INTERNATIONAL JOURNAL OF ENVIRONMENTAL SCIENCES Volume 5, No 2, Copyright by the authors - Licensee IPA- Under Creative Commons license 3.

Transcription

1 INTERNATIONAL JOURNAL OF ENVIRONMENTAL SCIENCES Volume 5, No 2, 2014 Copyright by the authors - Licensee IPA- Under Creative Commons license 3.0 Research article ISSN Study, design and fabrication of ductless fume hood Makarand A. Umarji, Nitin S. Shriram, Ruchira R. Takte, Nitish V. Topkar Department of Mechanical Engineering, PVPIT, Pune University, Maharashtra, India makarandumarji55@gmail.com doi: /ijes ABSTRACT This paper is about the solution on the problems of ducting and wastage of energy in chemical laboratories for just getting rid of fumes. This paper gives an effective model of ductless fume hood which demolishes the concept of ducts and helps recirculation of air in its purest form. This paper explores all the equipments used in a ductless fume hood with all its specifications and requirements for a specified usage. The paper gives a data about the filter used, the blower required and also the applications where this type of fume hood can be used. The data collected and the tests organized for the fabrication of the fume hood helped us to conclude with a fine helpful and effective ductless fume hood which can be used in various applications. The study in this paper helps to create a hood which can provide a protective environment for the user. Also this hood helps in protecting the environment from the hazardous fumes. Also this hood helps in saving the energy. Thus, the paper concludes with a better option for the fume hoods having ducts. Keywords: Ductless fume hood, activated carbon, London dispersion forces (van Der Waal s forces), polyurethane paint, hot wire anemometer, blower, carbon filter, baffle, pressure drop in carbon filters, face velocity, flow rate, ASHRAE, ANSI standard. 1. Introduction The Fume Hood Program of University of New Hampshire produced a fact that general room ventilation is not enough to provide adequate protection against harmful gases, particulates, aerosols etc. A fume hood is a local ventilation device that is designed to prevent or to limit the exposure to hazardous fumes. These are also called as Re-circulating fumes hoods. In order to draw all the necessary results we have carried out tests which are as described below: 1. Pressure Drop Test (in case of filters). 2. Face Velocity Measurement. 3. Discharge through the blower. 4. Flow Visualization test. 5. Material reactivity test. 1.1 Why Go Ductless? Most of the conventional ducted fume hoods require large space, maintenance and its cost, installation cost, proper ducting, proper filtering, piping losses, high capacity blower. In order to tackle these factors ductless fume hoods are used because of the advantages over conventional fume hoods. Moreover, the poisonous and toxic gases are filtered and re- Received on June 2014 Published on September

2 circulated back to the working environment thus preventing the atmosphere from contamination. 1.2 Advantages of ductless Fume Hoods 1. Less space requirement. 2. No ducting costs. 3. Low capacity blowers. 4. Low maintenance. 2. Material and methods Figure 1: Working of ductless fume hood The air moves from the ambient environment into the working space with an average velocity equal to 0.5m/s (i.e cubic feet per minute) according to ASHRAE standards. Negative pressure is created in the hood s work zone, which ensures the operator protection. The air passes through the slot and small slots are made in the baffles to the filter situated at top of the work zone. Air is taken through a pre-filter and an activated carbon filter mounted in the interior. The pre-filter is built into the activated carbon filter. This helps in longevity of the filter. The carbon filter removes all fumes from the exhaust air stream and filtered clean air is recycled again into the workspace. Filter used in this ventilation device is activated carbon filter along with a pre filter. 2. Activated carbon filter Activated carbon filter consists of activated carbon which is a form of carbon that has been processed to make it extremely porous, thus giving it a very large surface area for adsorption 252

3 or chemical reactions. One gram of activated carbon has a surface area around 500 m² (5400 square ft.). The effectiveness of activated carbon as an adsorbent is attributed to its unique properties, including large surface area, a high degree of surface reactivity, universal adsorption effect, and favorable pore size. It is used in filtration of chemical fumes, purification, deodorization, de-colorization and separation. It is usually derived from wood, coal, coconut shell, or peat. The carbon is activated either by chemical activation or steam activation. The adsorption phenomenon of activated carbon filter works on principle of London Dispersion Forces, which is a type of Van der Waal s Force. Characteristic properties of London dispersion forces are 1. Nonspecific - existing between all molecules. 2. Temperature Independence from -273 C to 1000 C. 3. Additive - the sum of all interactions. 4. Short ranged - the magnitude of the interaction is sensitive to the separation of the molecules. These characteristics make London dispersion forces analogous to gravitational forces, but short ranged. 3. Instruments Photo 1: Activated carbon granules The instruments used for measuring are hot wire anemometer and pressure gauge. 3.1 Hot wire anemometer An anemometer is a device used for measuring wind speed, and is a common weather station instrument. Hot wire anemometers, while extremely delicate, have extremely high frequencyresponse and final spatial resolutions compared to other measurement methods, and as such are almost universally employed for the detailed study of turbulent flows, or any flow in which rapid velocity fluctuations are of interest. 253

