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Energy Conservation

Heat recovery from turbine lubricating oil
 
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Steam turbine lubricating oil is circulated through bearings & collected in main oil tank at 60~65 C. From tank it is sucked by main oil pump & cooled to ~45 C by circulating through oil cooler and supplied to bearings. The heat transferred to cooling water in oil cooler is rejected in cooling tower. It is proposed to install heat pipe heat exchanger with property of fluid evaporation at ~55 C. This would cool oil to 55 C and can be used to preheat condensate collected in condenser from 45 C to ~ 53 C. This would not only recover energy directly but also help reduce cooling load on cooling tower.
vedarshi, Mar 04 2004

kcal to kWh http://www.chemie.f...&have=kcal&want=kWh
[Ling, Oct 04 2004, last modified Oct 05 2004]

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       You will never regain the energy (or the money) you expend in constructing and operating the equipment for this energy recovery to be worthwhile. You will find that energy recovery systems such as this one will have been evaluated, particularly for large-scale plant, and rejected on grounds of cost.
suctionpad, Mar 04 2004
  

       I'd skip the evaporator step - a direct plate & frame heat exchanger with condensate water should do the trick - add it right before the cooling tower system. Although [sp] may be right, it doesn't take much extra efficiency to pay back equipment on a continuous power plant. Those extra BTU's add up fast.
Worldgineer, Mar 04 2004
  

       aren't regenerative heating cycles very common ?, ie multi-stage turbines ?, exhaust heat from first stage preheats fuel for second stage.   

       For your specific app maybe you could use your extra heat for building heat ?
SystemAdmin, Mar 04 2004
  

       As [SysAdmin] states, these sorts of systems are widely known to exist in large commercial buildings. Typically, the condenser water from chillers etc will pre heat DHWS water via plate heat exchanger before hitting the cooling towers.
I assume that this system is applied from water to oil in a similar stylee.
gnomethang, Mar 04 2004
  

       Regenerative system is only for steam to water heat transfer. I don't know of any system capturing heat from turbine lubricating oil to preheat feed water. Lub oil heat is invariably rejected via oil cooler into atmosphere through cooling tower. Our country has cold season only for ~30 days( with minimum temperature hardly reaching 15 C). There is no economical possibility of putting room heating system. I would sincerely request SUCTIONPAD to visualize the proposed system again & give advise. Thanks.
vedarshi, Mar 04 2004
  

       [vedarshi] the problem I see with this system is that to achieve your 8degC temperature rise in the water you will need an oil-to-water flowrate ratio of 3:2. Now I am sure that the water flowrate will be many times higher than the oil flowrate (I stand to be corrected on this since I am unsure of the particular application you have in mind), hence the energy recovery would have minimal benefit. Given the capital and maintenance costs for the equipment that would be required I still think this would not pay.   

       The quality of the heat available is also rather low at ~55degC, meaning other energy recovery systems are also unlikely to be viable.
suctionpad, Mar 05 2004
  

       GNOMETHANG - My idea is to further upgrade heat content of steam turbine condensate by picking up major portion of heat from turbine lub oil itself before semding to regenerative feed heaters. SUCTIONPAD - (IN OUR CASE) Condensate is pumped out from condenser hot well at ~ 45~48 C, 4 kg/cm2 & returned to demin plant. So no extra pump & pumping cost would be there. Since condensate is highly pure, it won't clog heat exchanger (i.e. no maintenance). Condensate is nothing but demin water hence its specific heat would be almost 1 kcal/kg-C i.e. low flow rate for high heat capturing. I had mentioned HEAT PIPE heat exchanger which would have only some oil seals & it would be very compact due to extremely high heat transfer rates. YOUR COMMENTS PLEASE ? ANY FURTHER INPUTS REQUIRED ?
vedarshi, Mar 05 2004
  

       Isn’t there already far more low temperature waste heat around the power plant than can be put back into the feedwater? Please explain.
Fussass, Mar 07 2004
  

