dehumidification mode for 3,569 hours in a year. Zones 2 and 3 refer to the number of hours when the unit with a Rigid Media acting as IEC and DEC will provide the desired supply condition of 55[degrees]F without any use of refrigeration cooling. The IEC and DEC removes the need for refrigeration cooling 69% of the cooling hours annually. Even when the refrigeration system is operating (Zones 4 and 5), it is at significantly lower capacities due to the fact that most of the cooling load is handled by the IEC and DEC as first and second stage of cooling respectively. An important point to note here is that the IDEC system not only reduces the operating energy, it does so while bringing in 100% O/A. The DEC operating hours may be further increased when used with an Air-to-Air heat exchanger to provide winter humidification for Zone-5 (5,191 hrs) to maintain an acceptable relative humidity of 30% in the space. This eliminates the need for a separate steam generator and dispersion coil. The incoming dry outside air is preheated by the heat exchanger. This air is further heated with a heating coil to offset the sensible cooling effect of DEC and provide the supply air at 55gF. The face and bypass dampers across the DEC will provide precise humidity control to maintain the 30% relative humidity. Since the amount of Outside Air delivery is increased in each zone through the use of Air-to-Air heat recovery, the VAV terminal minimum airflow can be set lower and still meet the ASHRAE Standard 62.1 requirements, thereby reducing the fan energy in winter.
Peak Demand kW savings
An important contribution provided by the IDEC systems is in the reduction of Peak Demand Charges. Peak demand is the highest on the hottest days when the greatest cooling refrigeration tonnage is required for a vav system. However, in dry climates like Denver, CO where it is hot and dry, the larger "wet bulb depression" results in higher evaporative cooling capacity out of the IDEC system. The beauty of the IDEC systems is that on the hottest days when the peak demand charges are the highest, they provide most of the cooling and reduce refrigeration requirements significantly.
The Denver, CO design summer dry bulb condition is 93[degrees]F/60[degrees]F. Figure-4a shows the psychrometric chart which provides the detail of the processes taking place in the two analysed systems to achieve the desired supply condition. It is clear that IDEC VAVsystem, despite providing 100% outside air, uses absolutely no refrigeration while the economizer VAV system, providing only 30% outside air, uses 23.3 tons of refrigeration to reach the desired supply temperature of 55[degrees]F. There is a common misconception that IAQ and energy savings can not go hand in hand. This performance by the IDEC system shows that it is indeed possible to bring in more outside air and reduce energy consumption simultaneously.
Compressor Operating Hours
Like any other mechanical equipment, the compressors have a certain number of cycles and hours of operation before it fails and requires replacement or repair. With IDEC systems, the number of hours the compressors are operating is a fraction of the hours for a conventional system. As shown in Figure-5b, the IDEC system significantly reduces the monthly compressor operating hours, or in other words can increase the life of the compressors threefold. An important point to note is that even in those hours when the compressors are running they are in fact running at part load because the bulk of the cooling has been done by the IEC and DEC. Figure-5b shows that Air Handling Units with IDEC systems will allow the central chilled water plants to shut off earlier in the fall and put back into service only later in the spring. This results in savings in auxillary pumps, cooling tower fan energy and reduced cooling tower chemical treatment costs.
[FIGURE 5 OMITTED]
Annual KW-HR consumption
Finally, when the total annual electrical consumption for the two systems is compared, it gives a clear picture of the true potential energy savings out of the IDEC VAV system. In a given year, the annual electrical consumption of the IDEC VAV system is one third of what is consumed by the 30% economizer VAV system (Figure-6). Note that the additional parasitic losses incurred due to the addition of the air-to-air heat exchanger and the Rigid Media DEC in both supply and return air streams for the IDEC system were taken into account for the energy consumption calculations. Similar analyses were done for various other cities in the USA comparing the 100% O/A IDEC VAV system and the 30% O/A Economiser VAV system. Table-1 shows the summary of the total annual energy consumption savings through IDEC system for these cities. In extremely hot and dry conditions like Las Vegas, the potential energy savings will be huge. Even in relatively humid cities like Phoenix, when the Evaporative cooling hours are reduced, the annual kW-hr consumption could still be reduced by half.
IDEC systems will not eliminate the need for refrigeration cooling entirely but they can reduce the electricity consumption significantly. In certain dry climates, the IDEC systems used for Displacement Ventilation or Underfloor Air Distribution applications may in fact completely eliminate the need for refrigeration cooling. Along with the energy savings, the additional benefits of filtration and sound attenuation discussed above makes the Rigid Media Direct Evaporative Cooler very attractive but heavily under utilized technology. Typically, in HVAC applications, energy conservation and IAQ do not go well together but the discussions above prove that the IAQ can in fact be improved by the Evaporative cooling systems through the introduction of 100% Outside Air. Climate change poses a huge threat to the earth's existence. Finding alternate energy source is part of the solution but energy conservation should also play a big role. It is paramount that each industry finds different ways to conserve energy. For the HVAC industry, the evaporative cooling technology is a great solution which will go a long way in reducing the carbon emission levels and slow the effect of climate change. It is important that its engineering community embrace this technology.
I would like to thank Mike Scofield, P.E., Fellow ASHRAE, for his support and encouragement. His ideas and feedback provided valuable guidance towards the completion of this work. Thanks also to the Munters Humicool Division for providing the important test results data and for sharing their expertise on the Rigid Media Evaporative coolers.
ASHRAE. 2012. ASHRAE Handbook-HVAC Systems and Equipments. Atlanta: American Society of Heating Refrigeration and Air Conditioning Engineers, Inc.
ASHRAE. 2011. ASHRAE Handbook-HVAC Applications. Atlanta: American Society of Heating Refrigeration and Air Conditioning Engineers, Inc.
Lemmen, S.W. 2001. Hospital hygiene technical report on the evaluation of the Munters FA6 evaporative humidifier. Universitatsklinikum AACHEN.
Munters Corporation-Humicool Division. 2002. Sound Attenuation Properties of Celdek Evaporative Cooling Media. Engineering Bulletin EB-SA-0208.
Munters Corporation-Humicool Division. 2003. Particulate Removal Capability of Celdek Evaporative Cooling Media. Engineering Bulletin EB-PR-0302.
Scofield, C.M., and Bergman, J., 1997. ASHRAE Standard 62R: A Simple Method of Compliance. HPAC.
Scofield, C.M. 1994. California Classroom VAV with IAQ and Energy Savings Too. HPAC.
Sterling, E.M., Arundel, A., and Sterling, T.D. 1985. Criteria for Human Exposure to Humidity in Occupied Buildings. ASHRAE Transactions, Vol.91, Part-1.
Vijayanand Periannan is an Application Engineer in the Air Treatment division of Munters Corporation, Buena Vista, Virginia.
Table 1. Annual KW-HR Consumption for 10,000 cfm System US City 30% O/A Economiser 100% O/A Annual kW-hr System IDEC System Reduction in % Albuquerque, NM 35479.75 12128 66% Boise, ID 28294.78 7841 72.3% Denver, CO 26144.14 9627 63.2% Las Vegas, NV 68722.79 19998 71% Phoenix, AZ 87798.32 45031 48.8% Portland, OR 36495.29 11314 69% Salt Lake City, UT 34789.56 11391 67.3% Figure 6: Total Annual kW-hr consumption for VAV systems providing 10,000 cfm based on Denver, CO TMY3 data 30% O/A ECONOMISER VAV 26144.14 100% O/A IDEC VAV 9627.00 * Denver, CO TMY3 Data * 365/24/7 Duty cycle Note: Table made from bar graph.
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|Date:||Jul 1, 2013|
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