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High velocity in heat exchanger tubes
2

High velocity in heat exchanger tubes

High velocity in heat exchanger tubes

(OP)
In normal the operation of our cross flow heat exchanger the rate of heat transfer goes up proportionally to the cold side flow and we control exit temperature that way with a PID loop program operating the cold side throttle valve. But we seem to be able to reach a condition when the flow rate reaches a certain point the rate of heat transfer starts to drop, so the throttles open more and the rate goes up more so the rate of transfer drops even more and continues into an out of control situation.
I seem to remember something from an Advanced Fluid Flow course (about a zillion years ago) where high velocity flow drastically reduces efficiency but I don't remember any details or even what the phenomenon is called.
Can someone explain to me the mechanism by which cooling water flow above a certain velocity in the cold side tubes will drastically reduce the heat transfer ability?  And what solutions either in design or controls do you suggest?

RE: High velocity in heat exchanger tubes

tim02,

increasing velocity should increase heat transfer coefficient, not decrease it; but you know that...

a dumb question - how do you know the heat transfer rate is going down? by the outlet temp on the hot side?

it's just that the exit temp on the cold side may decrease with increased flow, but the heat transfer is increased...

is there any possibilty of fouling occuring on the hot side?

just some thoughts, no solution for you

Cheers,
John.

RE: High velocity in heat exchanger tubes



At the risk of misunderstanding your question, the minimum approach temperature for your exchanger design, limits what the control valve can accomplish.

RE: High velocity in heat exchanger tubes

Think that JOM's approach is correct. If you guess that heat transfer is going down (because exit temperature tends to decrease), your controller should reduce the flowrate, not increase it !!.
There is a first order effect  in the relation Flow-temperature and a second order effect in the Heat transfer coefficient with flow (because the other side and delta T also count), so , only the first should be used.
Regards

RE: High velocity in heat exchanger tubes

(OP)
Let me clarify some more.  
The hot side exit temperature starts to climb as the flow rate goes up.  
The cold side exit temperature (and delta T) goes down.
The more the cold side control valve opens to increase flow in attempt to bring hot side (process control) temperature back down into spec., the higher the hot side exit climbs and the smaller the delta T across the cold side until it is almost zero.  The normal control response only amplifies the problem and rapidly drives itself out of control.
We take manual electric control of the throttle valve and forced it more closed which drives the delta T on both sides back up, and process exit temps come back down with less flow.
I have been told by a corporate facilities engineer that there is a phenomena that occurs at very high cold side water velocities in the tubes that can cause an inability to transfer heat from the tube wall to the cooling water.  But he didn't have any more than annecdotal information and I cannot seem to find any documented corroboration.  But I also can't find any other explanation of this flow to heat transfer proportionality  reversal effect

RE: High velocity in heat exchanger tubes

tim,

if you can control it in manual then your controller settings need attention. inverse acting processes do exist. they are characterized by an initial dynamic action opposite to the steady-state behavior. without a very complex control scheme about all you can do is to detune the controller and accept the setpoint error.

restated: if you have a tight control objective on the hot side exit, then the control gain, integral, and derivative are set for rapid response. this can destabilize the valve operation.

suggest that you try, turning off the derivative and only using a little bit of integral. the controller gain should be 1 to 2 (to start with).

you can also add valve limits to precent the valve from going full closed: if the valve closes then you lose controllablity until the hot side exit temperature is returned below setpoint manually - has to do with so-called reset wind-up.



 



RE: High velocity in heat exchanger tubes

Tim

This is going to sound a bit dumb, but you've made sure your controller wasn't wired up backwards, right?  Sometimes that happens and the sensor is valiantly trying to tell the valve to close, but the wires are literally crossed....

it pays to check out the simple things before looking into the weird and unexplainable.

Patricia Lougheed

Please see FAQ731-376 for tips on how to make the best use of the Eng-Tips Forums.

RE: High velocity in heat exchanger tubes

(OP)
Thanks Guys,

Hacksaw, In manual we are closing the control valve to get more cooling which is intuitively backward but works. until we get back into a lower flow range and then starts working proportionally again.

Patricia, it's never a dumb question unless you don't ask it and check it out, because it will end up being the problem, right?  But we did ask and we have checked. They are all wired in the correct direction.  

