September 10, 2013

Strainers


Strainers are extensively used to trap foreign materials from different major rotary equipments mainly like pumps, compressors, fans, blowers etc or it may be used for protection of major instruments like meters, valves, and seals.

These are mainly neglected piece of equipment which are mainly important not only for protection but are MUST for many services and many chemical processes can not run without them. Still we do not focus on use of suitable design of these strainers.

Its a closed housing with cleanable screen element designed to remove & capture foreign particles.





Strainers are called strainers because they are used to strain or filter out debris / particles from liquid streams. They basically have one housing and one filter element nothing more than this. The discharge port is usually the intermittent outlet from it.

The main function of these strainers is to capture all particles above design specs reliably & in an uninterrupted manner with very little maintenance ans spare parts.

Currently the wedge wire construction of filter screen is very effective for all strainer applications. The advantage of wedge wire usage in any filter application is that it is non-clogging in nature as it is hard steel mesh of V-Shape in place of cloth and hence there is no pores choking during any sticky or slimy application.

Now the salient feature of a good strainer is that

1. It should have sufficient operation time before choking or clogging. Use wedge wire mesh for easy online cleaning.
2. Easily back washable. Wedge wire system can be easily washed online (in place with just one back flow). In case of cloth etc it has to be opened and closed after manual cleaning. So it actually becomes CIP filter easily.
3. It should pose minimum pressure drop in the system which is possible with wedge wire very easily.
4. long life span.
5. Wedge wire based Duplex system can be a very good option for continuous operation in non acidic media.
6. The only disadvantage of wedge wire is that currently we don't have any option to have it in nonmetallic MOC. I mean if nonmetallic MOC is required wedge wire is not possible today.


In next article, I will post about selection of strainers.

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August 30, 2013

Heat Pumps - 1

Industrial heat pumps are a class of active heat-recovery equipment that allows the temperature of a waste-heat stream to be increased to a higher, more useful temperature. Consequently, heat pumps can facilitate energy savings when conventional passive-heat recovery is not possible.

Therefore I am putting some basics for this useful device which can be utilized very effectively.

1. Introduction A heat pump is a device that can increase the temperature of a waste-heat source to a temperature where the waste heat becomes useful. The waste heat can then replaces purchased energy and reduce energy costs. However, the increase in temperature is not achieved without cost. A heat pump requires an external mechanical- or thermal-energy source.

The goal is to design a system in which the benefits of using the heat-pumped waste heat exceed the cost of driving the heat pump. Several heat-pump types exist; some require external mechanical work and some require external thermal energy. For the purpose of discussing basic heat-pump characteristics, this brief will first introduce the mechanical variety, and then address the thermal types.

2. Why can a heat pump save money? Heat pumps use waste heat that would otherwise be rejected to the environment; they increase temperature to a more effective level. Heat pumps can deliver heat for less money than the cost of fuel. Therefore, the cost of fuel of different types is very important in the selection of heat pumps.

Heat pumps operate on a thermodynamic principle known as the Carnot cycle. To aid understanding of this cycle, it is helpful to contrast the Carnot cycle with the more familiar thermodynamic cycle that underlies the operation of steam turbines, the Rankine cycle.

Degrading high-grade thermal energy into lower-grade thermal energy creates shaft work, or power, in the Rankine cycle. In a steam turbine, this is accomplished by supplying high-pressure steam and exhausting lower-pressure steam. In contrast, mechanical heat pumps operate in the opposite manner. They convert lower temperature waste heat into useful, higher-temperature heat, while consuming shaft work. See figure below.




The work required to drive a heat pump depends on how much the temperature of the waste heat is increased; in contrast, a steam turbine produces increasing amounts of work as the pressure range over which it operates increases. Heat pumps consume energy to increase the temperature of waste heat and ultimately reduce the use of purchased steam or fuel. Consequently, the economic value of purchasing a heat pump depends on the relative costs of the energy types that are consumed and saved.


3. How does a heat pump work, and how much energy can it save? Several types of heat pumps exist, but all heat pumps perform the same three basic functions:

a. Receipt of heat from the waste-heat source.

b. Increase of the waste-heat temperature.

c. Delivery of the useful heat at the elevated temperature.

One of the more common heat pump types, the mechanical heat pump, will be used to show how these functions work. Below is given a picture of typical system for energy saving.



Waste heat is delivered to the heat-pump evaporator in which the heat-pump working fluid is vaporized. The compressor increases the pressure of the working fluid, which in turn increases the condensing temperature. The working fluid condenses in the condenser, delivering high-temperature heat to the process stream that is being heated.

A key parameter influencing the savings that a heat pump achieves is the temperature lift realized in the heat pump. Temperature lift is the difference between the evaporator and condenser temperatures.

For example, if natural gas costs $3.00/ (MMBtu), the cost of delivering heat from fuel at 80% efficiency will be $3.75/MMBtu. Figure 1.3 shows that the effective cost of heat supplied by the heat pump is lower than the cost of purchased fuel that otherwise would be consumed.

However, this advantage erodes as the temperature lift increases, because more work is required to obtain the higher lifts. Also, because electricity is the work source for this heat pump, lower power costs result in greater benefits.

Under the right circumstances, a heat pump can reduce energy costs and provide an attractive cost-reduction project, particularly when:

a. The heat output is at a temperature where it can replace purchased energy such as boiler steam or gas firing.

b. The cost of energy to operate the heat pump is less than the value of the energy saved.

c. The net operating cost savings (reduction in purchased energy minus operating cost) is sufficient to pay back the capital investment in an acceptable time period.



