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Quotes - Engineering - SalesVent Condensers
Flash Steam and Solvent Recovery from Vent Streams
Every vent that plumes is throwing away energy, water or solvent. Vent condensers recover it — on deaerator vents, condensate flash tanks, storage tanks and blanketed vessels — and reduce the load on whatever treats the vent stream downstream.
A vent condenser handles a stream that is mostly noncondensable by design, with a condensable fraction worth recovering. That makes it a genuinely different design problem from a process condenser, and it is the reason vent condensers are so often undersized: the coefficient achievable on a gas-dominated stream is a fraction of what the same geometry would give on saturated vapor, and no amount of optimism changes that. These units are always engineered for the specific application rather than selected from a table.
Flash steam recovery. When hot condensate is let down to a lower pressure in a flash tank or condensate receiver, some of it flashes to steam and, without a condenser, leaves through the vent as a visible plume. That plume is latent heat and treated water at the same time. A vent condenser recovers both, heating make-up water, boiler feedwater, process water or air with the recovered latent heat and returning the condensate to the system. As a general guide the cooling fluid needs to be below roughly 160°F to condense flash steam effectively.
A condenser on a stream that is mostly noncondensable by design, with a condensable fraction worth recovering. A different design problem from a process condenser, and the reason vent condensers are so often undersized.
One practical caution: if there is not enough cooling flow for the flash load, the coolant itself can reach saturation and start to flash, which invites water hammer. A flash steam bypass or another means of limiting the energy the coolant absorbs is the usual safeguard.
Deaerator vents. A deaerator has to vent continuously to expel the oxygen and carbon dioxide it exists to remove, and that vent carries a good deal of steam with it. A vent condenser recovers the steam while letting the dissolved gases pass, and can be arranged to reheat the leaving noncondensables so no visible plume forms at the discharge — a small detail that matters more than it sounds when the stack is visible from a public road.
Solvent and VOC recovery. Vent streams from dryers, vacuum systems, nitrogen-blanketed vessels and tank breathing carry condensable solvent in a large volume of air or nitrogen. Cooling the stream condenses the fraction whose boiling point is above the achieved temperature, recovering saleable material, reducing emissions, and cutting the load on downstream carbon adsorbers or thermal oxidizers. Because those treatment systems are expensive to run, the condenser frequently pays for itself on the reduction in downstream duty alone.
Tank breathing and blanketing. Storage tanks breathe with ambient temperature and with filling operations, and a blanketed vessel pushes gas out every time it is charged. Both produce intermittent, low-flow vent streams where the design case is often the peak during a transfer rather than a steady average.
Exchanger selection. Shell and tube is the typical choice, with spiral, plate and finned coil units all appropriate depending on the stream and the heat sink. Where air or process gas is being heated rather than a liquid, a finned coil arrangement often makes the most sense.
Related pages: vacuum condensers, process condensers, and VOC recovery condensers.
Recovery questions
On a flash tank it recovers both the latent heat and the water, and the water is often worth more then the heat once you cost the treatment behind it. Take a receiver venting 1,000 lb/hr of flash steam: that is about 970,000 BTU/hr of latent heat, roughly 1 MMBTU/hr, which at $8 per MMBTU of gas and an 80 percent boiler efficiency is $10 an hour, or over $80,000 a year on continuous operation.
The same 1,000 lb/hr is 120 gallons an hour of softened, deaerated, chemically treated water that otherwise has to be bought, treated and heated again from cold. On a solvent vent the arithmetic is simpler: whatever condenses is product you no longer purchase, plus a lower throughput on the carbon bed or oxidizer behind it.
Because the rating is done on the condensable fraction alone and the air or nitrogen that carries it is treated as if it were not there. On a deaerator vent that is mostly steam the error is modest. On a tank breathing vent where the stream is 95 percent nitrogen and 5 percent solvent vapor, the solvent has to diffuse through the gas to reach the tube wall, and the effective coefficient can be a fifth to a tenth of the clean condensing value.
The second error is the design case: sizing on the average vent flow when the real duty arrives in a 20 minute burst during a transfer or a charge. A unit rated on the yearly average will plume and lose solvent every time the pump starts. We rate the peak event, the gas-side mass transfer and the condensable fraction separately, and the area that results is rarely a small number.
For flash steam, anything below roughly 160 F works; for solvent recovery the coolant temperature is set by the solvent's vapor pressure at the outlet, and lean streams need far colder coolant than people expect. A nitrogen vent carrying 10 percent acetone by volume at atmospheric pressure has an acetone partial pressure of about 1.5 psia. Cooling that stream to 35 F, where acetone still has a vapor pressure near 1.3 psia, recovers only around 15 percent of it.
Reaching 60 percent recovery means a stream leaving at about minus 5 F, which is a brine or refrigerant duty, not a chilled water one. Richer streams are kinder: at 50 percent acetone the same 35 F outlet recovers most of the load. Run the partial pressure arithmetic before choosing the heat sink, because it decides whether the project is a $20,000 exchanger or a refrigeration package.
It can, and the usual trigger is a drop in coolant flow rather than a rise in steam load. A flash load of 1,000 lb/hr is about 970,000 BTU/hr: heating make-up water from 60 F to 120 F absorbs that with 32 gpm. Make-up demand is not constant, though, and when the boiler is on low fire the flow may fall to 10 gpm, at which point the same steam load would drive that water past 212 F and it flashes inside the tubes.
The resulting slugs of steam and water hammer the tubesheet and the piping. The protection is either a bypass that spills excess flash steam when coolant flow drops, a temperature controller on the coolant outlet that throttles the steam side, or a recirculation loop that keeps a minimum flow through the condenser regardless of demand.
It removes the water vapor that makes the plume, but whether the discharge stays invisible depends on how far above its dewpoint the leaving gas is when it meets cold air. A deaerator vent condensed to 100 F still leaves saturated at 100 F, and on a 30 F morning that gas plumes the moment it mixes with the atmosphere. Reheating it 20 to 30 F above saturation, which costs very little duty because the remaining flow is only the oxygen, carbon dioxide and a trace of steam, keeps the mixture below saturation through the dilution.
On a solvent vent the leaving gas is nitrogen or air at the condenser outlet temperature and generally does not plume at all; what people sometimes see there is condensing moisture from the ambient air on a cold stack, which lagging the discharge pipe cures.
The split between condensable and noncondensable flow matters more than the total, and the peak during a transfer matters more than the average, so those two numbers deserve the most care. A deaerator vent is easy to state: steam rate and the gas it carries, which is usually a small fraction. A nitrogen-blanketed vessel is harder, because the vent flow during a charge may be 20 times the breathing rate between charges, and the condenser must handle the burst without losing solvent.
For solvent recovery the target recovery fraction decides the coolant, so state it as a percentage or an outlet concentration rather than as 'as much as possible'. Finally, what the recovered heat will warm, air, make-up water or feedwater, picks the exchanger family, since air on the cold side points to a finned coil rather than a tube bundle.
