Diet Coke Plus. A Cocktail of Chemicals?

Dr Janet Hull, a champion for the anti-aspartame movement, gives her thoughts on Coke's new diet soda."Sorry, folks, this new "vitamin" diet cola is another diet product with misleading marketing."



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Cooling Towers - Part II

This is the next one as I promised earlier in my first one "Cooling Towers: Not the Coolest One". This time I was motivated by the search of someone on cooling towers capacity assessment. I have already mentoned many factors & procedure to find out the capacity of cooling towers under the title Performance Monitoring - Item 9 to find out NTU. However, this search from some student led me to re-write it in simple way.

Let us understand how CT (cooling towers) capacity is fixed for design. (I am including this to give you an overview).

1. Assessment of all process loads. (e.g. Coolers, Condensers, etc.)

2. These process loads are then added with safety margins of 20-30% depedning on the confidence of design engineer either in terms of process temperature or in terms of flow approximation and then finally in terms of circulation pump capacity & head.

3. Thus, before hand we have ~20-30% margin in the CT flow rate compared to actual requirement.

4. Then CT is deisgned for worst case of WBT (Wet Bulb Temperature) which prevails only for 2-3 months in a year anywhere in the world in general. This in general gives you ~10% extra capacity in terms of heat load on CT.

Let us assume that these are the major prevailing factors which are contributing to the load on CT and no design or operating issue which we have discussed in the previous article is not there.

Now we can divide the loading of CT in two parts - One is flow loading & Second is heat Loading.

In General it is not advisable to increase flow loading beyond 10% or so of design value due to mechanical reasons for tower stablity & safety.

Thus the recommended way is to increase the heat load on CT utilizing the design margins available due to several reasons as discussed above & in my previous post.

Now the Mathematics is easier to find out the capacity of CT.

Case Study

Recently I need to increase the CW flow rate from a tower of 1050 M3/hr whcih was having a 3 cells of 350 M3/hr each. My requirement was 80-100 M3/hr.

When I estimated the total load on tower it was running with 1050 M3/hr flow rate but with 8°C range against design of 10°C range.

By simple maths, you can estimate that the

Heat load capacity was 1050 x 10 = 10.500 Gcal/Hr.

Actual load 1050 x 8 = 8.400 Gcal/Hr.

Additional Load 100 x 10 = 1.0 Gcal/Hr

Obviously there is margin in the CT & is possible to load it even the flow rate is running at design value of 1050 M3/Hr.

So total New load = 8.4 + 1.0 = 9.4 Gcal/Hr

With Design Flow = 1050 M3/hr

Delta T (Range required) = 8.95 ~ 9°C.

So I need to reduce the CW flow rate in other consumers such that the range of colling becomes 9°C from 8°C this will reduce the flow rate by ~100 M3/hr in other users giving me the flexibility or cushion to accomodate new load.

The purpose is to explain that CT should not be viewed as a stand alone flow machine it should always be treated as a combined entity for flow & temperature which is its heat removal capacity.

I have written it to help the users /students who are not clear in the concepts of CT. If you have any questio kindly post them in comments section & I will be happy to answer them as soon as possible for me.

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Surface Tension

Surface Tension

In the literature surface tensions of liquids are usually given at one temperature only (often 15 or 20 °C). As the temperature is raised, the surface tension decreases and becomes zero at the critical point. Based on this observation the surface tension at temperature T2 can be estimated for most liquids from the following simple equation if surface tension at another temperature T1 and critical temperature Tc of the liquid are known

This equation has been proposed by Kharbanda (Kharbanda, O.P, Ind. Chemist, Vol. 31, April 1955, p. 187). The results are fairly accurate. Best results are obtained for nonpolar compounds. Experimental values of surface tension of some pure liquids at one temperature are given in the following table. Critical temperatures for many compounds can be found in Appendix A of Properties of Gases & Liquids by Reid, Prausnitz, and Poling.

Table: Experimental values of surface tensions of pure liquids (from Jasper, J.J., J. Phys. Chem. Ref. Data Vol. 1 (1972), p. 841)

Compound Temperature Surface tension
°C dynes/cm

Compound Temperature Surface tension
°C dynes/cm
Acetic acid 20 27.6
Acetone 25 24.0
Aniline 20 42.7
Benzene 20 28.9
Benzonitrile 20 39.4
Bromobenzene 20 35.8
n-Butane 20 12.5
Carbon disulfide 20 32.3
Carbon tetrachloride 15 27.7
Chlorobenzene 20 33.6
p-Cresol 40 34.9
Cyclohexane 20 25.2
Cyclopentane 20 22.6
Diethyl ether 15 17.6
Ethyl acetate 20 24.0
Ethyl benzoate 20 35.0
Ethyl mercaptan 15 23.9
Formamide 25 57.0
n-Heptane 20 20.1
Isobutyric acid 20 25.0
Methyl alcohol 20 22.6
Phenol 40 39.3
n-Propyl alcohol 20 23.7
n-Propyl benzene 20 30.0
Pyridine 20 37.2



Example:

Determine the surface tension of aniline at 80 °C. From above table the surface tension at 20°C is 42.7 dynes/cm, the critical temperature of aniline is 426 °C. Then the estimated surface tension at 80 °C from Kharbanda's correlation is 35.2 dynes/cm. Compared to the experimental value of 36.15 dynes/cm the error for the estimated value is 2.6%.

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Cool it in.......... Summer

Perfect tips for the Summer!



Cooling costs can chill your budget in the peak summer months। Here are few tips for chilling out without going broke:

• Block out the sun. Using shades and blinds to block sunlight minimizes the amount of energy needed to cool your house. It's also worth noting that dark-colored exterior walls and roof tops can absorb up to 90% of the sun's radiant energy. To keep things cool, try painting your house a brighter color.

• Invest in an electronic thermostat. Most people are comfortable when temperatures range from 24 to 28 °C in the home. Try to avoid going any cooler by installing a programmable thermostat that automatically shuts off when it reaches a specified temperature.

• A few degrees can make a big difference. For every degree you shave off, you save roughly 3% on your energy bill, according to the experts at Progress Energy.com.

• Get your filters checked. Dirty air filters can make your air-conditioning unit less cost effective and shorten the equipment's life span.

• In most parts of the country, you can hire a heating and cooling technician to service your machine for Few Rs. A thorough cleaning may significantly lower your energy bills. So I suggest it doing every year.

• Hire a contractor to find and fix any air leaks and add extra insulation to basements, attics and crawl spaces.

That's All..........

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