Dr. Niccole Hyatt. Department of Mechanical Engineering. Date: 06-May-2016. Dubai ... developments and for maintaining healthy ecosystems. Increased ...
Research Methods GRCS.701.30 Technical Paper on
Reducing Carbon and water footprints with effective use of condensate water in hot and humid climates Submitted by
Smit Somani Under Guidance of
Dr. Niccole Hyatt
Department of Mechanical Engineering Date: 06-May-2016
Dubai
Abstract This study investigates the potential for chilled water energy, electrical energy and water sustainability and reduction in carbon footprints in hot and humid climates by utilizing the condensate water captured from large dedicated Fan coil units and Air handling units. In such climates, latent load is sufficient enough throughout the year to produce a substantial amount of condensate which is typically drained away unused. In this study, condensate is collected from several AHUs and FCUs in a storage tank. This is then used as make up in pressurization units to improve chilled water chemistry or used in coil to precool the outdoor hot air in air handling units. Also the same condensate is used in cooling towers to lower the inlet water temperature and reduce the blow down losses. Thus the condensate primarily serves to reduce the chilled water energy and electrical energy consumption and secondarily used to reduce water consumption meanwhile improving the chilled water chemistry.
Introduction Water is among the most essential elements in our day to day life. Water is essential for socio economic developments and for maintaining healthy ecosystems. Increased population and industrial development have put pressure on the current water resources creating an imbalance between the supply and demand of water (World water council, 2012). Conservation and efficient reuse of this precious and versatile resource are mandatory if such areas are to achieve proper development and the need of water conservation does not limit itself only to arid regions. My ability to be proactive in this matter makes me stand apart. In addition to finding out how a system operates, I’ve always been interested in design aspects and monitoring the incipient faults occurring within systems. This urge has strengthened my ability to recognize the scope of improvement and also motivated me to look beyond the conventional functionalities of any system. One helpful means to that end is reuse of condensate to reduce the total energy consumption in a District cooling plant. The goal is to assure the implementation of a combined system, which will satisfy all design and environmental requirements with sound engineering and responsible cost. My inclination towards energy savings began, when I started my career as an Energy Engineer at Burj Khalifa, Dubai, UAE. This included retro commissioning of a system such that it became energy efficient. Having undergone such an exposure, I was keen to carry out research in this field.
Problem Statement 1. Higher chilled water energy consumption 2. Higher electrical energy consumption. 3. Water losses In Hot and Humid climates inside a District cooling plant.
Detailed description of problem 1. Higher chilled water energy consumption: In hot and humid climates, the outside air at 45˚C and 85% Relative humidity is to be cooled at Room temperature of 20˚C and 50% Relative humidity.
This leads to higher chilled water energy consumption.
2. Higher Electrical energy consumption: The cooling tower inside a DCP is required to cool the exhaust steam or hot water at a required temperature. The cooling efficiency of cooling tower depends on the speed of the induced draft fan. Thus higher the inlet water temperature, higher the speed of fan leading to higher energy consumption and vice versa.
3. Water losses Major water inside a DCP is evaporation losses due to higher inlet water temperature in a cooling tower as well as blow down losses due to hardness of water. Also at some places, the condensate from Air handling units is not used and is wasted in drain instead.
Purpose Statement Thus, “The Purpose of this ethnographic study is to collect performance data of energy savings that can be calculated by making effective use of condensate in mechanical systems such as air handling units, fan coil units, pressurization units and cooling tower at a district cooling plant in hot and humid climates.” Return on Investments shall be calculated based on energy savings and costs of modified system.
