August 26, 2008

Thermic Fluid Vs Steam for Heating

Recently I came across an article on above topic suggesting the use of thermic fluids in place of steam as heating medium even for lower temperature range where usually steam is employed. The article was from Hydrocarbon processing Oct'1991 issue.

The article suggest that we should consider thermic fluids in place of steam for new projects as they are more efficient & cost effective.

Do you agree with this?

I also doubt, so I decided to do some calculations as they are presented in the paper.

Let us assume that we need steam for a process heating load of 1,000,000 Kcal/hr i.e. 1.0 Million Kcal/Hr or 1.0 Gcal/Hr. Steam temperature required is 200°C.

So the corresponding pressure & other parameters are given below in the table.

In the above table it is clear that though the fired heater duty requirement is more for boiler, the efficiency is ultimately playing the role in higher fuel consumption for thermic fluid heaters. Even if we consider other system components the cost of operation will be higher for TF heaters as shown below.

The said article also shows that area requirement for TF based exchangers for process heating is almost similar due to gain in LMTD, but if I consider reasonable temperatures then the following table suggest that area requirement increases by at least 2 fold.

The other factors which should be considered in the selection between the two are.

  • Higher Flow rates are required for TF (as shown you need only 2000 Kg / Hr steam but 66 M3/hr TF) resulting in higher power consumption in pumps.

  • Need more heating fuel due to low efficiency of fired heaters @60% compared to large boilers having efficiency range from 88 - 92%

  • Steam can be economical after utilizing its pressure energy for power generation which has conversion efficiency of ~94%. In such cases even if we consider the overall steam generation station efficiency as said in the article, it will be economical

  • Generally heat transfer coefficients are large for phase change compared to sensible heat transfer & hence you need more surface area for TF heaters

  • Practically fouling is the biggest problem for TF heaters as they degrade easily due to temperature fluctuations. This is rare with steam

  • Careful design & selection of TF is needed for each application. Sometimes they can be dangerous & hazardous also

  • For large heating loads steam is more economical compared to TF as shown above

  • TF may be economical at very high temperature requirements >250°C where steam system hardware becomes uneconomical or where steam facility is a limitation.

  • Capital cost wise you need to replace TF after a specified time limit which add to your capital cost

  • TF system can not be used where live steam can be used.

  • In the process where cooling & heating cycles are required its very complex & response time is very low. It is always better to use steam in such cases.

  • Safety is an issue with the use of TF as in case of leaks they easily absorbed by insulation & cause sudden fire due to higher temperatures.

  • Since only sensible heat transfer is there & because of low specific heat of TF, the temperature control in the narrow range is quite difficult.

The above discussion suggest that one should very carefully consider the option of using TF system in place of steam system. In fact, the pressurized hot water system is more safer & better if high temperature is required. So always compare TF with HP hot water system as benchmark before suggesting any TF.

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August 19, 2008

Flow Device Upstream or Downstream of Control Valve

Often when you prepare a P&ID, a question is raised by my young engineers where to place any flow orifice or any other flow measuring device when there is a control valve in the system. And of course, Why????

What do you think? Should it be upstream or downstrem of control valve.
The answer depends on many factors e.g. the condition of fluid, type of fluid, operating parameters, purpose of flow mesurement etc. but most practical situations permit it upstream of the control valves.

The first important thing is that what type of flow meter you are going to use & what are the possible conditions of flow upstream as well as downstream of control valve.

It is easier to put it in upstream of any type of control valve because of two simple reasons. One is that you always know the flow condition (whether it is single phase or two phase flow) at upstream but you never know all possible variations downstream of the control valve.

Second thing is that due to variations in flow conditions your flow element may not be suitable or may give you erratic results due to larger variations in P & T parameters compared to updtream condition which is almost fixed & can be envisaged for different scanrios.

If we list these possibilities then probably it will be a list like this.
  • Variation in pressure downstream of CV may change FE reading, may lead to phase change, may lead to flashing etc.

  • Downstream of CV may have problems of full bore flow also in case of extreme limit operation

  • Downstream of CV may have expansion & sudden cooling in case of vapor as well as liquids e.g. in Cryo processes

  • Downstream of CV may have vibrations problem & therefore type of meter is important (Vortex meters may not work)

  • In case of compressible fluids the low pressure side may or may not have large flow element requirement. As density goes down DP across FE goes down & may result in higher bore size which may lead to larger inaccuracies (Depending on beta ratio).

You may also add more reason to the above list.

The evolution of good practices is a result of large experience of many & every process engineer over a period of time. There is no written rule for them, but logical common sense is required. To develop the habit of good reasoning I ask lot of step by step questions to my team engineers to give them right direction so that they themselves feel the practical situation & learn these small things.

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August 12, 2008

EconoFrost - Save Energy in Super Stores

This post is based on the suggestions from our vistior Samantha, who pointed out this source. So I thought to share it with all of you.

