Heat Recovery

I give you a GA for your approach and understanding.

We only return 5% of steam because the rest is consumed by the process. This 5% is sent back to the blowdown heat recovery device. We used to send it back to the deaerator but somehow the chemistry was never correct.

Ours is a steam chest molding operation so steam does the work and then water takes away the heat. The steam and water all flow away down the same drain to the tower sump which by now has been exposed to the air and the molded plastic. It is not suitable for reuse in the boiler due to the contamination. We try to recover some of the heat before it reaches the sump to preheat the boiler feed as well using stack economizers, but think we have a lot to learn here since the sump temp before it goes to the tower is 120F give or take.

Our boilers are equipped with TDS sensors and blowdown is automatic through a flash recovery unit, but we still need to cool the water B4 returning to the muni. Bummer!

Further, we are unable to run our boiler TDS beyond 2500 or we get carry over due to our erratic demand.

We haven't insulated the condensate return as yet. Never though of it as anything other than hot water. The best we did is paint it yellow for caution. There is insulation on the steam lines, but is it enough? You are probably right. Being a Yank I don't follow your GBP notation. Can you clarify?

The things you point out about the shop lights being on for hours are probably the best of the low hanging fruit that take a a few minutes to effect and pay off forever. Thanks for the response.

BeriBeri

Abbreviations in an international forum can cause communication problems.

I think GBP = Great Britain Pound also known as Pound Stirling.

What is a TDS?

BAB

Total dissolved solids.

We have a similar process in our pulp mill where steam is injected into our digesters to break down the fibers. This water is now unrecoverable to the boiler system. I'm also looking for ways to recover that waste heat. On a similar note though, the digesters run in batch mode and we have several surges of steam per batch which can double our steam usage. We are investigating a steam accumulator to flatten out the peaks on our boiler and possibly eliminate the need for a second boiler during the winter when our heating load takes the main boiler beyond its capacity. Reducing the high fire needed to catch up quickly to load changes will save fuel as well.

Wet accumulators are the way to go. Our machines are capable of huge draws over the course of a few seconds and we have multiple machines which often draw at the same time. If we didn't have accumulators we would suck the boilers dry. In fact, we have to be very mindful of our boiler chemistry and keep our tds below recommended levels or the sudden hits can still pull water. We also have a back pressure valve in line which will sense the pressure between the valve and boilers and shut off flow if boilers are losing pressure. Sometimes the draw is so sudden and large that the valve can't react in time. Since we have multiple boilers usually only one goes off on low water at a time.

Do you have a sizing formula for your system? if not, I can provide one.

Thank you, I do have a sizing formula but my challenge at the moment is gathering data. Our powerhouse has very little automation and I'm working off of daily circular charts for the last 3 years to get enough data to size it properly. Last winter in the Northeast U.S. was unusually warm so I can't only use one year. I have plans to at least gather the process variables electronically but even today many of the transmitters are 100% pneumatic. The time and cost to replace transmitters and install conduit and wiring isn't there at the moment so I'm still in the "grunt work" phase of the project.

The backpressure valve is a great idea and we are considering it as a stopgap measure until we can get the accumulator installed. At the moment we have a PLC and hardware timers to select different preset regulators that step the steam valves open over time to prevent sudden pressure drops. That system was installed in the late '80s and I'll be changing that portion to a loop controller soon to smooth out the surges. We also have some flexibility at this time to schedule our batches across 5 digesters so that only one is pulling at a time.

Is your digester steam demand weather sensitive? I am unfamiliar with what triggers demand in your situation. Our approach has been to calculate the instantaneous demand of each machine and then estimate how many could hit at the same moment and use that for the desired surface area. Since you can double the surface area of a wet accumulator for a lesser factor in price, it pays to err on the large side, as long as you have the floor space. They just sit there and work. Aside from a condensate trap at the half way level there are no moving parts.

The digester steam demand seems to be independent of the weather, at least it seems so in the short time I've been able to spend with the charts. I've only been at this plant 6 months so I haven't been here for a winter to see.

Our demand is greatest during the purging cycles. When the digesters are loaded they are open to atmosphere. We charge the digesters in 3 cycles with a vent between charges. This eliminates enough air that we can cook without air insulating the product. Once the last charge is complete and the cook is in progress usage is only what it takes to maintain temperature.

