Pressure drop and velocity for gas application
Pressure drop and velocity for gas application
(OP)
Hi,
Okay this seems like same question asked so many times but I'm still new in heat transfer field and wanted to know between these two parameters. So when sizing for HE, the pressure drop shall be kept below the allowable pressure drop as specified for example allowable is 50 kPa then when design I get somewhere around 41 kPa so I know that my pressure drop is under the allowable limit however my velocity gets too low and I know that the velocity of gas should be somewhere around 10 m/s.
What I would know is there any guideline when sizing for gas application mainly on the pressure drop and velocity? If no then what are the criteria I should consider when sizing for gas application?
Okay this seems like same question asked so many times but I'm still new in heat transfer field and wanted to know between these two parameters. So when sizing for HE, the pressure drop shall be kept below the allowable pressure drop as specified for example allowable is 50 kPa then when design I get somewhere around 41 kPa so I know that my pressure drop is under the allowable limit however my velocity gets too low and I know that the velocity of gas should be somewhere around 10 m/s.
What I would know is there any guideline when sizing for gas application mainly on the pressure drop and velocity? If no then what are the criteria I should consider when sizing for gas application?





RE: Pressure drop and velocity for gas application
David Simpson, PE
MuleShoe Engineering
In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist
RE: Pressure drop and velocity for gas application
Gases have notoriously low heat transfer coefficients so maximizing velocities within given pressure drop should be utilized to boost the performance. Dave Gulley has published some empirical equations that correlate between gas heat transfer coefficient, exchanger pressure drop, and tube length - the results are claimed to be within 25% of the real value. You can play a bit and see how increasing velocity and pressure drop affects the gas (and the overall) heat transfer coefficient, and what kind of performance difference is between 41 and 50 kPa pressure drop.
Ho = 430.Cp(ΔP/L x ρ)1/3
where
Cp = specific heat (Btu/lb-F)
L = tube length (ft)
ΔP = shell side pressure drop (Psi)
Dejan IVANOVIC
Process Engineer, MSChE
RE: Pressure drop and velocity for gas application
RE: Pressure drop and velocity for gas application
RE: Pressure drop and velocity for gas application
actually I am sizing for plate and shell heat exchanger (PSHE) and this kind usually gives low calculated pressure drop compared to shell an tube however the limiting factor here is the velocity.
PSHE have two different velocity where one is on the connection side before reaching the plate packs and the other one is between the plate packs so that is what I am trying to figure out for gas application case what usually the optimum velocity.
RE: Pressure drop and velocity for gas application
Your recent post says you have low pressure drop utilisation..
Anyway, the other key factor is to ensure that the design case duty Reynold's number is above the transition flow regime as fas as heat transfer is concerned. This transition occurs at approx Nre 4000 for shell and tube type, but could be lower for plate type HX with corrugations of some sort or other.
RE: Pressure drop and velocity for gas application
Commercially, if you are bidding from a spec that was distributed to other bidders, then the specified DP cannot be exceeded , and the technology used to beat the competition needs to be better ( in terms of surface area per ft of HX length) or the manufacturing techniques changed to provide a lower cost of construction. Extended finned surfaces or surface treatments can be used to improve the effective surface area per ft of HX length .
manufacturing techniques that reduce the fabrication cost could include use of robotic welders or outsourcing, or just a smart review of the fabrication sequence.
Economically, some process engineer had to decide on the value of pressure drop vs operating and capital cost. That economic analysis can always be re-calculated and reviewed based on changed economic circumstances, such as the changing cost of power, fuel, materials, capital ( interest rates), labor. For example, the rule of thumb of 10 fps as the economic pipe size for typical liquid piping, valves, pumps seems to be longstanding but can be recalculated for each job. Erosion/corrosion is often a reason for velocity limits in liquid pipe sizing.
Maximum gas velocities are often proportioned to the soundspeed or mach number , or are presented as a percentage of the inlet pressure. A spec may ask that the HX DP be less than 2% of the gas inlet pressure, or that the max gas speed not exceed 15% of soundpeed over the range of process conditions. Above some mach number ( 0.2?) the noise and vibration become excessive and damaging.
Overall, the bidder that consistently wins is using a technical approach that has been optimized, so studying those past winning bids will give some direction.
"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick