6 replies to this topic

[Jome128]

Dear professional:

Please bear with me if you don't understand my question or this trivial.

Variable frequency drive (VFD) of Blowers(2 is on/1 is spare) to be controlled

the airflow distribute to the 2 aeration basins and each basin has a dissolved

oxygen (DO) monitor on top.

There are two control options.

1. DO control

By varying the speed of a motor and its driven load, the airflow capacity

are adjusted in direct proportion to the DO level. As the flow is proportional

the blower speed, the DO level in the basin is reach the set point, the

blower speed will be adjusted to lower speed. But, the question is

What is the set point of DO level for the air demand in aeration basin?(fig 1)
 

 fig 1.png   293.71KB   36 downloads

2. Cascade flow control

The control for positioning the blower inlet valve, and the final challenge is the

coordination of multiple blower operation. The method for controlling the blowers is

Parallel and cascade control.

The problem is the in parallel control, whenever the next blower is required to come

on line

cause operating blowers are out of capacity, the following occurs :

- running blowers are adjusted to minimum airflow by closing the inlet valve.

- The next blower is started and as air demeaned increases, the inlet valves to running blowers are modulated simultaneously to increase air flow, the process is reversed as blowers are dropped off.

So, one blower may have more airflow than the other one blower.  The unstable blowers running condition may occurs.(fig 2)

 

 fig 2.png   542.42KB   35 downloads

I wonder these two control option description that I wrote is correct or not.

Please kindly correct and comment on these.

Really appreciate for your kindly help.

Thank you very much.
 

[Erwin APRIANDI]

You miss some importance info,

 

1. What is the purpose for the control?

2. What is the limitation? (Power Limitation, etc...)

3. Is this two sketch are for same system or different system? why the first figure has 2 aeration basin while the second figure has 3?

[Jome128]

Dear Sir

 

Thank you very much for your reply.

 

1. The purpose for this control is to adjust the airflow from blower into the aeration basin through monitoring the dissolved oxygen in the basin.

My question is that is possible to control the blower motor directly instead of modulating air inlet valve or guide vane located before the blower.

 

2. The power limitation is not decided yet, The VFD or VSD motor will be used.

 

3. The fig 1 and fig 2 system are the same. The difference is that there is two aeration basin in  fig 1, there basin in fig 2.

And the control scheme is different between these two option. there is no control valve in the suction or discharge line in the fig 1 for VFD motor.

But, there is inlet guide vane and flow control valve in fig 2. So, I wonder is possible to used fig 1 design instead of  installing any control valve, like fig.2

in this kind of design.

 

Thank you very much for your time and consideration.

[Erwin APRIANDI]

Hi Jome128,

 

First you have to know that if the type of blower that you are using is rotary blower as you have said before, based on my experience

You should never attempt to control the capacity of compressor by means of throttle valves in the intake or discharge piping. As this will increases the power load on the motor and may seriously damage the compressor.

 

To change capacity (outlet flowrate), it is necessary either to change speed (energy saving) or vent some of the air into atmosphere (not energy saving).

 

So first configuration should be used, to control the compressor using VFD or VSD, and for the process parameter. And I think it is better have a flowmeter rather than an DO meter as DO meter will be quite expensive and the logic of online reading of DO might be quite complicated and as I remember during practicum back in college, to have an accurate DO value there will be some correction on temperature and pressure.

You can set the required flow to the aeration basin by predicting the amount of oxygen dissolved in the liquid, you can do a trial and error for this, and just to check the DO now and then, no need for online meter for the DO.

Edited by erwin.apriandi, 02 April 2013 - 09:06 PM.

[Jome128]

Hello Erwin.apriandi

Thank you very much for your kindly help, and its nice to hear you generous share and precious comment.

 

As you advised the DO meter is quite cost, the capital costs for instrumentation, controllers, and programming necessary to acquire efficiency gains exceed the value of the benefit for a reasonable return period of many years.

