calculating Normal-flow per hour.

[knutabru]

Hello.

I'm about to start on a project about regulating a blower fan after a SetPoint where the customer enters a required Nm³/h, and the blower delivers it. The PV are coming from a Differential Pressure Transmitter measuring over a "constriction-orifice" in the air-supply before it enters the blower. I'm told that I need to take the square-root of the measuring signal, and I will have the air flow for the PV to the regulator. So far so good.

But there are also a measurement of the temperature to compensate for the temperature affecting the volume-flow and this must affect the PV coming from the DP-cell. As I understand it, the normal flow is what defines the volume the air would occupy at a pressure of 101.325 kPa, and a temperature of 0 °C (may be 20°C).

Toady there are an old HW-regulator taking care of the regulating-loop, and the documentation does not show how the temperature affects the PV. I enclose a PDF showing the principle of the regulating loop as I understand it to be.

I would be very happy if someone could show me the direction of how to calculate the temperature compensation in this loop.

Regards From Knut A.B.

 

[Pandiani]

Hello Knut,

Normal cubic meter is defined under standard temperature of 0°C (273.15K) and pressure of 1 atm (101325 Pa). From your post I can conclude that someone else has done orifice sizing and calculation so you know what flow corresponds to what differential pressure at some design temperature. Is this true?
Please note that usually it is not sufficient to take a square root of your measuring signal. You need to know exact replationship between differential pressure and flow at some conditions.
Now, I'll assume you know for sure that "DP" corresponds to "Q" at some design temperature (working point). Now, when temperature is changing, density of medium is also changing. This is responsible for the fact that now same value of DP corresponds to some other value of Q (flow).
Distinction is made between volume and mass flow. Since you're measuring volume flow, then DPc (compensated) is calculated to the following formula:
DPc = DP * Tc/Ta
where Tc is temperature at design (working) point of operation and Ta is actual (current) temperature (in absolute scale - K). Please, keep in mind that flow doesn't only depends on the temperature but also on the pressure as well.
Now actual flow would be
Qa = sqrt(Dpc), or Qa = Q * sqrt(Tc/Ta). This is valid for volume flow. For mass flow, replationship is inverse.

For example,

Differential pressure of 100 mbar corresponds to an air flow of 3581.8 m3/h at temperature of 20°C and pressure of 1 atm (orifice sizing calculation)
Now, if temperature is increased to 35 °C, same DP of 100 mbar corresponds to the following air flow:
Qa = 3581.8*sqrt(308.15/293.15)= 3672.3 m3/h.

Note, this is due to change in fluid density. Have a look at this:.
What is density at 20°C and 35°C. Try to find out ratio and look how it will fit with the above's formula.

In my experience (mostly steam applications, pressure is at least as much important as temperature for calculating exact mass or volume flow with compressible fluids).

That's all from me.
Good luck with your setup.

Pandiani

BTW, what is the purpose of "linearization" block at the output of PID controller?

 

[knutabru]

Hello Pandiani.
Thanks for your most valuable input.

I have not found any documentation that describes the relation between flow to a differential pressure at some design temperature, but I must search the old scrolls at the customer some more.

The HW-regulator does have an input designated for the pressure, but there are no pressure transmitter connected to it, so I assume the pressure are omitted in this application.

The linearization block at the output are meant for making the characteristic of the pump linear to the output from the regulator, but maybe this is best done in the frequency converter.


Knut

 

[Pandiani]

If it is a VSD pump, I don't see any real need for linearization block. If you're not 100% absolutely sure about this, probably you can make things worse. I've seen many regulation of level and flow with VSD pumps and there were no "linearization blocks". Most of the time PID controller (properly tuned) is sufficient.

Please, keep us updated with your application.

 

[Tom Jenkins]

The attached includes relative humidity correction. If this isn't important you can substitute standard values.