In the next post we will discuss about different type of heat pumps with specific variations.

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August 20, 2013

Basics of Ion Exchange Resin - 1

Resins are generally used for Ion exchange reactions which is a powerful technology to not only treat water to extremely good quality but to process many industrial operations also today.

This ion exchange technology is now well proven for many chemical industry applications also.
It was developed initially in 1950s, and today, it is still the best to produce ultra-pure water, i.e. to remove all traces of contaminants. For example, it can be used for

• color removal from sugar syrups to make white sugar
• Purify of antibiotics and other pharmaceuticals
• Extract uranium from ores
• Separate metals
• Remove harmful substances from solutions
• Catalyse reactions

Let us start this post with basics only for understanding of ion exchange resins. So I will start with some terminology and types of resins and how do they work?


So first focus is what is ION?

Ion is nothing but small ionised substances present in water, which are electrically charged atoms or molecules. The positively charged ions are called cations, and the negatively charged ions are called anions. Because water is neutral electrically (Else it would give electric shock) the number of positive charges are same as the number of negative charges.

Ions can have one charge or more, the most usual range being 1 to 3. Ions can be made of one atom only (monoatomiic ions) , or several atoms linked permanently together, like molecules (polyatomic ions). For Example...

1. Monovalent - Monoatomic Ion - Such as Na+, K+
2. Divalent - Monoatomic Ion - Such as Mg2+, Ca2+
3. Trivalent - Monoatomic Ion - Such as Al3+, Cr3+
4. Monovalent - Polyatomic Ion - Such as NH4+, NO3-, NO2-
5. Divalent - Polyatomic Ion - Such as SO4--, CO32--





Now I am explaining the basics one by one. So What is an Ion Exchange Resin first of all. The resin as name suggests are basically small polymer molecule which you can visualize as small plastic beads or granule. Now these beads are made of one fixed component which is generally polymer part and longer in chain (not necessary but effective if it is) and the second part is mobile and can leave the structure with other similar type of ion only.



Now for the purpose of making it simple, the general ion exchange use is for softening of water in any process plant because hard water can not be used directly for many reasons. So simply the hardness means Ca++ ions are removed and replaced with Na+ ions thus calcium ion from water is removed. The equation can be represented as below.

2 RNa + Ca++ ---> R2Ca + 2 Na+




Now let us see how resin looks like. it is clear from the two pictures given below.





Once you are ready to study more you can go thru the following website

Learn to Do It Right the First Time: Guidelines for...
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August 10, 2013

Boiler Efficiency - Parameters affecting it

Energy savings does not always imply efficiency improvement in boilers therefore, we need to understand the factors which are affecting it. So basically one should know all those parameters that influence efficiency.

For this one should know how to calculate the energy savings from parameters that do not influence thermal efficiency.

For very clear understanding between an increase in steam generation and improvement in efficiency, I have always emphasized that process engineer should be very clear about his options, definitions, & actual process happening due to modification proposed in the name of energy efficiency.

Recall the general efficiency equation of a boiler

(1) Eff = Absorbed Heat / Energy Input

The equation says nothing about the fact that not all adsorbed heat is useful heat. For instance the blow down is certainly “adsorbed” heat but rarely “useful”. In other words the energy in the blow down will be lost to ambient but any change in the blow down rate will not affect the system efficiency.

Equation (1) was converted into another equation


(2) Eff = 1 - Losses / Energy Input


It was shown that equation (1) and equation (2) are equal and should give the same result.

The energy input to the system is in the most simple case the energy of the fuel and the enthalpy of the combustion air. However one may add to the ener0gy input the steam necessary to atomize the fuel, or the electricity needed to power all electric motors of the boiler. In large power plant boilers it is especially important to draw a system boundary and prepare a list of all energy flows that enter and leave the system boundary.

Similarly the sum of losses depends on what we call a loss. Some obvious losses are the energy in the stack gas, the radiation and convection loss, and the refuse loss. However blow down is not considered a loss and therefore excluded from the sum. In fact the norms state no blow down is allowed during efficiency testing.

Using the efficiency simulator one will notice that system efficiency does not change at all if the following parameters are changed

• The steam pressure
• The steam temperature
• The blow down fraction
• The percentage of condensate return
• The condensate return temperature

The above parameters do not enter equation (2) and consequently the system efficiency will not change.

Nevertheless lowering the steam pressure or temperature or increasing the condensate return and temperature will certainly save fuel. There is absolutely nothing wrong with the definition of efficiency except the fact that we may save fuel by not changing the system efficiency at all.

Whenever Ps , Ts, xBD, xcon,Tcon, change the steam output changes as well, but the efficiency stays the same. Consequently we cannot calculate the fuel savings by the equation

(3) Fuel Saving % = ( Eff new - Eff old ) / Eff new


Another peculiarity are the savings one may achieve by preheating the air or the feedwater. The efficiency definition does not provide for entering the temperature of preheated air or feedwater, because preheating devices such as an economizer and air preheater are inside the system boundary. In other words preheating of the combustion air and the feedwater is taken into account through lowering the stack gas temperature.

So basically, the major purpose of writing it to again clarify that fuel saving does not necessarily mean the increase in efficiency. Currently I have worked an oil water emulsion technique which makes nano particle mix of fuel saving 3-5% fuel but there is no change in the efficiency of the system as none of the boiler parameter is changing. This makes saving just by using water as fuel.

So be Careful.


This Article is reproduced from the paper of GTZ from BEE India website.

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