We have several of these heat exchangers with 2 parallel, independently throttled, stages in each HX and they all drive into this situation on occasion.  And it's always the hottest stage that goes first.

But it's beginning to seem like no one else has heard of this supposed "high water velocity - inverse thermal efficiency" effect.

Tim

RE: High velocity in heat exchanger tubes

Tim.

Looks like a job for Sherlock Holmes (or Mulder n Scully).

Just to be crystal clear, can you tell us the typical temperatures on both sides at both inlets and outlets when the HX is performing as desired? Also, what are the two streams? What is the internal arrangemt - no of passes on tube side?

>But it's beginning to seem like no one else has heard of >this supposed "high water velocity - inverse thermal >efficiency" effect.

My thoughts are that whatever phenomenon was occurring was been masked and the observations, taken at face value, led someone to jump to a wrong conclusion. "There's a logical, scientific explanation, Mulder."

We musn't abandon our confidence in the basic heat transfer principles. Increasing velocity will increase heat transfer co-efficient - believe it. If somehow the oppposite occurs, there will be an axplanation, but it won't be a reversal of the basic principles.

In milk pasteurisers (a heating job), milk proteins deposit on the HX surfaces, heat transfer reduces so the controller calls for higher temp of the heating stream, the protein deposition gets worse cos of the higher temps and the controller calls for more heat....unsustainable and the HX eventually has to be cleaned.

I know that's not your situation, but I just use it as an example to show that something other than simple heat transfer can be at work.

Smart of you to ask for fresh ideas. Have you ever found the harder you stare at a problem the more the solution avoids you?

I know of an example of a HX behaving opposite to plant personnel's intuition, and the answer was surprising and only discovered through expert analysis. I'll see if I can write it up in detail. The answer was in leaking tubes which no-one knew about. When was the last time your HX was inspected internally?

C'mon, Scully, let's go.

Cheers,
John.

RE: High velocity in heat exchanger tubes

Tim02,

The inverse action you have described is normally found in reboilers and other such equipment where a phase change is possible. That problem is well known.

What is unique to your problem is an apparent lack of phase change.

What is the fluid state of the hot stream and where is the temperature sensor located realtive to the discharge of the exhanger?



RE: High velocity in heat exchanger tubes

hi guys,
in my opinion,there can be a turbulance effect due to high velocity which can make the heat transfer goes down.
may be you know in many cases, this effect should provide the heat transfer to increase up to intended values.
but,according to high velocity,there can be no useful attitude for the water or generally let's say refrigerant to release its thermal energy to go out.
we have to check the pressure drops and piping diameter if it is enough for the heat transfer within the sufficient turbulance effection.

RE: High velocity in heat exchanger tubes

2
Hacksaw may have indeed hinted at the possible reason of the strange cooling water behaviour when he speaks of phase changes. On this line of thought I'd venture:

If the H/E is located sufficiently high so as to create a vacuum at the water return nozzle, air may be released from warm water.

The geometry of the exchanger may be so that at higher water superficial velocities, we get some kind of segregation, so some tubes may be "seeing" a water-air mixture, and others only water, with a consequent drop in HTC.

Don't take this idea too seriously, it's just an hypothesis easy to be checked.    

RE: High velocity in heat exchanger tubes

25362,

I do have the similar sentiments as u, that the possibility of a reduced HTC as a result of some vapour-liquid mixture within the HX. In fact, I am in the midst of doing some testing on this possibility for one of the HX in the plant i'm working in. Any comments on this subject, anyone, because i'm definitely not an expert on this.

RE: High velocity in heat exchanger tubes

I too had a problem of that sort once in our vapor absorption system condenser. What I thought at that time was that the contact period between the two fluids was less(it was a guess and was not backed by any calculation). I controlled the valve towards closing position and the thing worked.  

Yet I agree with JOM that the real occurence might have been masked for I still think my idea was a weird one.

RE: High velocity in heat exchanger tubes

I want to upload an unusual heat exchanger behaviour case, but it really needs a diagram.

Does anyone know how I can insert a .gif file in the message. I've noticed .jpg pix in other threads. How do you do it?

Cheers,
John.

RE: High velocity in heat exchanger tubes

Perhaps this could be of help if the picture is on a webpage.

