Pressure Related Fan Speed Control

It sounds like energy saving to me.

The pressure drop is an indirect method of measuring the air speed.or flow rate.

The VFD fan speed control maintains constant flow via pressure feedback to the VFC controller.

The pressure sampling would occur in a straight section of pipe where the flowrate is required to be controlled. It could even be on a branch pipe where the flowrate is most crtitical.

Is this a new (still in design review phase) or an existing installation Yuri?

I asked because I have seen a strange installation(s) on one cite. At lest at 2 supply/exhaust AHUs the samplings were taken at the all fans inlets ! I have found this installation senseless: should, for instance, a supply diffusor somewhere in the system be closed, this would lead to the pressure rise in the supply fan's sampling point, triggering the fan's speed increase, so causing even more air to be supplied - while the opposite is required ! Thinking a bit on it I have found that, at lest, at the supply fan side the outlet should have been used as the sampling point. Need more to study at this qwestion. Thanks for your answers !

Hi,

Obviously the Fans outlet point is where the sensor needs to be connected.

There are mainly 2 reasons why the pressure control is useful :

- For Process Requirements, one instance which I know is in Polyster Yarn Plants for Cooling the polymer by Air. As Wal says, here the flow is controlled by controlling the duct pressure. The system is very critical and the Duct pressure needs a very vey precise control as even a minor variation in pressure and hence the flow affects the quality of yarn and its evenness.

- Second application is for Energy saving which is quite obvious and well known.

Thanks

Energy saving can be obtained by reducing the speed . Adjustment to actual flow or pressure demand is done by dampers or guide vanes is not energy efficient. Also this will reduce the wear and tear.

Usualy we measure pressure on two places in duct and that is on begin (by the supply fan) and on end of duct and try to maintain same preassure. Main reson to control fan this way is to maintain positive preassure in a building.

For close loop circulation control this type of sensor locations are some times used. There is some details from the Danfos training manuals:

5) Closed Loop/PID Control

In this arrangement a 4-20mA static pressure

transmitter is wired directly into the VFD. This is a

Feedback signal and is always referred to as

Closed Loop. The VFD monitors its own signal

and result.

Up to this point most of the previous control arrangements have been

closed loop, which means that there is feedback signal monitoring the

controlled variable, going directly to the VFD. Closed Loop is used for stand

alone control. In the example above, the VFD monitors the signal coming from

the 4-20mA static pressure sensor in the supply duct. In a variable air volume

(VAV) system it is important to maintain Static Pressure in the duct for proper

operation of the VAV boxes.

In all closed loop applications, additional parameters must be

programmed. These include a setpoint, and PID settings. In this application,

the VFD is constantly comparing the static pressure setpoint, 2.5"wc, (625

pascals) with the actual feedback value coming from the pressure transmitter.

The VFD modulates the speed of the supply fan to maintain that pressure.

Controller action is one of the parameters that must be checked in the

VFD. There are 2 selections which are as follows:

Normal Control (Reverse Acting) which increases the speed of the fan

when the signal decreases from the pressure sensor, as in the example above.

Inverse Control (Direct Acting) increases the speed of the fan when the

signal increases from the sensor.

Not knowing your application for the equipment, just knowing that it is an air handler leads to other questions however the basic answer is air flow control = energy savings. The SA - Supply Air sample should be taken at the outlet of the AHU - (air handling unit) 6 - 10 feet from the main trunk before the first bend or drop into the building or zone.

If it is for just Duct static then the second sampling tube should be taken from the atmosphere outside the unit in an area where the wind can not blow against it causing a false outdoor reading. Use a Static tip in a can.

If it is a constant volume system then the SA static reading should be taken in the buildings lobby to assure that all zones are fed properly to all the supply air registers as required, and balanced for proper CFM. Again you will need the same atmospheric reading for proper VFD control.

The common reason for VFDs and air flow are two fold. If it is a VAV system (variable air volume) it will have zone control. Once those zones reach set point temperatures the VAV boxes will close down to their preset minimums, building a Back Static Pressure on the duct causing the fan to slow (saving energy) while still maintaining static pressure to meet the other zones CFM demands. When the boxes open the duct static drops causing the fan to pick up speed to supply a greater volume of air to the duct.

Constant volume works the same but it uses the shell of the building as its controlled duct work and the reference point is usually located in the lobby or first floor. As the building or structure the fan is serving builds positive pressure the fan reduces speed to maintain that pressure. This is to be sure that the doors or windows do not blow open wasting energy but maintaining a slight positive so that dust and pollutants are not sucked into the building.

Thanks for your comments ! The main difficulty for me, in this current aspect, is understanding the work of a whole, balanced supply-exhaust system (it is more easier if there is just a supply or just an exhaust)

Yuri,

Your question has many more variables than the data you have given. Indeed, without seeing the total system's spectifications and drawings, I doubt that any of us could give an adequate answer. We don't know if the use is industrial, food processing, health care, places of assembly, or whatever. Each of these would have different codes and requirements the complete system must satisfy. I remember working in a food processing plant where one room's exhaust air system failed but the supply was still running--the entire dropped ceiling was lifted, broke sprinkler pipes, and you can imagine the rest.

