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mfqd (Mechanical) (OP)
20 Jul 09 13:32
Hi,

Can anyone explain to me, for example, in a plate heat exchanger system, where  we want to control a exit temperature of a fluid, what are the reasons to choose between a 2 way or 3 way valve? (the fluid is water)

Thanks!
 
TD2K (Chemical)
20 Jul 09 22:12
Turndown on a 3 way valve would be better I think.

Assuming you mean by a 2 way valve you are considering automating the bypass around the exchanger, as you open the valve and reduce the flow through the exchanger the available dP for the bypass valve also decreases so you can't totally bypass the exchanger (you may be able to bypass enough depending on the requirements of the process). The other problem with a 3 way valve is that you have a pressure drop with both 'paths' so total dP loss through the system is going to be greater.

A two way valve would be less expensive.
mfqd (Mechanical) (OP)
21 Jul 09 6:05
Hi,

TD2K, thanks for the reply.

Still i would like to know more arguments to sustain a good opinion. I've attached 2 images that shows the application that i am reffering to.

Wich would i choose? Wich one is better for wich kind of applications?

Thanks!
gerhardl (Mechanical)
25 Jul 09 8:37
Hello Mfqd,

I presume your two skeetches show on the lefthand side the steam/condensate circuit in and out of the plate heat exchanger, and at the righthand side the separate hot water circuit forced by pump circulation through the room/water heating system?

We must also presume that both systems has an adequate steam pressure regulating valve upstream from the application, able to maintain the steam input pressure.

Further that the condensate outlet in both cases have an adequate sized steamtrap, letting out condensate created , and keeping steam inside the system.

The purpose of a steam heatexchanger is as known to utilize the steam energy in a best possible way. As most of the energy is given by transferring back from gas phase (steam) to liquid phase (condensate) the art is to balance the steamside of the system in such a way that the heat exchange surface is exposed to steam.

In addition energy used should be kept at a minimum for the total system.

In 5.2.19 with a two port valve you have two ways of regulating your input: one is the steamtrap, wich we assume is dimensioned to let out the condensate at maximum rate necessary for the heat exhanger at maximum startup conditions.

The other control is the two-way valve which will open to full capacity steam inlet when necessary, and turn down when not needed, and can be turned further down by regulating down the temperature sensor at periodes if wished.

In 5.5.3 the steamtrap will in the same way let out what condensate is created, but you do not have an additional valve to override the steam input.

For 5.5.3 energy will be taken out to heat and maintain the whole water side of the circuit to a temperature somewhat below the condensate temperature at the given steam pressure.

Pro and cons for both systems. Your 5.5.3 will be faster acting at your user/room level by sudden changes.

Pros and cons depending for both of your total steam systems on the total userpoints and type of use, balance and after utilization of rest-heat in condensate, and the possible need for extra steam capacity elsewhere in the system, correct sized and selected steamtraps, correct sizing, layout and isolation of all steam and heat transport components more than the valve differences themselves.

 

roydm (Industrial)
26 Jul 09 1:44
It may be that the flow rate is fixed by another loop e.g level but you also need to control temperature, you do this by diverting the flow. One advantage of the 3 way is it doesn't change the flow rate, just diverts it.
Roy
mfqd (Mechanical) (OP)
31 Jul 09 10:22
Thank you all,

I think i understood. It depends in the application and one of the most important thing to consider is that there is a flow rate change whe we are applying 2-way valve and not with a 3-way valve. I think this is very important for the pressure drops in a water circuit.

Thanks!
 

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