[img http://www.sitename.com/picture.gif]

Just above Submit Post button there are 3 options. Process TGML is one option where you will find some tags to put into your post for subscript, superscript, bold etc,

Regards,

RE: High velocity in heat exchanger tubes

While trying to find a reasonable solution to this puzzling question I imagined the H/E as a square box. tim02 speaks of a (one-pass?) crossflow exchanger. In these exchangers both fluids usually enter and flow at right angles to each other, and tubes sometimes have perpendicular radial fins to enhance heat transfer on the shell side.

As a result they behave in a strange manner: the shell side unmixed fluid crosses the unit leaving it with a profile of temperatures, showing a coldest "corner" where the outlet of the warm fluid crosses the incoming cold water, and a warmest corner at the other shell outlet end.

Water also leaves  with a profile of temperatures, the highest at a corner diagonally opposed to the "cold" corner of the shell side fluid.

This translates in a peculiar delta T map: two diagonally opposed corners with minimum delta T (the "cold corner" and its diagonally opposite) and the others with a maximum delta T.

Because of the particular geometry, the large number of tubes, and typical header arrangements, water distributes unevenly between the middle tubes and those at the periphery.

The overall coefficient of heat transfer influenced by the shell side fluid is low, and the increased water velocities don't improve it much. If we admit that the increased water flow rates asked for because of higher oulet shell fluid temperatures, tends to re-distribute among the tubes, we may find that the overall heat transfer drops even when the outgoing mixed fluids show a larger delta T.

This is only an hypothesis trying to show that there is nothing wrong with heat transfer theory and that hydraulics may be the reason for this fata morgana trying to lead us astray.




RE: High velocity in heat exchanger tubes

Hi ,

I just also want to write down my propably stupid thought,
can temperature increases due to high friction between water and tube wall caused by extremely high velocities ?

RE: High velocity in heat exchanger tubes

hartof,

that's a brilliant thought, not stupid at all

basic physics says there ought to be friction-generated heat, but I've never, ever heard it mentioned in fluid flow

my guess it would need extreme velocities to have a noticeable effect

Cheers,
John.

RE: High velocity in heat exchanger tubes

Especially when timO2 says that water comes off cooler at high rather than at low flow rates.

To me it seems that somehow cooling water doesn't seem ""to see" all the warmer tubes at high (beyond some limit) flow rates as it does at lower flow rates.

RE: High velocity in heat exchanger tubes

tim02

Getting back to the original question - heat exchanger efficiency and velocity are related through the Reynolds number, and if you normally operate at a very low flow rate (such that you're in the laminar region) and the increase is just enough to put you in the transition region rather than get you to turbulent flow, then you could experience a large drop in heat transfer.  JOM asked you to give us some data.  If you have information such as your normal operating temperatures, normal flow rate, the flow rate when things go whacky, and the type of heat exchanger you could check for this.

Patricia Lougheed

Please see FAQ731-376 for tips on how to make the best use of the Eng-Tips Forums.

RE: High velocity in heat exchanger tubes

(OP)
More System Details and Description

The HX is has 4 cross flow tube bundles in a row separated into 2 parallel stages (a "hot end" stage and a "cold end" or process controlling stage.  Each stage has its two tube bundles connected in series (1st bundle flows across and the other flows back).  
Cold supply header water enters each stage into the bundle furthest downstream relative to the process flow and discharges out of the upstream bundles. The cold side fluid is relatively clean, chemically treated, closed loop water. The hot side process flow media is air-fluidized sand. The air serves predominantly to keep the sand bed of approx 7 tons fluidized so it will flow and only incidently provides 3-4% total cooling.  Cooling water supply temperature ranges from 50 to 90F and discharges with a wide range of delta T greatly dependent on flow rate.  The process sand flows at 45 tons +/-2 per hour at 650+/-25F and discharges at 120+/-5F.
Under normal steady operation the cooling water flowrate throttles in a range of 200 to 300 gpm per stage.  But when the unit begins to lose controllability, it gradually opens throttles up to near 500 gpm and at that point seems to lose all control driving throttles completely open and achieving up to approx 1000-1200gpm per stage with a negligible to 10F Delta T, while sand exit temperatures continually climb at a slow rate.  We then use the program to drive the throttles to close to about 400 gpm and the Process sand exit temp comes back down.  We have since programmed a throttle max clamp position but the system  still gets into a condition where it drives both stages throttles to the max positions and still won't cool.
I'm chasing my tail trying to figure out what's happening.  