VFD's--energy conservation, design simplification, air quality assurance, convenience, and a host of other factors to consider. Happy learning.

Your kind comment prompted me to ask you a couple of qwestions - if you don't mind.

Do we use a some kind of barometer to measure the pressure inside a building /a room to assess a ventilation system work ?

Are static pressures at an AHU's outlet (air to room) and inlet (air out of room) the same - with inverted sign (positive and negative) - in a balanced system (when the outside and the inside-the-building pressures are the same) ?

Yuri,

First a disclaimer--I am not an expert in HVAC work--mine has been on controls and control systems, not ducting and air flow. However I have studied this some when I took a year's evening classes (decades ago), and have worked around them before then and many times since them.

Read catalogs, ask questions, find and read text books on the topic, go to trade shows, study technical literature. A lot of this might not make sense the first time around, so go back to it again as needed. Eventually you will get to the position where others see you as the expert, even though you still feel like you have so much to learn. Oh, and be assured that you will make mistakes--sometimes many of them.

Air is considered to be a fluid, a compressible one. For it to move from one location to another, there has to be a pressure difference. Everywhere--down the entire length of a duct and not just suddenly at one point and nowhere else. If you want to move the same volume through a small duct, the pressure difference to do this will be much larger than if the duct is large. Get the picture?

Texts say that the ideal ventilation system design has equal total pressure loss along the length of a duct system and all its branches, when the system is delivering the desired amount of air out of each register or grille into each location. Programs, tables, and charts allow the designer to determine the best size of duct for each section and branch to get close to this ideal. Then the installer does the work. Finally at the time of commissioning, an air balance is done to measure actual air flow at each outlet and dampers are adjusted to bring the final performance to the calculated ideal. Supply ducts and return ducts are entirely different systems, so there is no way to say that the pressure difference between the conditioned space and the inside of the two ducts is the same (or any other arbitrary value). However, the net pressure loss by friction of the moving air has to be made up by the blower or fan that is at the HVAC unit. If not, the amount of air moving will be either greater than calculated (if the blower is more powerful or more efficient) or less (opposite cause).

Manometers (a type of barometer) can be used to measure the pressure within a duct system (where the pressure is compared to the room), but usually are connected so they measure the pressure difference at some piece of equipment. Most commonly, this is at a filter, so the maintenance people know when it is causing too great a loss in pressure and therefore when the filter needs cleaning or changing. A second typical use of them is across a location with a known constant amount of friction, so the pressure difference can be used to give a reliable value for the rate of air flow (such as in cubic feet per minute or similar suitable metric units).

If you have a system with variable volume dampers on automatic controls, the total need for air movement will change according to the settings of these dampers. A typical use would be to reduce the air flow when a room is unoccupied or to increase it when the room's use requires purging of air contents for some reason. Under these conditions, the designer has two choices--either design the system for the maximum flow (and recognize that when dampers are closed, the system will be working with poorer energy efficiency) or include a way to change the fan speed so the total pressure drop in the duct system is constant (a variable speed drive and controls that are giving it a feedback signal from a pressure transducer reading the duct pressure). For decades, the first option was the one used, but now the second one is the most commonly used one--legacy systems are frequently retrofitted with the v.f.d. and the necessary controls for this. The reason is simple--look at the so-called "fan laws" which relate changes in fan speed, energy use, pressure drop, and air flow; you will see that very significant energy savings are possible.

I hope my somewhat long answer has addressed your questions and prompted you to study and learn more on this topic.

--JMM

GA!

Yes - Yuri - research and learn all you can.

Take a look at the Veris site www.veris.com/PXULN05S.aspx this is what you will need to control your drive. It has a digital display so that you can monitor you actual DSP or SSP.

Oops, this one.

http://www.veris.com/Category/Pressure.aspx

Then hover and click on Dry Media types. You can locate your desired set points there or pick the universal transducer so that you can configure it in the field as required.

I was taught a control man (I stay also in this field) should know a process better than anyone else. Thanks for your comments anyway !

Yuri,

I disagree. Much more important is to know how to question the information you are given, to determine what is valid and what is without basis. After this, a very good education will tell you where to find the answers to the questions. My job is to take what the person knows and add to it my understanding of equipment or controls so they can achieve their needs. I always am learning and the teachers are the people I work for and the equipment I work on.

Your post implies a much greater knowledge than I had assumed when I last wrote--please forgive me if my words were a problem.

--John M.

(What problem ? You should not be sayng this seriously)

I am asking on forums many basics - what one studies at a college, in a due time - because I encounter in the trade, indeed, here and there, ones doing "professional" work but who do not know elementary concepts. So, no use to ask THEM.