Boiler & Furnace

Replies

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  zaveri

  Hammer_Time

  Firstly I noticed that neither the pipes or the boiler is insulated, Now they could save a truck load of energy by insulating, I'm right in thinking fibre glass is the way to go with steam pipes and boilers? and how do I work out the thickness needed?

  One odd thing about insulating cylinders is that the greater the thickness of insulation, the the more is the heat transfer.
  
  There is something called the "critical radius " of insulation . this is the thickness of insulation at which the maximum heat transfer will take place. any increment of thickness beyond this value will decrease the heat transfer.
  
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  engg.ashish

  and can you upload these pics to this community so that we dont have to register ourself to a new forum
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  Kaustubh Katdare

  #-Link-Snipped-# : You may use 'Upload File' facility to upload your photos/files to your post 😀
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  Hammer_Time

  Think I've sorted the pics out now, sorry for that. let me know if you need better explanations for any of this.
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  Ramani Aswath

  The loss from an exposed surface for the given conditions will be about 15 KCal per sq.mtr of surface per degC temp diff between the surface and ambient. You can calculate the loss.
  Vidoflex is the optimal insulation for this situation based on our own extensive experience.
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  At 1barG steam pressure we found a six mm thick foam to be adequate. Higher pressures may need 9mm or 12mm. More than this may not be called for. The manufacturer can advise.
  If the plumbing is not extensive there may be no 'truck load' energy saving by insulating.
  I did not get the furnace bit. Are the vats heated by flue gas from a furnace? Why cant they be heated by the steam from the boiler itself. Sealing the vats is not easy. May be a cylindrical tank is a better option. What is the desired temperature? Are the vats inseries? Parallelling will make all see the same heat.
  It is difficult to suggest without more details.
  Sealing does not prevent heat loss. All that happens is that the pressure goes up. For a given heat input the amount of water evaporated is the same at all pressures up to a few bar pressure. You will still lose the same amount of heat through the steam release valve. You can consider a floating blanket of polypropylene balls to reduce losses. However, this requires that there has to be some kind of temperature control mechanism is in place. Without that the losses cannot be controlled. Thi is why the initial suggestion for some kind of water heating was made.
  I do not know the area of the vat. Assuming a pressure of 1barG, you have to design for a cover separation force of about 30 to 40 tons/sqmtr cover area as per current regulations. A non starter in my opinion.
  The optimum solution is indirect water or steam heating.
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  Hammer_Time

  #-Link-Snipped-# I'll try and explain the furnace side of things a little better. They have 3 vats in series, each hold around 240litres of water. I believe 8% of this space is taken up by the raw material, the rest is water. The raw material (which is banana leaves, some where between grass and wood) has to be cooked to soften it ready for producing paper. The furnace door is at one side and so the first vat receives the majority of the heat and the last vat very little, they use mainly hard wood which they buy in for the furnace and boiler (cheaper for them then L.P.G). The cooking times for the vats are the 1st vat 4 hours, 2nd vat 6 hours and the last vat takes 24 hours!!! so you can see there is a massive difference. I feel one of the problems here is that there is not substantial area under the first vat to allow the heat to get under and round to the other vats. Maybe they could dig lower? but if a re-design of the furnace was happening then I thinking adding another fire between the 2nd and 3rd vat would help a lot.
  The reason I suggested a 'pressure cooker' design is I believe it will dramatically reduce the cooking time. But I think I understand what you mean about the heat loss being the same.
  You mentioned possibly running the steam pipes from the boiler to the vats but one of the problems they are having here is how quickly the steam is losing heat and condensing (hence my suggesting insulation to them) I'm going to check the flow rates tomorrow as I fell some thing is very wrong here. They seem to think the temp of the steam entering the rad's is 70oc so they have lost 30oc before the steam has even done any work! I will also try and ascertain a correct temperature reading as none of the gauges are working correctly. Pressure is around 20psi. And as for speaking to the manufacture. I will do but don't expect much help as it's pretty much a guy in his shed scenario. I have added some more pics, hope they help and thank for you reply.
  
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  Ramani Aswath

  Hammer_Time

  #-Link-Snipped-# I'll try and explain the furnace side of things a little better. They have 3 vats in series, each hold around 240litres of water.

  Hard wood has a calorific value of about 3000 Kcal/kg. For all three vats together, the total heat needed to raise the temperature from ambient to near 100 Celsius in one hour is only about 50,000 KCals, which translates to just about 17 kg wood. Quite modest. Even at 1/3 efficiency of transfer only 50 kg wood will be needed. The problem is to get a good heat transfer area. Instead of digging deeper, it might be better to make space around the vats by moving the walls out ward. The chimney mut also be located beyond the last vat at the highest point. It is also possible that a lot of the sensible heat is lost in the chimney. Some kind of damper to regulate combustion air inlet may be useful.
  It should be remembered that it is the temperature and time that cooks. Once the vats reach the cooking temperature, excess fuel is just wasted heat. Only enough wood to keep the pot (vat!) boiling need be used.
  
  Even in the series arrangement indicated, all vats should reach cooking temperature at the same time.
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  akpower

  Mr.Hammer_Time,
  Since modification of furnace will be a costly,time consuming affair as well as redesigning will delay the process, changing the vats position from series to parallel though technically right is ruled out. I think only option will be change the material of vats from lower heat transfer to high heat transfer material. This will properly utilise heat generated and time taken for cooking may approach uniform.Since we discussed all issues related to heat generation, enthalpy,transfer,furnace design,the main issue centers around vats & its construction material will be playing dominant part. Heat transfer co-efficient will decide selection of vats material of construction. The manufacturer may guide on this aspect.
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  Ramani Aswath

  akpower

  Mr.Hammer_Time,
  I think only option will be change the material of vats from lower heat transfer to high heat transfer material. This will properly utilise heat generated and time taken for cooking may approach uniform.Since we discussed all issues related to heat generation, enthalpy,transfer,furnace design,the main issue centers around vats & its construction material will be playing dominant part. Heat transfer co-efficient will decide selection of vats material of construction. The manufacturer may guide on this aspect.

  I am afraid not. In the situation described, the overall heat transfer coefficient is decided by the gas side film coefficient, which is very low. The conductivity of the vat material will be orders onf magnitude higher and will play negligible role in th overall coefficient. Changing the vat material may be futile.
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  Hammer_Time

  #-Link-Snipped-# The furnace is in pretty bad shape (cracked bricks and such) and although the budget is very limited I think they will have to bite the bullet and spend some rebuilding. I will suggest as #-Link-Snipped-# suggested that they build the walls out slightly to allow better flow in the furnace and that they insulate better. I will also be looking at and suggesting better materials for the vats that have a 'high heat transfer'. Have you any ideas on this? as speaking to the manufacture hasn't really got me any where, no disrespect to him but it pretty much is just a guy in his shed (quite impressive really)
  
  I was also looking at (on the boiler side of things) trying to heat the fed water tank a little so the boiler doesn't have to work so hard and there is less thermal shock. The tank is outside and I thought of adding a glass roof to it and using the sun to heat the water a little? would this work? I was thinking of how hot it gets in a car with the windows up. Maybe though the sun would have to be focused like a magnifying glass? just a thought.
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  Ramani Aswath

  There is nothing gained by changing the vat maerial for the reasons I have explained. Insulating may not help much either. I still feel that the heat is being lost elsewhere (chimney?).
  Soalr heat of the kind indicated by you will not help either. Can you try partially closing the air entry point to the furnace?
  If you can link all three vats welding pipes both near the top and bottom of the water level you will find that all tanks will be at the same temperaature. This is not possible if the vats are removable.
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  Hammer_Time

  Yes unfortunately the vats are removable so welding them together would not be an option. I was looking a possible using a damper on the chimney to stop too much heat escaping but I'm not totally sure how this works considering the smoke still needs to exit the chimney. I could look at reducing the air flow into the furnace yes, do you think that there is too much air entering?
  
  The solar heating I mentioned is on a separate operation. It would be used on the fed tank to a boiler (they use the steam produced to dry paper). I was thinking just a cheap way perhaps of warming the water in the tank slightly before it enters the boiler.
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  Ramani Aswath

  You can certainly reduce air entering. If too much air gets in, the chimney heat loss goes up. There is still something odd. How does the flue gas exit the furnace. By any chance is the chimney also at the first vat end? If so, all can be explained since the flue gas get short circuited and exits without seeing the other two vats. In this case all that is needed will be to shift the chimney to the third vat location. Can you please check this out?
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  Hammer_Time

  No the chimney is at the other side of the furnace to the fire, the side where the slow cooking last vat is.