RE: High velocity in heat exchanger tubes

tim,

I think perhaps we all assumed it was a liquid/liquid shell and tube exchanger(s). Fluidised sand? That's certainly two-phase!

Is there any possible alternate path for the water to take at the high flowrates? Can it be bypassing the heat transfer surfaces?

"We then use the program to drive the throttles to close to about 400 gpm and the Process sand exit temp comes back down."  

Bizzare.

Cheers,
John.

RE: High velocity in heat exchanger tubes

I have not designed a heat exchanger for some time but referring to a book "Compact Heat Exchangers" by Kays and London, Fig. 10-1 you find that at Reyonlds No. 1500 to 3000, transition range, the heat transfer rate drops below the laminar or turbulant range-i.e heat transfer does not always increase with increased velocity. Maybe your flow works in this range, as some of the others have suggested.

RE: High velocity in heat exchanger tubes

(OP)
Yes it is fluidized sand but...
... that doesn't make it a two-phase heat exchanger.  It has 2 different fluids (the sand flows essentially as a fluid) but neither changes phase. Two-phase HX implies a phase change (i.e. a turbine exhaust condenser).  
Dooron, thanks for the reference. I think that might put me on the trail of the phenomenon I was looking for.

Ciao for now,
Tim

RE: High velocity in heat exchanger tubes

tim02

i guess i have to eat my words - increasing velocity can result in a decreased hx co-efficient according to dooron.

You go from 500 to 1000 gpm, so the Reynolds no. is doubled. Dooron's transition zone is over a factor of 2 as well. If that is the problem, then is your solution to push the flow past 1000 gpm and ensure flow is turbulent?

Cheers,
John.

RE: High velocity in heat exchanger tubes

Using Sieder-Tate and other formulas and graphs I couldn't confirm the strange behaviour mentioned by Dooron on laminar or transitional flow regimes. The only possibility of getting lower HTC at higher  Reynolds number was to reduce the Pr number. But on using cooler water Pr values go up, so does the viscosity to the 0.14 power. The only value that drops at lower (high velocity) water temperatures is the thermal conductivity, but it's too little to compensate the others going up. So, my conclusions were that the higher the Re No. the higher the HTC for the case in question.

Of course, there may be new formulas that contradict older conventional approaches. And I may be wrong.

Anyway, I'd like to raise two more options:

1. Is there an enthalpy-and-mass balance showing that one set doesn't steal water from, or receives more process fluid than, the other set ?

2. Are the inlet nozzle water linear velocities lower than the tube average velocities ? Water maldistribution happens when axial entrance velocities are greater than tube velocities.

tim02, please keep me posted. Thanks.

RE: High velocity in heat exchanger tubes

(OP)
Whatever is causing this, 25362,
I believe it must be flow related on the cooling water side.
The fluidized sand has only one flow path available, entering down a chut at one end, through the heat exchanger shell horizontally across all 4 tube bundles in series, and out a chute a the other end.  The water side, however, enters from an constant pressure (20psi) 8" header then splits into each of 2 independently throttled 6" parallel stages, and through 286 tubes per tube bundle.  It seems that there may be some possibility that the is developing a very uneven flow distribution profile through the bundles above a certain velocity. My theory is that it is then effectively not using all the tubes, and therefore bypassing some heat transfer surface area, so the overall capacity of the unit effectively goes down as does the efficiency of the "working" portion of the HX.  (i.e. using the basic heat tranfer equation Q'=UA(Tout-Tin)ave, the UA decreases in the equation).  Even so, I don't know how to prove this is what's happening.  

RE: High velocity in heat exchanger tubes

I pressume you can not measure the water flow rates to each set and the temperatures around each set on both (process and water) sides to make the needed heat balances.

You must find a way of measuring these to corroborate your theory.
Good luck !

RE: High velocity in heat exchanger tubes

tim02, I found an old (late 1960's) article in CE by Scaccia and Theoclitus (McGraw-Hill, New York) that deals on "types, performance and applications" of heat exhangers, in general, and cross-flow types, in particular.

They say that a cross-flow exchanger losses effectiveness when the fluids are "mixed". In "unmixed" tubeside fluids (water in this case) there would be temperature profiles across the bundles resulting from "cold" and "warm" corners in the exchanger. When the tubeside fluid becomes "mixed" because of high turbulence (larger flows in this case) the thermal effectiveness drops and the temperature profile disappears.