In the figure 1, DO is measured in each basin and the DO controller adjusts the blower VFD motor set point at its corresponding basin DO controller.(This control is still unclear to me, so please correct me if I am wrong) The VFD motor controller modulate the air flow into the basin to matin the set point airflow.

 

In the figure 2, there is no DO loop, and the set point for the airflow controller at each basin are adjusted with flow-paced controller proportional to plant influent flow. DO meter monitored periodically with a portable meter to correlate DO levels with influent flow.

 

To reduce the risk of process upset from under-aeration, operators over-aerated flow-paced systems to prevent DO levels from falling below minimum setpoint when DO is not monitored. The amount of over-aeration required is subject to some controversy.

 

Please advise me if my false logic or miscomprehension about these control

 

Thank you.

[Art Montemayor]

  

Jome128:

 

Erwin hasn’t pointed to several contradictory and questionable issues with your flow diagrams, so I will:

• Contrary to what you state, the two figures (1 and 2) are not equal.  Figure 1 shows the symbols for positive displacement (PD), lobe air blowers (“Roots” type) while figure 2 shows the symbols for centrifugal blowers. Which do you propose?  The two types of blowers operate under totally different principles and you must understand that before trying to control them;
• You never control the capacity of a PD blower by throttling the discharge.  Therefore the butterfly valves depicted on figure 2 are not going to work and the whole control scheme is invalidated if the blowers are indeed PD blowers.
• If you want to regulate (“control”) the air flow generated by the blowers, then the best and most practical choice from a control standpoint is the PD type of blower.
• PD blowers are easily and accurately controlled with a Variable Frequency Drive (VFD) on the induction electric motor drive and this gives the ability to vary the amount of air delivered from the blower with a high degree of accuracy.
• If you are planning centrifugal type of blowers, then this raises more issues.  Centrifugal type blowers normally cannot be depended upon to vary their delivery with a VFD because of the inherent characteristics of their performance curves – the variance of the air delivery is not proportional to the speed (as in a PD blower).  Plus, the head characteristics of the blower change with speed as well.  The PD blower will maintain the same head under different speeds and different air capacities.  This makes it ideal for the application.

[Jome128]

Thank you very much for Mr. Montemayor and Erwin kindly help, and really appreciate your generous mind to share your comprehensive process knowledge and experience to the forum members.

 

Mr. Motemayour is correct. The rotary screw blower (fig1) uses two rotors to push air through the blower, which creates pressure. There are three basic type of drives for air compressor or blower, V-belt, Direct, Gear-driven. The variable speed drives(VSDs) usually alter the frequency of the incoming motor power. This approach is a simple way to vary compressor output and can be efficient if the airend has a wide efficiency range or flat efficiency curve over a wide speed range.

 

VSDs can boost the motor power factor to eliminate penalties from utility companies. The soft start feature reduces high input starting amps, another cost-saving aspect. Some drawbacks are additional initial cost and a power loss of 2~3% during full load operation. There is also two basic types of capacity control before VSDs control became popular: inlet throttling and rotor length adjustment.

 

They both automatically regulate to match system demand with no over or under pressure. The centrifugal blowers (fig2) : unlike positive displacement blower, which are essentially constant volume devices, centrifugal blowers deliver virtually constant pressure over a wide volume range. The discharge volume can be reduced by restricting the blower's inlet or discharge with a butterfly valve. The centrifugal blowers also are possible with variable speed blower operation.

 

The horsepower draw of multi-stage centrifugal blowers is a function of the speed cubed, so a speed reduction of 20 percent reduces the power input by more than 70 percent. To match the air delivery closely to the aeration needs, a dissolved oxygen (DO) probe can be used with a variable frequency motor speed controller. Electronic drives are most effective for blower speed variation, but also are expensive.

 

Please bear with me that most of above is refer from article because I was limited to these blower basic process knowledge. If anything is logically incorrect, please correct me.

 

Thank you very much for your generous help.