       [FUSSASS] Steam turbine lub oil serves two functions - 1) lubrication, 2) dissipation of excess heat from journal bearings (heat is generated due to shear forces in oil as well as conducted along turbine rotor). In this process, its temperature increases from 45 C to ~ 65 C across the bearings. The oil at 65 C is cooled to 45 C in oil cooler & recirculated. In oil cooler, water circulated at ~ 35 C from cooling tower picks up heat from oil & gains temp. rise of approx. 4 C. The heat picked up by water is dissipated to atmosphere through cooling tower. In proposed HEAT PIPE heat exchanger system, approx. 50% of heat in oil would be picked up by turbine condensate (& not cooling water) thereby raising condensate temp. from ~48 C to say 55 C (before going to regenerative heater). After HEAT PIPE heat exchanger, Lub oil would be further cooled to 45 C in oil cooler in existing system. The net effects of interposing HEAT PIPE heat exchanger would be : 1) condensate preheating by recovering some heat from lub oil, 2) reduction in cooling load on cooling tower which translates to blower power saving/ circulating water pump power saving, lower water evaporation rate leading to saving of treatment chemicals & lower blowdown rate from cooling tower. Why HEAT PIPE heat exchanger, please see my post dated Mar 5 addressed to GNOMETHANG. IF I AM MISSING SOMETHING BASIC, PLEASE CORRECT ME. THANKS !
vedarshi, Mar 07 2004
  

       [verdarshi], What oil flow rate do you have, and how much power do you intend to recover?
Ling, Mar 07 2004
  

       [LING] Oil flow rate = 30 m3/hr, specific gravity = 0.9 so mass flow rate = 30*0.9*1000=27000 kg/hr; specific heat of oil = 0.4 kcal/kg-c; oil temperature drop in proposed heat exchanger = 10 C hence heat lost by oil (energy recovery in propsed heat exchanger before going to existing oil cooler) = 27000*0.4*10 = 108000 kcal/hr. If it is assumed that 75% of heat lost by oil is effectively used through preheated condensate, energy recovered = 108000*0.75 = 81000 kcal/hr If you know fuel calorific value & cost, you can easily work out monetary savings & payback for proposed heat exchanger. ALL READERS ARE REQUESTED TO PLEASE VOTE. I WOULD REPEAT, PLEASE CORRECT ME IF I AM MISSING SOMETHING BASIC. THANKS.
vedarshi, Mar 08 2004
  

       You mention cooling tower. Is this a nuclear power plant under discussion?
bristolz, Mar 08 2004
  

       [bristolz] No !! This is a captive thermal power plant with 7.1 MW extraction cum condensing steam turbine. The cooling tower is counter flow induced draft type of 1500 RT capacity.
vedarshi, Mar 08 2004
  

       [verdashi], sorry, the first thing I have to do is convert kcal/hr to kWh/hr, otherwise I have no feeling for the numbers.
1 kcal = 0.001163 kWh
You mention that 81,000 kcal/hr might be recovered which is equivalent to 94 kWh per hour.
Working in Dollars and Cents, and assuming 5 cents per kWh (equivalent electrical heating!), this would save 4.7 Dollars per hour, or 40,890 Dollars per year, taking into account downtime.
How much your system would cost: I don't know. But you get a bun if the payback is 6 months or less :>)
Ling, Mar 08 2004
  

       [ling]thanx for very positive & optimistic picture. Actually, this would be low grade heat recovery & as such cannot be directly converted to power. But, by preheating condensate, we would be saving some of the regenerative heating steam which in turn would lead to reduction in turbine live steam input (quantity not proportionate but depending on enthalpy ratios). Reduction in live steam would result in boiler fuel saving. So there would definitely be monetary saving coupled with savings for cooling water system as explained earlier. Honestly, I have yet to do monetary saving calculations on above lines. I would surely share results with you all. Moreover, I have considered very conservative temperature values for oil. In fact, TTD would be much less for oil & water & savings would be much more than mentioned earlier. cheers !!!
vedarshi, Mar 08 2004
  

       Moving away from all these details, and the dodgy use of units (kWh/h is the same as kW for example - why can't everyone just use the SI system?....)   