Literature Review Although the world’s fresh water supply is abundant, there are certain areas where water usage demands are heavily out of balance with natural replenishment. Conservation and efficient reuse of this precious and versatile resource are mandatory if such areas are to achieve proper development. And, the need of water supply does not limit itself only to arid regions. This study investigates the potential for energy and water sustainability by using Condensate recovery systems in different parts of the world. The increasing cost of water and sewer services rates were the main reasons for the first successful installation of condensate recovery system in Texas San Antonio by K. Guz in June 2005 and proved that condensate recovery systems are more cost effective that the former one. Thus San Antonio became the first city to require all new commercial buildings to design drain lines which facilitates condensate capture. The equation developed by Guz is known as Equation 1 which is as follows: Condensate water production (gal gal/hr/1000/ft2) = (Tons of capacity) x (Load Factor) x (0.2 Gallons) Guz recommends various applications where condensate water can be used such as cooling tower, Irrigation and landscaping, fountains, decorative ponds and golf resorts. The author also explains the qualities of distilled water when formed on the coils, but it can get contaminated during the transport process and hence recommends chlorine or ozone treatment for the recovered condensate water. (Study conducted by K. Guz, Condensate water Recovery, Ashrae Journal in June 2005) Bryant and Ahmed in 2008, investigated the sustainability of condensate recovery system installed in international buildings located on the education city campus in Texas A&M University in Doha, Qatar. In this study Bryant and Ahmed collected the condensate water during 4 weeks period and from various Air handling units. Based on their practical results, they developed a formula to estimate the quantity of condensate water that can be obtained from the normal Air Handling unit system. (Study conducted by Tausif Ahmed, Student of Petroleum Engineering at Texas A&M University Qatar under guidance of Professor John A. Bryant in 2008) The formula can be used in commercial buildings and is given as follows: Q = (1.08 CFM x Delta T) / 12000 The Study by Dusan and Chandra Sekhar investigated the potential of energy and water sustainability in hot and humid climates by utlilizing the condensate captured from large dedicated air handling units for pre-cooling air in another Air handling unit and subsequently offsetting cooling water needs. It was found that the condensate production was large enough to make pre-cooling cost beneficial with energy savings of approximately 10% and the ability to offset cooling tower water demand in excess of 50%. The condensate water was used as make up water to reduce overall chemical treatment and improving cooling tower water chemistry. (Study conducted by Dusan Licina and Chandra Sekhar, School of Design and Environment, National University of Singapore in 2011) According to research conducted by Atul Kajale, the condensate water in the storage tank can be used to offset for the water losses due to evaporation occurring in cooling towers due to higher return temperatures. The condensate water is added in the chilled water system and it lowers the water demand of the chilled
water plant and hence lowers the operating cost. (Study by Atul Kajale, Graduate student at University of Alabama at Birmingham in 2013 under guidance of Professor Robert W. Peters.)
Methodology and Evaluation An Ethnographic study has been carried out of various systems based on practical observations in a chilled water plant. This section includes Qualitative methods employed for design, data collection, analysis and interpretation of the research. Before proceeding to data collection, analysis and interpretation procedure, let us understand the Conventional design. Retro-fitting and Retro-commissioning of the conventional design is carried out to observe the results.
Design of Condensate Recovery System (CRS) A chilled water plant consists of various system such as Fan Coil Units, Air Handling units, Pressurization Units and Cooling tower. The proposed condensate recovery system uses the same systems as in chilled water plant. The Single Line Diagram which shows the design of Condensate Recovery System is as follows:
Description of Condensate Recovery system (CRS) In this system, the recovery medium is cold condensate water that is typically drained away from numerous Air Handling units and Fan coil units to the nearest sanitary drain. During cooling process, air passes through the cooling coils of the Air handling unit and Fan coil unit prior to entering to the facility or conditioned room. As air cools, its ability to hold water in the form of vapor decreased. When air is cooled below its dew point temperature, transition from gaseous state into liquid state begins and moisture gets released in form of condensate. This condensate is collected in the drip tray below each unit from which and is collected in a storage tank. The condensate is redirected to Pressurization unit which pumps the make-up water in the chilled water system of pipes through a condensate pump. Pressurization units maintain the pressure of chilled water line by pumping additional water into the system in case of possible chilled water losses from the system in form of leakages. Due to lower temperature of condensate, an alternate arrangement is also made to precool the outside air temperature in air handling units when condensate is not used in pressurization units by means of 2 way valve. In the precooling process, condensate flows through the precooling coil and transfers heat with high temperature air to bring its temperature down. The air is then cooled to its set point temperature by means of main cooling coil. The condensate output after air precooling process is then directed to mix with cooling tower high temperature inlet water by means of thermostatic expansion valve. The condensate along with cooling tower inlet gets accumulated in cooling tower sump and the unused condensate is directed to use for irrigation purpose. This system is specifically arranged for Chilled water plant in Cities like Dubai, Abu Dhabi, etc. in Middle East due to its Hot and Humid climates. The Dry Bulb temperature and Relative Humidity for year 2015 is as shown in graph below:
The amount of condensate produced from the dehumidification process is calculated using mass balance equation as below:
Mcon = Mamb (Wamb – Woff) The heat transfer between the moisture droplets and water inside the cooling coil governed by conduction. The temperature of the condensate collected in the tray is assumed to be average of 13◦C. Condensate collected does not account for any heat gain during the transport process to the pre-cooling coil. Therefore during the precooling process, outside air temperature can be brought as low as 25◦C which reduces the use of primary cooling coil and thus reduces the chilled water energy consumption.
OA Temp ◦C 40 41 42 43 44 45 46 47 48 49 50
Precooled Air temp considering condensate at 13 ◦C 26.5 27 27.5 28 28.5 29 29.5 30 30.5 31 31.5
Set point Air temp ◦C 22 22 22 22 22 22 22 22 22 22 22
Precooled air Chilled water energy consumption (KJ/KgK) 18.9 21 23.1 25.2 27.3 29.4 31.5 33.6 35.7 37.8 39.9
Outside air chilled water energy consumption (KJ/KgK) 75.6 79.8 84 88.2 92.4 96.6 100.8 105 109.2 113.4 117.6
The difference in temperature of outside air and precooled air is as shown in graph below:
Temp Difference from Outside Air to Precooled Air 60 50
Difference in Inlet air temperatures
◦C
40 30 20 10 0 Outside Air temperature
Precooled Air temperature
Energy Recovery Process: Chilled water energy consumption in the Outside Air Handling unit can be significantly reduced by cooling the outside air to room temperature. This also reduces the pumping energy (electricity consumption) and thus maintain the chilled water Delta T of the system. Delta T is the difference in Primary Return temperature to the Primary Supply temperature of the chilled water. Chilled water energy savings can be observed as shown in graph below:
Chilled water energy consumption 140 120
KJ/Kg K
100 80 60
Chilled water Energy savings
40 20 0 Precooled air Chilled water energy consumption Outside air chilled water energy consumption
Due to comparatively lower temperature of hot water in the cooling tower, the electrical energy savings can also be observed from VFD fan in cooling tower. The above graph indicates 70% chilled water energy savings using precooled air. The difference in Electrical Energy consumption from Normal Hot water inlet to condensate water mixed hot water inlet is as shown in graph below:
Electrical Energy Consumption 200 180 160
KJ/Kg K
140 120
Electrical Energy savings
100 80 60 40 20 0 Electrical energy consumption using condensate Normal Electrical energy consumption
The above graph shows 45% electrical energy savings using a condensate water mixed in the cooling water inlet.
Water Recovery Process: The condensate collected, after recovering energy in the precooling coil of the Outside Air Handling unit is routed to cooling tower inlet using thermostatic expansion valve. When the inlet temperature goes beyond certain setpoint, the thermostatic valve opens to allow condensate to flow into the cooling tower. This maintains a constant inlet temperature to cooling tower and a constant desired outlet temperature using VFD fan. Mixing of condensate with cooling water inlet will evaporation losses to a great extent. Also frequent blow down losses can also be reduced by use of condensate.