4 Photo 2: Hot wire anemometer 3.2 Pressure gauge A Pressure Gauge is used weigh flexibility like fluid, gas flow, speed, water level, and altitude. Pressure sensors are also called as pressure transducers, pressure transmitters, pressure senders and pressure indicators. Differential pressure is the pressure between two points in a fluid system. It is measured in relative difference to a reference side. It is often measured in PSID (Pounds per Square). The actual difference between a normal pressure gauge and a differential pressure gauge is that a pressure gauge indicates actual pressure and a differential gauge indicates the difference in pressure. 4. Calculation for blower Photo 3: Differential pressure gauge The selection of blower requires Q and P of the total system through which the fumes are absorbed. 4.1 Calculation of discharge Q Q for the blower can be calculated using the Continuity Equation, Where, A= face surface area V= face velocity 254

5 Face Surface area= = 0.372m 2 According to the ANSI/ASHRAE , US, the air face velocity at any point of the front openings must be between 0.4 m/s and 0.6 m/s. Considering the Face Velocity = 0.5 m/s. So Using the Continuity Equation, Q= m/s Q= m³/s 4.2 Calculation for pressure drop before carbon filter P₁ At Slot, Q remains constant and area changes changing the velocity. Open surface area = ( ) + [33 {( )+(π 0.004²)}] = m² A =0.066 Q=A V V= Q/A= m/s According to Bernoulli s theorem, kg/m³, g = 9.81m/s² P1 = Pa For MS duct, Q & A remains constant, but has friction loss and change in datum. f= f=1.44 Equivalent diameter of rectangular duct = = = m Losses = = = m According to Bernoulli s Theorem, P2= Pa 255

6 According to standard velocity at carbon filter should be m/s, Considering 3.3m/s, Calculation of No. of Holes: Q=A V A=0.056m² Area=No. of holes Area of available hole, 0.056=No. of holes π 0.06², No. of holes = 496. Again using Bernoulli s Theorem, P₃= Pa the pressure drop in the fume hood before the carbon filter is, P - P₃= = Pa. Using the equation, Pa = mm H₂O = mm WC P₁= = 2.71 mm WC = m WC. 4.3 Testing for pressure drop in carbon filter P₂ The following method is followed to test the pressure drop created by the carbon filter if the filter is filled at 100mm depth of carbon. The carbon filter is attached in the testing case in vertical position. The drum is fitted with a blower outlet which delivers air from one end of the carbon filter to the second end of the filter. A manometer is attached to the testing case to examine the pressure drop created by the carbon filter. The blower is connected to a power supply and operated to suck the air from the casing. At first the pressure drop of the casing without the filter is noted down from the two ends of the filter. Then the pressure drop created by the filter is noted down. Two readings are taken and the average of both is considered. 256

7 Figure 2 : Test rig for calculation of pressure drop across carbon filter The results are as follows: Table 1: Results of pressure drop in carbon filter Velocity of air Lengths of carbon filled in filter 50mm 75mm 100mm 0.5 m/sec m/sec The results are in (mm WC) By interpolation we calculate pressure drop for 3.2m/s, P=13.02 = m Thus the pressure drop of the total fume hood = P₁+ P₂ 4.4 Selection of blower For selection of the blower data considered is: Air output = m³/s = m³/hr Pressure drop P = 15 mm WC= m of WC = = mm of WC = m of WC Thus considering the data from the following Blower Catalog (Appendix B-1) we select a blower: 5. Result and conclusion 5.1 Material reactivity test 257

8 Observation after 336hrs: There is no effect on the metal painted with polyurethane paint. Conclusion: As it is seen that the paint or the metal is not affected due to HCl it can be concluded that polyurethane paints are acid-corrosion resistive and the can be used to protect the metal from corrosion, thus making it useful for painting the fume hood. 5.2 For Smoke test Observation: It is visually observed that the smoke is sucked inside the fume hood. Conclusion: Thus the Fume Hood manufactured is safe in working for sucking the fumes inside and not letting them flow out to harm the operator. Figure 3 : Graph of Air output vs Static pressure According to the requirement and considering the above chart RF225S2 [2] is selected for the Fume Hood. 5.3 For velocity test Table 2: Observations of velocity test Required Velocity At the Opening 0.5 ±0.1 m/s Required Velocity At the Baffle 2.5±0.1 m/s Observed Observed Conclusion: As the Observed velocity at the different locations is approximately similar to the required velocity, the Fume hood passes the velocity test. 5.4 For pressure test 258

9 Pressure Drop created by the baffle and carbon filter Table 3: Observations of pressure test Required 15±5mmWC Observed Conclusion: As the observed pressure is approximately in the limits of the required pressure drop, the Fume hood passes the pressure drop test. 6. References 1. American society of heating, Refrigeration and air- conditioning engineers (ASHRAE), 1995 ASHRAE handbook heating, Ventilation, and Airconditioning, Applications. 2. Laboratory Fume Hoods: A user s manual by G. Thomas Saunders. 3. The fume hood program of university of New Hampshire. 259