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# Steam temp set point for Rebiolers

## Steam temp set point for Rebiolers

(OP)
Hi,

i'm working on figuring out how much flow of steam we need in our reboilers

currently the steam temp to our reboilers is much higher the design temp of the reboilers which is affecting the efficiency of the reboilers

but working on the flow to the reboilers is the key solution. i need to change the set point of flow to the reboilers?

the calculations will involve LMTD and Q=mU(DeltaT)

but if someone know the procedure for the calculation

thank you

### RE: Steam temp set point for Rebiolers

You can't set the steam temperature and flow rate independently of each other. You will have to use the Q=UA(LMDT) relationship. If U and A are fixed, then to increase Q you have to increase LMDT by the same proportion. If you want a lower steam temperature in the reboiler then you need a larger reboiler with more area. Or maybe your reboiler is dirty and cleaning it will increaase U and decrease the LMDT required.

Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

### RE: Steam temp set point for Rebiolers

There is a phase change (at least in part) on both sides, right? I assume the steam is at or near being saturated, but I might be wrong.

Depending on inlet and outlet temperatures of both fluids, you might run into trouble with the LMTD being undefined. I would try to see if you can get the results you want by equating the heat of vaporization rate on the boiling side with the enthalpy surrendered on the steam side.

Would NTU effectiveness work here?

### RE: Steam temp set point for Rebiolers

There are really 3 heat transfer sections in a steam reboiler:

1. steam desuperheating
2. steam condensing
3. condensate subcooling

To calulate the steam flow rate you need, the heat supplied by the steam will be:

delta H steam = Fsteam x [Cp steam x (Tsteam-Tsat) + latent heat + Cp water x (Tsat-Tcond)]

where:

Tsteam = steam supply temperature
Tsat = saturation temperature of water on the steam side of the reboiler
Tcond = condensate temperature leaving reboiler

The highest heat transfer rate occurs for steam condensing. This regime also releases heat most efficiently, pound for pound. Therefore, a higher steam supply temperature is not beneficial because the steam needs more surface area to desuperheat, leaving less area for the condensing regime. This may be why you have an "efficiency" problem. You want to maximize the condensing regime to best utilize the available surface area.

The pressure on the steam side will depend on the regime heat transfer coefficients and surface area. As the pressure rises, Tsat increases which increases the heat transfer rate across the tubes in the condensing regime. The steam side pressure finds a value so that the heat balances.

So, the steam supply should ideally have just enough superheat to ensure there is no condensate formed in the supply lines. The flow of steam should be adjusted to give the desired heat duty based on the process side requirements. If your steam side pressure is very high, you may have condensate flooding the tubes and/or a lot of fouling.

### RE: Steam temp set point for Rebiolers

You really need to look at maximum heat flux, maximum bulk fluid temperature, superheat, pressure drop, and a bunch of other factors. The flow is determined by the duty of the unit including condensing (the majority), desuperheating, and subcooling. Ultimately the exchanger will reach a natural balance where the flow matches the duty and the LMTD tightens up until the exchanger is 100% matched. Get yourself a copy of the Spirax Sarco steam heating handbook - it's brilliant and explains all this quite nicely.

### RE: Steam temp set point for Rebiolers

Have you considered desuperheating the steam so it's at the design temperature?

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