Data Collection and Recording Qualitative Data collection was done by visual observations of the system and qualitative documents were used for recording data and analyzing it. It includes: 1) Amount of condensate generated from Fan Coil unit or Air handling unit. 2) Temperature logger used to continuously record the temperature of Condensate. 3) Outside air temperature was assumed based on Average temperature during peak summer. On the basis of recorded data, qualitative analysis was done using Heat Transfer equations to calculate the energy savings in the system in terms of chilled water and electricity. The percentage energy savings shall be calculated against its cost and compared with the cost of retrofitting the system to get the Return on its investment. For 70% chilled water energy saving and Electrical energy savings, the cost savings come to be 10767.48 AED Per day. Please note that the Cost savings are calculated on basis of above Energy savings. Actual figures may vary while carrying out practical approach.
Conclusion The analysis performed shows the condensate produced from Air Handling unit and Fan coil units is sufficient to be effectively used for the chilled water and electrical energy recovery process. Almost 70% of the chilled water energy savings and 45% electrical energy savings was observed using the Condensate Recovery system. This results in overall reduction in carbon footprints of the system. Although energy savings is often the primary focus, another important part of sustainability is water conservation. Water conservation strategy is coupled with energy efficient features to provide a better overall design. In hot and humid climates, about 50% of water savings can be observed by using condensate water as make up for blow down losses. Also the blow down losses can be reduced as the condensate water is soft form of water without any mineral salts in it. The chemistry of chilled water system can also be increased as the makeup water for the chilled water system from pressurization units is again a soft water with zero TDS in it. Thus, adopting the positive attitude of water being the next “oil” like commodity in Middle East countries due to its scarcity and understanding the reuse potential of condensate make its very valuable to simply be drained away is very proper approach that can help in managing a very precious and necessary resource.
References 1) http://www.allianceforwaterefficiency.org/Condensate_Water_Introduction.aspx 2) https://weatherspark.com/averages/32855/Dubai-United-Arab-EmiratesEnergy and water conservation from AHU in Hot and Humid climates by Chandra Shekhar and Dusan Licina. 3) Condensate water recovery, ASHRAE Journal by K. Guz 4) American Society of Heating, Refrigerating, and Air Conditioning Engineering. (2009). ASHRAE 2009 Handbook of Fundamentals. Atlanta, GA: American Society of Heating, Refrigerating, and Air Conditioning Engineering. 5) Environmental Leader LLC. (2013). Air Conditioner Condensate Recovery-Environmental Management and Energy News. Retrieved 9 8, 2013, from Environmental Leader: http://www.environmentalleader.com/2013/01/15/air-conditioning-condensate-recovery/ 6) Energy Solutions Professionals LLC. (2008). Maintaining Comfort While Optimizing 7) http://www.gewater.com/handbook/boiler_water_systems/ch_19_Condensate.jsp 8) http://www.gewater.com/handbook/cooling_water_systems/ch_32_closed.jsp 9) http://www.tlv.com/global/TI/steam-theory/introduction-to-condensate-recovery.html 10) http://energy.gov/sites/prod/files/2013/10/f3/epa-scesd_watercs.pdf 11) https://www.epa.gov/sites/production/files/2015-05/documents/cs2-gsa-condensate-recovery.pdf 12) http://www.districtenergy.org/assets/pdfs/2012-Campus-Arlington/Presentations/TuesA/4A2PRUITTWILKISONGUCCIONENEWUsing-Air-Handler-CondensatePGuccioneFinal.pdf 13) http://www.environmentalleader.com/2013/01/15/air-conditioning-condensate-recovery/ 14) Condensate water collection for an institutional building in Doha, by Tausif Ahmed. 15) Estimating and Forecasting of AHU Condensate recovery using multiple regression analysis and time series autoregressive models by Atul Kajale. 16) Qualitative methods of Research by John Creswell. 17) https://www.plantengineering.com/industry-news/mechanical-news/single-article/a-quick-guideto-condensate-recovery/ecd6fbf8f2f4a785b5976bb211f9ab60.html