EconoFrost are nothing but reflective night covers made of woven aluminium for display boxes/cases with commercial refrigeration system in Super Stores. What is the need?

The need is based on the cold losses & heat gain from ambient condition during hot summer nights in these stores. If some cover is avaialble which can easily reflect the heat back to atmosphere, the load on thses refrigerated machines can be reduced during off-peak hours & night. This will definitely result in Energy Saving.

EconoFrost is a cover made by weaving of aluminium wires. Aluminium is having very high reflectivity compared to other metals & is easily available. Thus it can save ~37 - 50% on energy bill. The woven fabric reflects 70% of the heat that would normally enter open refrigerated display cases. The woven pattern of the aluminum refrigeration blinds disperses reflected heat in multiple directions, effectively maintaining optimum, even temperatures throughout the refrigerated display case

ECONOFROST insulating night covers also protect all heat-sensitive merchandise from exposure to radiation during closed hours. UV radiation from supermarket lighting penetrates the surfaces of refrigerated products, heating them and causing premature decay and discoloration. Enclosing your refrigeration display cases with ECONOFROST night curtains extends the shelf life of your merchandise and reduces produce shrinkage.

Finally good option to save energy in Super stores where lot of saving potential exist.

EconoFrost is a product from Market Group Ventures Canada. Source: Their website.

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August 07, 2008

Chemical Professionals among Top-100 Science Blogs

Congratulations to all the readers.

Chemical Professionals appeared on top-100-cutting-edge-science-blogs on X-ray technicians website. Click here to check it out.

The important thing is that 'Chemical Professionals' is the only blog related to chemical engineering among top 100 useful sites. This has been possible only with you overwhelming support & interaction of more than 450 readers by now in different bots & feed aggregators.

Thanks for your support.

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August 03, 2008

Adjustable Speed Application - Free Excel Calculator

Most pumps operating today are selected to meet a maximum system demand, or potential future demands. This means that most pumps are oversized, rarely operating at their full design capacity. In addition, pumps are often installed in systems with multiple operating points that coincide with process requirements.

A throttling valve is usually employed when the process flow requirement is less than the flow at the pumping system’s natural operating point. Throttling valves control flow by increasing the system’s backpressure or resistance to flow. This increase in pressure or head requirements shifts the pump’s operating point to the left along its performance curve, and, typically, away from its best efficiency point. The result is a loss in efficiency.

Adjustable Speed drives (ASDs) provide an efficient flow control alternative by varying a pump’s rotational speed. These drives are broadly classified as mechanical (fluid or eddy current) drives and variable frequency drives (VFDs).

Related References

Today, the VFD is the most frequently specified type of ASD, and pulse-width-modulated VFDs are the most commonly used. In centrifugal applications with no static lift, system power requirements vary with the cube of the pump speed. Small decreases in speed or flow can significantly reduce energy use. For example, reducing the speed (flow) by 20% can reduce input power requirements by approximately 50%.

In addition to energy savings, VFDs offer precise speed control and a soft-starting capability. Soft-starting reduces thermal and mechanical stresses on windings, couplings, and belts. Also, VFDs reduce voltage fluctuations that can occur in starting up large motors. Induction motors with across-the-line starting draw as much as six times the full-load current during start-up. During acceleration, a VFD-controlled motor’s locked rotor current is limited to one and one-half times the full load current. Operating at reduced speeds results in other benefit, as well, such as lower bearing loads, reduced shaft deflection, and lower maintenance costs. Estimating Performance We can use the affinity laws to predict the performance of a centrifugal pump with little or no static head at any speed, if we know the pump’s performance at its normal operating point. The affinity law equations are as follows:

Q1 / Q2 = N1 / N2

H1 / H2 = (N1 / N2)^2

P1 / P2 = (N1 / N2)^3

The affinity laws show that the pump head decreases significantly when the pump speed is reduced to match system flow requirements (see figure). Pump shaft horsepower requirements vary as the product of head and flow or as the cube of the pump’s speed ratio. Note, however, that the affinity laws will not provide accurate results for systems with static head. In that case, constructing a system curve to calculate new duty points is essential.

The Issue
We operate a pump with VFD, the significant drop in flow rate comes with significant drop in head also whereas in actual operation sometimes system curves are changing which results in lesser drop in head compared to calculated from affinity laws. In such cases, the VFD alone can not provide the desired head & flow combination becasue speed variation alone can not change system curve.

So Be Careful while selecting or recommending VFD. DO NOT CONSIDER IT without the knowledge of system curve. In such case, we need to consider other alternate also e.g. combination of conventional throttling with VFD.

A Very useful & versatile ASD Calculator for estimating Energy Savings from ASD based on running hours & requirement is Available Here FREE.

This calculator is available free from owners website & we do not guarantee or owe any responsibility for the ocnsequences out of its use.

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