Strange, isn't it ?

In short, it appears that this seems to be a characteristic of cross flow exchangers. It would seem appropriate to look for specialized literature on this type of compact units.

RE: High velocity in heat exchanger tubes

25362,

This is one interesting finding. I'm not sure whether I get it intepreted correctly though. Are you saying that for cross-flow HX, it would not be advantageous to operate the shell side at higher RE because thermal effectiveness decreases? What do you mean by the temperature profile disappears? Do you mean that the profile is linear in this case?

RE: High velocity in heat exchanger tubes

tim02,

you've got a 560 deg F delta T across the tube surface.


that is a bit excessive don't you think?

water at 20 psi boils at 258-259 Deg F.

At low cooling water flow, everything is more or less okay, but you have a conditionally stable process.

Open the CW disch. valve and you drop the pressure, poof!, you immediately develop local hot spots in the tube that prevent nucleate boiling,local superheated zones).

The air/sand exit temperature immediately rises (sees an effective loss of surface area), at which point your controller asks for max CW water flow. You eventually drive the temperature down....

you need to put a pressure transmitter on the cw dsch. (up stream of the valve) to allow temperature modulation of the CW valve with a low pressure constraint.

you can also achieve same with electronic limits on the controller output or mechanical stops on the valve.





RE: High velocity in heat exchanger tubes

to reactorshell, the article defines the temperature (or recuperative) effectiveness as the ratio of the temperature drop (cooling) on the process (hottest) fluid to the difference between the extremes: the hottest (fluid) and the coldest (water) inlet temperatures.

It also says that cross-flow units show their effectiveness to be in between those of counterflow and parallel flow heat exchangers.

A temperature profile or better a gradient of temperatures appears at the cold fluid outlet, when water is not "mixed" meaning that the water leaving the tubular shows a range of temperatures from coldest at the lowest point to warmest at the highest. According to the article, when this happens the thermal effectiveness is highest, when the streams mix (as by turbulence) this effectiveness drops and the oulet gradient disappears.

As the mixing takes place the effectiveness lowers and becomes nearer to that of a parallel flow exchanger.

RE: High velocity in heat exchanger tubes

The CE July 1990 issue contains an article under Engineering Practice named: "Quick design and evaluation: H/E" by Bowman and Turton, that brings worked out examples on cross-flow exchangers.

Using their own data and graphs, when doubling the flow of cooling water, we obtain, of course, a drop in water outlet temperature, with a rise (!) in the hot fluid outlet temperature, contrary to expectation. The same as in the case in question.

They base themselves, among others, on Taborek's "Heat Exchanger Design Handbook" (ed. by E. Schlunder - Hemisphere Pub. Corp. Washington, D.C. 1983) and on "Design of Heat Exchangers"  by  Turton and others in CE Aug.18, 1986, both of which I don't have access to.

The trick offered in the article is based in using graphs (as usually used for LMTD correcting factors) showing precalculated values of NTU (number of transfer units) = UA/mc, where m,c represent mass flow rate, and specific heat, of the cold fluid, respectively. These graphs are meant to circumvent the hard work of calculating T2 and t2 via NTU.

Readers are invited to comment on the subject.

  

RE: High velocity in heat exchanger tubes

tim2

Do you know the diameter of the tubes?  I have a spread sheet that does a quick and dirty calculation of Reynolds # for cross flow heat exchangers, and other heat transfer factors but  need the tube diameter.

Patricia Lougheed

Please see FAQ731-376 for tips on how to make the best use of the Eng-Tips Forums.

RE: High velocity in heat exchanger tubes

(OP)
Tube dimensions:
4.5 foot long tubes with diameter 29/32".
Thanks,
Tim02

RE: High velocity in heat exchanger tubes



she'll need the wall thickness too

RE: High velocity in heat exchanger tubes

If I got my figures right all water flow rates are in the laminar regime and
WT is 0.5(31-29)= 1". Right?

RE: High velocity in heat exchanger tubes


at nominal 1 gpm /tube you a nominal pipe reynolds of 5000, if i have consolidated the design details properly. it is definitely not in the regime of well developed turbulent flow.

tim02, what kind of cycles(temperatures) do you see in the cw exit of the hot stage?



