Fan Static Pressure

[Samarjit Sen]

With a propellor fan , what would be the approximate difference between the Static Pressure at the air pull side and the Static Pressure at the air blow side. The air pull side is free air, whereas at the blow side there are coils. Will this type of fan be able to deliver a higher static pressure than if the process was reversed.

Would any one kindly help me with answer.

[The Viking]

If I understood your question correctly, then it's too many variables to give you an answer.

Imagine the fan hanging in free air, then it wouldn't be any static pressure at all but you would have maximum airflow.
Then put a box with a coil before the fan, you would then get some pressure differential across the fan and slightly reduced airflow.
Then fast forward a couple of months, to when the coil got dirty and create a high resistance, then your pressure differential would be high and your airflow low.

If you contact a fan manufacturer/supplier, then they should be able to provide you with pressure/flow graphs for their range of fans.


It is generally accepted that if the coil are before the fan you achieve a more even (and higher) airflow across the coil than if it been designed the other way around.

[US Iceman]

Hi Samarjit,

The fan volume flow is usually based on zero static pressure, which means free delivery with no resistance. The fan performance is also based on a tip clearance and placement within the fan shroud.

Now when the fan is placed on a coil there are other things that have to be accounted for. These are known as "system effects". In other words, the fan performance is derated to allow for the interference/resistance created by the fan placement and surrounding objects. In addition, the fan volume flow is also derated for the static pressure resistances imposed by these factors.

The fan by itself will have the highest volume flow. With the friction losses due to flow through the coil, the fan volume flow is decreased.

Some axial flow fans are better than others. A stamped aluminum blade flex's a lot and will not be capable of producing a decent static pressure. A cast aluminum fan on the other hand is more rigid and offers more performance capability in this regard.

In most cases I am aware of the fans are usually arranged to draw-thru the coil (rather than blow thru the coil). A draw through coil allows more uniform air flow and more predictable heat transfer. If a coil has varying air velocity vectors through the coil, the results may be unpredictable.:o

Does that help a little?

[frank]

Have a look here (http://www.vent-axia.com/knowledge/handbook/section2/curves.asp), it explains some of the points about manufacturers fan curves.

[John Hunter]

The energy developed by a fan is known Total Pressure,
This total pressure has two components if you neglect friction*, they are Static Pressure and Velocity Pressure.
A fan blowing from and into free space is free blow and is all velocity pressure. Put an object in front of it resisting the flow some of the flow is stopped and that portion of the original velocity pressure is converted in to Static Pressure, some times known as regain .
So Total Pressure = Static Pressure + Velocity Pressure.
In simple terms if you blow up a balloon , you are exhaling air , Velocity Pressure and when the balloon is inflated all this has been converted to Static Pressure.
When a fan is installed in to an air using system the system for any given air flow condition will have what is known as System Resistance. If the system has an air cooling or heating coil installed the designer must ensure the fan selected has the capacity to deliver the design air flow under normal operating conditions, if the unit is a finned evaporator in a chiller or freezer allowance has to be made for fouling, icing etc.
The fan selection is made by selecting a fan with a performance curve that best suits the system curve for the required duty.
There are many good publications on the basics of Fan Engineering my bible is Woods Practical Guide to Fan Engineering . Trane publications are also excellent on the subject.
* Friction is never neglected as it is main cause of losses in ducts etc. and always has to be incorporated in to the concept at design.
Most successful set ups arrange the air flow direction such that it flows through a coil. This normally has the effect of delivering better heat exchange.

[Samarjit Sen]

Thank you all for the valued information provided. I am facing a very peculiar problem. In the field of Precooling of Fruits and Vegetables, I have adopted a system wherein I install amn Air Cooling Unit with Pull through fans in front of the DX Coil. The units are of Heatcraft. With the proper system design, I get very good results and get the desired temperature and the RH. Further as these units are ceiling hanging type, more floor space are available. I get about 90% RH without running the additional humidifiers after which the adiabatic Humidifiers starts to operate and enhances the RH to 97%. My clients are very happy and so are the authorities.

Now till now there are other Refrigeration firms, who has been using a system wherein they suck the air from the room , pass it over banks of bare coil tubes over which water is being sprayed and then discharge this treated air to the room. The products are situated in rows in front of the fan to form a tunnel and is covered with polythene sheet or tarpaulin. This is to let the return air flow through the product containers. This system is also specified for Precooling.

However the draw back in such a system as told to me ny the users, is that they do not attain the required conditions.

To counteract the existing system and offer a better system for Precooling , I need your help to provide me with some knowledge on Fan Engineering so that I can prove it that a Pull Through system with a DX Finned Coil is more effective than the previous system with air being sucked and passed over a bare coil bank with ater spray.

The system installed by us have been very much appreciated by the users and the authorities, but I am fighting a lone battle with the veterans in this field who would not like to change or make an improvement to the system.