Air Balancing of Operating Theater

[mechbuild]

Hello Everybody,
I am facing problem in air balancing of operating theater. I measured air flow directly below the Hepa filter of an operating room and also at the air intake through Ultra clean ventilation fans which are located around the hepa filter.
After subtracting the supply air flow from the UCV intake air flow which is the actual air coming from the Air handling unit, I got 1840 cfm while the design air flow is 2100 cfm.

On the other hand, when I measure the air flow by the traverse method on the supply duct, I got 2700 cfm.

As you can see there is a considerable difference between these two air flow results without any considerable air leakages in the supply duct.

I would appreciate if any body suggest me the correct method of air balancing of operating theater.

Regards,

 

[ChasBean1]

The 2,700 cfm by traverse is probably more reliable than your other readings. Look at velocities relative to instrument range and you'll see that you can accumulate error at low velocities. Adjust and balance based on the 2,700 cfm (tweak down to 2,100 cfm) and maintain ORs positive by about 0.02" wc. (work with the GC to adjust leakage/door sweeps as necessary).

CB

 

[mechbuild]

Thank you Chasbean1 for your valuable comments. After lowering the air flow to 2100 cfm by traverse if the theater becomes negative, then it is the indication of air leakage through duct riser in the shaft which is outside the scope of TAB.
However, I will recheck and get back to you soon.

Thanks and best regards,

 

[ChasBean1]

The room will go negative if the return/exhaust from the space is not proportionally reduced.

 

[mechbuild]

Thanks Chasbean.
We need to maintain the design air flows in which supply is 300 cfm more than exhaust.
Supply air: 2100 cfm
Exhaust air: 1800 cfm

I already checked exhaust and it is almost equal to the design.

 

[mechbuild]

I also found problems with fan unstability and lower efficiency and the curve is not complying to the fan laws because on a perticular fan curve, as the resistance increases, the flow also increases which is against the fan laws.

The fan efficiency is only 48% which is quite low for forward curved fan.
I would appreciate if you can share your expertise in this matter.

 

[ChasBean1]

mechbuild, tough to decipher that in any meaningful way with your givens...

 

[hoodguy]

Your traverse may be inaccurate depending on the location. A reliable traverse should be located in a long straight duct which often is not available.

 

[mechbuild]

The traverse reading were taken on 7 meters straight duct located on the roof top with a liquid type anemometer which does not require any factory calibration.

 

[mechbuild]

The theater is positive when UCV fans are running and becoming neutral when UCV fans are off. If the supply air is more than exhaust, then it should be always be positive.

Can any body advice wheather above situation is acceptable or not.
I think theater should always be positive wheather UCV fans are running or not.

 

[tys90]

Operating rooms should always maintain a positive pressure in relation to surrounding rooms.

 

[mechbuild]

Pulley Data:

Motor pulley diameter: 94 mm (3.7 ” ) (Variable pulley, at present, not fully tight)

Fan pulley diameter: 88.9 mm (3.5 “ )

Note: The motor is VFD compatible and VFD current rating is 8.8 Amps. It means motor cannot be operated above this rating.

Motor name plate data:

Make: Brooke Crompton

Phase: 3

Enclosure: IP55

Duty: S1 IEC-34-1

Temperature suitability: 40 C

Cos ø: 0.87

Class: F

Voltage: 380 V

Current: 8.2 Amps

Hz : 60

Speed: 3500 Rpm (On the motor box: 2915 Rpm)

Power: 4 KW


AHU Name plate data:

Serial No: 39FX-240-955 711SP

So No: C-0402829

Item No: 5

Section: 1

Air flow: 1039 L/Sec

Cycles: 60 Hz

Voltage: 380 / 3


As per fan curve, the design operating point is at the extreme left side of the best efficiency point, in fact it should be selected at the right side of best efficiency point.

Motor speed (measured): 2829 RPM

Blower speed (measured): 2006 RPM

Design Air flow: 2100 cfm


Measured air flow: 1840 cfm

Measured air flow by traverse 2450 cfm

Note: Theater is negative at above conditions.

Motor pulley diameter: 94 mm (3.7 ” ) (Variable pulley, at present, not fully tight)

Fan pulley diameter: 88.9 mm (3.5 “ )

1.Based on the fan first fan law, new fan speed required is

V2/V1 = N2/N1

V1 = 1840

V2 = 2100

N1 = 2006

N2 = (2100/1840)X2006 = 2289 rpm.