RE: High velocity in heat exchanger tubes

(OP)
Hacksaw-
CW supply temps cover a wide range (40 to 90F) as they are driven by ambient. Exit temps are nominally at a 50 to 70F delta.  But I've seen exit excursions up as high as 175F and down to 90F during normal operations and almost regardless of inlet temp.  There is no control on the water exit side temp and every cooler seems to settle into a different delta range even though they share a common supply header.  When one starts losing controlability the delta drops to 10F or less.  
And 25362, the tube I.D. is 29/32" with wall thickness .0183".

RE: High velocity in heat exchanger tubes



Is the fluidizing air stable during this exercise? Some fluidized beds will be stable and others will pulsate or surge in a cyclical manner.

Does the cw supply pressure hold up during the upsets? We've had HEX's (coolers) lose control when the cw header pressure dropped as a result of other demands in the plant.
Having several coolers in parallel can lead to similar demands. We had to install restriction plates to limit excessive user demands. Mecanical stops on the valve also worked (max flow limit).

good luck,

RE: High velocity in heat exchanger tubes

After correcting my assumptions of the tubes diameter, all Reynolds numbers appear to be beyond the critical regime.

It still seems that one bundle is stealing water from the other when CV's open. Can you find a way to check that ?

RE: High velocity in heat exchanger tubes

By the way, I agree with 25362, that cw maldistribution is taking place, whether a hydraulic imbalance or an triggered by changes on the process side.  

It is a tough problem

RE: High velocity in heat exchanger tubes

Tim (and others)  Here's my two cents:

Based on the information you've given me, I've calculated Reynolds numbers which are in the 3,500 - 38,000 range, dependent on initial temperatures and flow rates.  The first number is for 50 degree water, 200 gpm flow, the latter for 90 degree water and 1200 gpm flow.  So I think you may be getting some transition range instability.  Have you noticed this happening more on days when the inlet water temperature is hotter?  At 90 degrees and 300 gpm, I calculated a Reynolds number of 9,500, which is right at the transition point.  (Over 10,000 is definitely turbulent flow).  If you don't have a way to control the inlet temperature, is it possible to limit the flow to under 250 gpm?  That keeps further away from the instability regime.

Basically:  Re= [rho]*di*v/[mu] where [rho] stands for density and [mu] stands for viscosity, both of which are functions of temperature.  Density varies from about 62.4 to 62.1 lbm/ft3 while the viscosity varies from about 8.8E-4 to 5.1E-4 lbm/ft.sec.  

For the velocity, because I used a spread sheet,  I first converted gpm to lbm/hr by multiplying by the density, dividing by the conversion factor for the weight of water (7.48 lb/per gal) and multiplying by 60 minutes/hour.  I then used v = mdot divided by the density, divided by 3600 (hours back to seconds), divided by the area of the tubes.  Of course, you could save a step by taking mdot in gpm and just dividing it by 60 and the area of the tubes.

The area of the tubes is pi*di/4 times the number of tubes (286) divided by the number of passes (2).

Patricia Lougheed

Please see FAQ731-376 for tips on how to make the best use of the Eng-Tips Forums.

RE: High velocity in heat exchanger tubes

(OP)
Patricia,
We are actually in the process of changing the supply system over to a newer cooling tower system that has bypass mixing capability so we can control inlet temperature where we want it.  We actually use only about 600gpm total at 90F inlet temp. Usually about 2/3 in the first stage (hot end) and 1/3 in the second stage (cooler end, process control).
Also, this effect has happened at different times of the year, with water at various inlet temperatures.  I think it starts with a surge of sand or at least hotter sand into the first stage that suddenly makes that stage struggle to drive the cooler sand midpoint control temperature back to its control setting.  The control valve, which is a butterfly-type, starts to open more. (I know, I wanted real throttle valves with more linear characteristics but got told these have always worked in the past so why change now).  It gets to the point (about 1/2 open) where further opening of the disk has very little effect on flow.  At this point the second stage inlet sand temp starts to go up and that stage's throttle goes through the same thing.  These sudden high flow excursions probably have as much to do with the poor throttling capability of the valve in the upper half of it's open position as much as anything else.  But even now, nobody will even consider changing them to a different style. In our plant, maintenance and production don't believe any plant engineers without flying in "experts" from the OEM or consulting firms from at least 100 miles away to corroborate.

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