All of our building heat is steam and the plant was built in the early 1900's so it is an all-brick construction and glass blocks for windows. Our plenum area above the paper machines also needs to be heated to keep condensate from dripping on the paper. On average, I'm seeing a doubling of usage from Summer to Winter. The surges last about an hour and there are 10 - 15 a day. They are appoximately 25,000 lb/hr surges that step up over 10 minutes or so. In other words, a +25,000 lb/hr rise over baseline over a ten minute period, hold at that rate for almost an hour then drop off over 5 minutes. Nothing drastic, but I'd like to keep those surges off the boiler directly if I could. This should help fuel usage if we don't have to keep going from low-fire to high-fire to maintain pressure. The big payoff I see is that we need to start a second boiler in the winter on cold days. If we could keep the surges off the boiler then the one boiler could maintain the load alone.

As for area to place the accumulator, it's going to be an interesting tradeoff. The powerhouse has very little floor space due to an "abandon in place" mentality. Our pulp mill has the floor space, but it on the third floor and the concrete is not what I would consider structurally sound enough for an oversized tank.

My plan is to get worst case mean usage for each surge and worst case time between surges for recharging the accumulator. I'll need to go a bit higher on the usage rates since the accumulator should be able to let us charge the digesters faster than we currently do. Our current rate is determined by how much system pressure drop we can stand while charging. Placing the accumulator closer to the digesters should help with the system piping induced pressure drops that limit us now.

I'll be crunching numbers for a few weeks at least but I have several other projects in progress to help knock off some of the usage as well.

Thanks for the ideas.

Sounds challenging, but you have a good grasp of the situation. I don't know what your boiler pressure and system limitations are but if you can raise your pressure to the accumulator and prv down from there, you may be able to down size your accumulator and save some floor space and weight. Good luck!

GBP is to the Pound Sterling as USD is to the United States Dollar. Eire, Malta and Egypt have also used the Pound as a unit of currency, though the spelling can vary.

Loads of countries use dollar units and they are all different. It is important to be specific on these units in a global forum, for full understanding.

The best guy (dollar wise and smarts) is at johnsmagala@sbcglobal.net

If he gives you an idea and a price it will work period.

Mike

I know one maintance Super who ran some of the heated water into heating units for the facture then that water continued on for use flushing toliets. The compnay made this investment during a slow period and many workers from around the plant were used as pipe fitters under maintance supervision. No contactors were harmed in this project.

Hello,

Your invitation is being responded with our experience, in using Waste Heat

We have a Waste To Wealth operations in our Organic Composting Plant at Bangalore

We get Organic Slurry which is Dense semi solid despite high Moisture. We used to DRY the Slurry in Open,under the Sun; but limitations in Ground Space forced us to think.

We have a boiler which offers Waste Heat, Recovered to DRY Powder of Organic Waste Mineralised separately, for Bricquett Manufacturing. All this is automated.

We were knowing that a lot of Waste Heat from Boiler is still unused.

We made a Tap and allowed The Heat with Closed Loop COILED COIL, (Panel was very hot to be handled by bare hands), to return Gas to System.

This Closed Loop Coiled Coil Panel was Placed in a Old PVC Tank in which Slurry was pumped and We found that the Slurry was Drying pretty Fast and Thick Solid Condensate Removal was another problem faced by us; which we did manually, though we had an Out Let to tank, expecting Slurry will still remain liquid to drain from bottom of Tank.

Well this experimental success gives you any insight then, I suppose I could have been of some assistance to help solve your problem

Regards:Kishore

First idea, as Mr. Slack I believe mentioned, is energy conservation. Is insulation in good shape and at recommended thicknesses? You do not mention steam temperature or pressure, but if you check http://www.engineeringtoolbox.com/pipes-insulation-thickness-d_16.html , therre is a table of minimum insulation thickness. Also, OSHA has a minimum "touch- temperature" of 140º F for the outside surface of insulation.