       In my opinion this is just a very specific application of a widely baked concept of waste heat recovery. I am not sure this post adds to the sum of human knowledge - any good process engineer would include your system by default if it were economical.   

       Oh, and please don't type anything in capital letters - it is taken as the textual equivalent of shouting, which tends to get peoples' backs up a bit. As does asking people to submit a vote - it is up to the reader whether they want to comment or express an opinion.
suctionpad, Mar 08 2004
  

       You want your electric bills in Joules?
BTW kWh/hr was used for convenience, since I later wished to use a price per kWh. I was trying to be careful since some people cannot understand kW vs kWh.
Ling, Mar 08 2004
  

       [suctionpad] thanx for the tips. since i belong to asia, i really did not have any idea about when to use capital letters. regarding asking for vote/ opinion, i admit these should not have been requested for. i would do thorough techno-economic analysis & share results soon.
vedarshi, Mar 08 2004
  

       All right, you did ask. I think that you are missing something basic. No doubt the calculation is correct, but that is not the same as having 81000 kcal/hr of fuel. Oil makes a lot of heat at only 65C deg. It could be used to heat feedwater but there is more than enough exhaust steam and flue gas to do that, making the oil heat redundant. It is not hot enough to substitute for any regenerative steam bleed from turbines.   

       That is a layman’s view, now you will have to ask someone who knows.
Fussass, Mar 08 2004
  

       [Ling] yes I do. My gas bill already comes in Joules, would make it easier to compare prices gas vs. electric. My wider point on the units was the use of such things as kg/cm2, kcal/kg.C, BTU etc. The SI system is the only coherent one for cross-calculations, and none of these units fit (cf N/m2, J/kg.K, J etc.). Excuse the off-topic rant.
suctionpad, Mar 08 2004
  

       Conservation?!?!?!?!? You damn hippies just never learn...
whatastrangeperson, Mar 08 2004
  

       [fussass] boiler blowdown heat recovery is a proven system somewhat similar to proposed system by which there is a reduction in regenerative steam consumption. the only major differences are that with proposed heat exchanger there will be 1) heat recovery at lower temperature but still of fair magnitude, 2) reduction in cooling load of cooling tower & associated benefits. the real innovation (as perceived by myself) is to use HEAT PIPE heat exchanger which would give following benefits 1) fairly constant oil temperature after extracting heat 2) totally isolated oil & water circuits to avoid mixing in the event of leakage 3) compact size. oil waste heat will never be redundant because 1) exhaust steam at least in our case is under vacuum & low temperature so it doesn't have potential for heat recovery 2) only a fraction of flue gas heat can be recovered at higher temperature say 130~140 C unless you put condensing heat exchanger with flue gas scrubber.
vedarshi, Mar 08 2004
  

       Running your jets a few degrees cooler will make them last longer and require less frequent teardown periods, so by all means sacrifice and improve your system's efficiency before waiting on the market to call with a solution. Cheers.
dpsyplc, Mar 08 2004
  

       [verdashi], sorry, the first thing I have to do is convert kcal/hr to kWh/hr, otherwise I have no feeling for the numbers.
1 kcal = 0.001163 kWh
You mention that 81,000 kcal/hr might be recovered which is equivalent to 94 kWh per hour.
Working in Dollars and Cents, and assuming 5 cents per kWh (equivalent electrical heating!), this would save 4.7 Dollars per hour, or 40,890 Dollars per year, taking into account downtime.
How much your system would cost: I don't know. But you get a bun if the payback is 6 months or less :>)
Ling, Mar 08 2004
  

       You want your electric bills in Joules?
BTW kWh/hr was used for convenience, since I later wished to use a price per kWh. I was trying to be careful since some people cannot understand kW vs kWh.
Ling, Mar 08 2004
  
      
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