VAV terminal mode of operation 2

[Teto]

I have a question pertaining to VAV system operation/setup and would value your comments. First some background:I operate an 8 year old building with a VAV system. The AHU units have variable air volume controls on the fan (inlet vanes controlled by duct static pressure controller to 2" wc) and supplies air at 55 degrees to the building. The AHU units are strictly cooling units with economizers. In the building, the duct distribution system has terminal units on DDC with a reheat coil, pneumatic n.o. control valve at every terminal. I have no information on how the terminals are supposed to function as far as air flow is concerned. I have the prints with terminal box schedules showing 2 CFM designs. Example 2040/340 CFM. Is it safe to assume that the terminal will have a minimum air flow at 340 cfm? For heating, the control valve will start to open with the terminal damper staying at minimum? For cooling, the control valve will be in the closed position and the damper begin to open for and a maximum of 2040 CFM? Should the damper stop at the minimum position by mechanical means (it's a pneumatic actuator)? Should anything above the low CFM flow be considered cooling mode?. I would be happy to read any thoughts/comments on VAV terminal box setup and design.

 

[tombmech]

You need to review the Sequence Of Operations, first. If you can't find that, you may have trouble determining the logic. The sequence should exist in two places - on the original plans, and in the DDC controls documentation. If you can't find the original plans, or talk to the original design Engineer, then refer to the DDC vendor. The system has to be operating - right now - from program logic according to a Sequence Of Operations.

One of the historical issues with VAV was that they were allowed to completely close if no cooling/heating demand was present. Unfortunately, that meant no ventilation air, too. The smaller CFM you cite may be the minimum closed airflow, it may be the heating airflow, or it may be an unoccupied setback airflow. Chances are, you are correct - but what determines the heating vs. cooling changeover?

If you have an Equipment Schedule on the drawing, perhaps you can check the temperatures with the heat capacities of the heating coils. Using 1.08 X CFM X deltaT(deg.F.) should give you an idea if the airflows match with your suspicions.

It is still possible that the VAV closes completely in both modes, though. I would really try to find that Sequence - that is the key.

 

[haneyrm]

It sounds like most of your assumptions are correct. I would assume that the VAV box CFM's are max and min. In heat mode with heat at the box, the box would actuate to it's min. position and modulate the control valve on the heating coil to maintain space temp at setpoint. The min setting on the box should also be your ventilation rate for the space or the quantity of air required for heat, whichever is greater. The heating control valve may also operate during cooling mode if your box reaches its minimum position and you continue to overcool the space. If I understand your last question correctly and your system is using this "typical" operating sequence, I guess you could assume that any valve position on the box above the minimum position would mean that the box is in cooling mode. I never though of it that way but it makes sense. There should be a mechanical stop on your box to keep the box from going to the full closed position. If this were to occur, you are providing zero ventilation air to the occupied spaces which is a code violation. It sounds like you have a good handle on the operation of your system.

 

[Teto]

Thanks for the input.
Another question on the same subject is: what is the "normal" duct main trunk pressure for such systems? I'm talking upstream of the VAV. How is it calculated? Is it based on the total of the max air flow through each terminal? Read any books on the subject?
Thanks,
Hector

 

[designbuild]

SMACNA says ducts upstream of VAV boxes should be installed to withstand a minimum of 2" w.c. ESP. In practice what you want to do is minimize the static pressure in the duct but still maintain full air flow. By minimizing static pressure you minimize fan HP and hence energy costs. The problem is most engineers pick an arbitrary duct static pressure set point (or worse don't pick one and leave it to the control contractor/installer). What I see a lot is 1.5" w.c. ESP to be measured 3/4 of the way down the main duct run.
If you have a DDC control system and are constantly measuring duct static pressure, air flow through the VAVs, and valve position, you can add some control logic to automaitcally reset your duct static pressure to match the conditions in the duct at that moment. Basically what it does is looks at the box positions and air flows and finds the worst case box. The worst case box is the box that is open the most at any particular time. Say you have 30 boxes and most are around 50% but one is at 70% the 70% box would be the worst case. The controls would slow the VFD down lowering your static pressure until the worst case box was about 90% to 95% open. All of the remaining boxes would open more as well to maintain proper air flow. If a box started to open past the 90% to 95% point you would in turn speed the fan up. This sequence is usually canned for the major control people and referred to as static pressure reset.
Hope this helps.

 

[HVACctrl]

Most everything said seems right to me. I used to work for one of the major VAV box mfgs as a controls engineer. Who makes the boxes on your site? You mention DDC and pneumatics. Are the boxes controlled by DDC controllers and dampers by pneumatic actuators? Many times, with all pneumatic controllers, the mins and maxes are calibrated into the pneumatic controller. With DDC controllers and pneumatic actuators, the same should be done.

Let me know who the mfg is. If it is the company I used to work for (Nailor Industries), I might be able to provide some pretty good insight. Even if not, maybe I can help. I should also be able to put you in touch with someone in the factory who can give you some help.

Ed

 

[Teto]

Thanks for the input above,
Sorry it took so long to respond, I do appreciate the interest. The box manufacturer is Metalaire series 400 air terminal. It has an optional hot water coil. The hot water coil has a pneumatic control valve and the damper actuator is also pneumatic. The controller is by TAC (formerly Andover) and is a model TCX850 with 4 universal electrical inputs and one air flow sensor. It has 3 form A relay outputs and one Tri-State relay. The software for the controller is "Plain English" by TAC.
As for the roof top unit and main trunk pressure control, it is a Carrier 50Dk034 without electronic controls on the static pressure control. No VFD on the motor, only fan inlet vanes that open and close for pressure control. The control is a static pressure switch with a closed, open and null position. There is electronic control on this unit, but I can't see that the vane position control is tied in to it. Having said that, I do have a static pressure sensor in the trunk tied in to the facilities FMS system. I could have some programming done and use an output to control the vane position.
Going back to the trunk static pressure: what I am seeing is that cooling is the main determinant for pressure level. Low system cooling load means low trunk static as there is less need for flow. Inversely, high cooling load means high static pressure as more flow is required. So how can I calculate a starting point on my static pressure? I say starting point as I know that reality demands I will need to adjust it either for seasonal or other reasons.
Keep the thoughts coming.
Teto

 

[cme]

use fan laws to get the system curve

i have not seen a design other than using 50% of max as the heating setpoint

off topic: a vav system is no good if you don't have the right diffuser ..... i design around anemostat's xdp w/ the throw reducing vanes

 

[willard3]

snip
Inversely, high cooling load means high static pressure as more flow is required. So how can I calculate a starting point on my static pressure? I say starting point as I know that reality demands I will need to adjust it either for seasonal or other reasons.
snip

The Engineering approach is worst-case, ie, maximum cooling, maximum airflow, highest total pressure.

Lowest point can be had by comparing delta-t of air. Heating delta is usually 50- 80F (and up depending upon OA) and cooling delta-t is in the range of 20-25F. Average difference in air temperatures suggest a 30% turndown which meets practice in my climate where it is cold in winter.

 

[haneyrm]

I design systems on a regular basis that have heating minimums less than 50%. I also live in Florida. Our design standard is to set our minimums at our ventilation rates (for both heating and cooling) unless we are using electric heaters that require higher airflow. In that case, the electric heater min. airflow becomes my new box min. The comment about diffuser selection is dead on. ASHRAE 62.1 addresses this with the calculations that basically state that the lower the minimum box setting, the higher the o.a. requirement. This is primarily due to a loss of ventilation efficiency at low airflows. This has caused us to change our thinking on minimum box settings.

Mike