2.new fan pulley diameter required =

Motor Speed X Motor pulley = Fan Pulley X Fan speed

Fan pulley = (2829 X 94)/2289

= 116mm

So the new fan pulley required is 2 SPZ 112

At present the motor is already consuming 8 ampers which is almost near to the full load current.
In fact as per fan curve, the motor current should be 5 amperes at design air flow.

I think the motor is taking high amperes due to incorrect pulley combination and can any body advice me if I change the blower pulley to bigger size as calculated above,will the current remain same or increase.
If the current stays same and rpm and flow increases, this problem can be resolved.

 

[ChasBean1]

Mechbuild, you've been completely clear with all the leaves, but not the trees and I'm missing what forest you're in...

Starting basically, fill in the blanks: supply traverse measurement with UCV fans on: ______. Supply traverse with measurement UCV fans off: _______. Exhaust traverse measurement: _______.

Also, are the UCV fans in series with the supply AHU? Are there any flow controlled terminal boxes on supply and exhaust at the OR?

 

[mechbuild]

Thanks Chasbean1,
Please find below answers.

supply traverse measurement with UCV fans on: _2700 cfm Supply traverse with measurement UCV fans off: 2450 cfm Exhaust measurement by balometer: 1760 cfm.

There is one supply grille (with 100 cfm design air flow) within the ceiling space of OR just for ceiling ventilation and we found that it was supplying 380 cfm instead of 100 cfm and after balancing this outlet, we got 2000 cfm by a balometer but still the theater is negative / neutral when UCV is 'off' .
If this measurement is correct, the theater should have been positive but unfortunatily it is still neutral and some time negative.
Can we get any success after changing the pulley combination.Please bear in mind that we do not have any spare ampere and any increase in speed will also increase the motor current and it will exceed the motor full load current rating also the VFD is rated for 8.8 ampers that means we cannot operate the motor more than this current.

I think the motor is taking high ampere due to incorrect pulley combination i.e the blower pulley should be bigger than the motor pulley.

Thanks and regards,

 

[ChasBean1]

The room pressure will also be a function of the pressure of the surrounding areas. If the OR is used 24/7, run the UCV fan 24/7. By the way, I didn't see anything on your fan curve dwg (may be my PC). Something more helpful than a fan curve however may be a system schematic (even in pencil)...

 

[mechbuild]

We have balanced OR-1, 3rd floor with 10% less supply and exhaust air at 55 HZ, 92% speed, 5.5 amps. With this 10% less air, we are able to maintain 22 supply air changes which is much more than the minimum required (15 ACH ) and also the theater is positively pressurized with differential pressure of 5 Pascal when UCV is on and 3 Pascal when UCV is OFF.

The above condition is complying to the ASHRAE and JCIA requirement and the results must be acceptable. More over, this is a standby fan which run for only some time when duty fan fails.



As you can see, there is an allowance of 8% for the fan to speed up in order to compensate for dirty filter condition and also to maintain the same air flow. This condition will supersede our previous condition of achieving design air flow at 80 % speed. We do not have such standards yet for the fan to achieve design air flow at 80 % VFD speed, even 92 % can do the job.

 

[AnotherEllis]

Has the design engineer looked at the FC fan curve?

It sounds like the fan is operating in the dip on the left side of the curve, which is a bad place to be. The manufacturer should have a system curve labeled something like "select fan performance to the right of this curve."

 

[mechbuild]

This problem is sorted out now, I will display the air balancing detail soon.

Thanks

 

[mechbuild]

Please see attached excell file of operating theater air balancing data.

The main problem in balancing was incompitable pulley combination. The blower pulley was smaller than motor pulley and after replacing the required blower pulley, we could achieve the design air flow at 55 HZ, (92 % VFD SEEED).

Apart from this, there were other problems related to the fan selection, but nothing can be done about it since it is already installed and because of this we could not achieve the design air flow at 48 HZ (80 % VFD speed).

Regards,

 

[mechbuild]

Hi,
Can anybody tell me the operating theater pressure requirement based on approved standards.

I need mimimum as well as maximum pressure requirement.

Regards,