Is every piece of equipment well maintained, in good operating condition, with heat exchange surfaces kept clean as recommended by equipment supplier? Keep equipment operating at high efficiencies, clean heat exchanger tubes, surfaces, boiler tubes, cooling tower fill and drift eliminators, etc. Keep water clean and any inline filters/strainers clean.

Air cooled compressors can have the waste heat ducted into the plant during the winter to reduce heating costs. Do an air-flow balance to make sure you do not cause any adverse effects with ventilation revisions.

Actually, do some sort of energy balance for the overall plant and for each piece of every major piece of equipment. Are operations continuous or intermittent? I worked near an engine-test area, and every few years, they would investigate energy recovery. Unfortunately the test cycles were too spread out to make heat recovery a viable economic option during that particular time frame.

Remember, if you cannot make the economics work out it probably is not worth doing. Keep it simple!

Remember, if you cannot make the economics work out it probably is not worth doing. Keep it simple.

This is the key. A lot of things that don't work out at $3.50/dekatherm gas will pay off at $12.

So, can anyone predict where the price will be over the coming years? Are we in a bubble like crude oil at $140/ barrel or are we faced with continually higher natural gas pricing? I don't pretend to understand the dynamics of the natural gas market as well as I do the market for crude oil which seems to be half demand, half speculation and half syndicate manipulation. Oops, that's over unity!

For example, there were a lot of ideas to recover energy or heat from an engine test stand, but the operation was so intermitent that their was not enough energy recovered to make the investment worthwhile.

First make sure everything is well tended and operating efficiently.

Then, do your energy or heat balance on the equipment, and then on the plant over all. There is no magic solution. Understand how, where, why, and how much energy is being used. Know what goes into the product, and what is "lost". Determine if the lost energy is at a location or in a form that can be recovered. Keep it simple. Too much complexity will lead to breakdowns in the system.

Change your processing methods so that the primary heat in the steam cycle is closed loop uncontaminated. The secondary heat will still have to be injected and therefor contaminated, drained condensed and recovered. But, adding closed loop recycling is the only way to really change the thermodynamics of your process.

At the end of the closed loop look for a recycle system that will compress the vapor and so recover the heat without passing through condensation. This compressor pump should feed the vapor side of the circuit after the expansion valve.

You will also need a separator and condensate pump that will feed the condensate back to the water side of the boiler.

You may be tempted to try and plumb the condensate back to the inlet pipe before your pressure pump for the make up water, but this would be quite tricky to regulate. Better to leave the steam line all high pressure and have a separate condensate pump to the high pressure side with appropriate switching so it does not try to pump steam.

This will be a fairly expensive custom system but, you may recover 50% of your condensate and a large amount of the heat that is in it. The condensate won't pay for the system very fast, but the heat may. Ball park the cost figures yourself, and then if the project has an estimated 4 year or less payback consult an expert for system design and justification.

Since you will be adding a heat exchanger to every piece of process equipment you have you will certainly want to see if you can buy this feature from an existing vendor or if it is custom modify only one peice to check for troubles before improving the whole line.

US systems are often monolithic, while european systems tend to be individualized. Monolithic systems have great advantages but stand by heat losses are very high. If you have continuous production then standby is not such an issue if production is daily cyclic then a system that pumps hot water to a gas or electric boiler sized to the given peice of equiptment in use would match the individualized low standby loss approach.

Number of ways, one at a time

We only return about 5% of our steam in the form of condensate.

first you mention your are only reclaiming 5%, there may be reason not to return your condensate, depending on the quality condition of your condensate, you are only running 1200 HP boilers which isn't very much, any contamination (and it wouldn't take much) to the condensate such as proteins in the return will cause valve slip and your boiler would trip.

I'm looking at other possible savings.

phoenix911

We only return 5% because we inject steam as a curing agent right into the polymer which is essentially open to contamination from cooling water and compressed air and polymer lube, etc. The 5% is condensate from header traps, etc.

the previous guest entry is mine. I got a new computer and CR4 didn't recognize me as anyone but "guest".

Hello beriberi,

You make mention of steam in your forming mechanism. Spray cooling is the most efficient. However heat recovery needs know more of your layout. Also these days there are far more efficient hot gasses than Steam. This also applies for air compressers should the air be used a force rather than a fill.

Cheers Peter