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Condenser Flow Rate and Efficiency


Aaron Tompkins

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Can anyone tell me which is the most efficient between the following two:

High flow rate in the condenser or Just enough(slow) flow rate through the condenser?

For example the water going in is 60 degrees and exiting at 61 degrees vs. going in at 60 and exiting at 100 degrees.

This would also be a recirculating system with water reservoir.

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I generally look at having a client pump 50F chilled water to his condenser and throttle the amount of flow to the condenser to achieve best results. throttling valves can be "hand" type or automatic sensing output temperature.

Normally output water temperatures are around 90F or higher, depending on what the manufacturer advises for their condenser design.

Mike

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It depends on what you mean by "most efficient". The high flow rate and low temperature rise option will be the best option in terms of condensing the vapors, but at the cost of the energy to circulate all that cooling water. Generally the choice is to allow the water to rise to around 40 to 45 C (104 to 113 F) to limit the amount of pumping energy required. If you allow the temperature to go higher than this you accelerate fouling on the water side.

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When you say efficiency - what do you mean? Are you concerned with the energy efficiency of the overall system? As in, I'd like to be green? Or are you talking about being able to support a faster product take off rate? As in I don't want to find myself having to run at a slower take off rate because my product condenser can't keep up? These are different.

Short answer, the higher coolant flow rate the greater the "efficiency" (TO AN EXTENT), but there are dozens of other variables too, and some of these are tradeoffs.

The "Just slow enough" regime is ideal when you are not recirculating, as it utilizes the least coolant possible. This is especially good if you can find a use for the hot water created. However, in the real world, being able to run faster means turning up the coolant flow rate, which wastes more water, or generates an overabundance cooler water. Tradeoff.

However, if you are recirculating, then it's a different story. You aren't wasting coolant, so you don't need to worry about that tradeoff. You now have the luxury to run increased flow rates, and if you increase flow rate, you increase the heat transfer rate (TO AN EXTENT). This assumes of course, that you have the reservoir capacity for this to make sense.

Don't take this to mean that you can blast cold coolant through an undersized condenser to infinitely increase take off rate, won't happen. Yes, you'll gain some headroom, but you'll hit a wall (where you'll witness a head scratching operating condition where very cold distillate is produced alongside hot vapor chuffing out).

Keep in mind though, with most simple recirculating coolant setups, the flow rate in the coolant loop is determined by the process controller opening and closing a valve. So, the flow rate that's required to maintain a given set point is going to be the variable that's being adjusted. You don't have the ability to increase the flow rate beyond this, without adjusting or impacting other variables in the mix.

Also, if your chiller can't keep up with the reservoir (or you don't have one) the reservoir temperature increases during the run. You'll notice that through the run the flow rate through the condenser will need to increase to maintain a given set point (As Temp-In increases, Flow rate needs to Increase to maintain the same Temp-Out, this is probably obvious). So again, coolant loop flow rate is only partially in your control.

Now, there is a different way to plumb the coolant loop so that it's "Constant Flow Rate" - this is a little bit more complicated as it requires the pump to be in the cooling loop, and it also requires the use of a 3-way mixing valve, and not a simple 2-way. However, in this scenario you can increase or decrease the flow rate through the loop (TO AN EXTENT) without impacting the process set point.

This was probably way too much information.

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The difference between the two way and three way is mostly in control, the three way will let you change flow as a function of the set point (output temp of product) where the two way is more an automatic shutoff, allowing chilled water to be used elsewhere (like fermenter jackets). Now, if you have a variable speed process pump, that would be the most expensive option.

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