VFD needed?

[Designer69]

Thanks guys. I can see now that it is not overkill.

Also, if the HVAC system is designed without throttle valves operation then the electrical engineer has no choice but to specify VFDs anyway right? (Is this a true statement?)

I can see this system has Air Handler Hot and Chilled water pumps, Boiler pumps, Cooling Tower, Condenser Water Pump, Heat Recovery Water Pump. All on VFDs. (understandable now since the HVAC system is prob. designed this way?)

Also are electric reheat coils. These must be a needed part of the overall HVAC system? (Also on VFDs)

 

[GoldDigger]

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Also are electric reheat coils. These must be a needed part of the overall HVAC system? (Also on VFDs)

Reheat coils are installed on air ducts feeding specific areas to allow the air delivered through vents into those areas to be warmer than the initial air temperature leaving the evaporator coils. Other ways to do a similar thing is to zone the evaporator coils to allow different set temperatures for different areas or to mix a variable amount of heated air with the cooled air before delivery. I once worked in a building with separate cooling and heating air sources and the two air streams were mixed based on a thermostat for each HVAC zone. It was horribly inefficient and did not work very well either.
For some reason the Variable Air Volume (VAV) boxes (basically smart dampers) that mixed the two air streams tended to always dither back and forth between full heat and full cold instead of controlling proportionally.

 

[Designer69]

Thank you. Is it possible they put these reheat coils on VFDs bc the HVAC system is designed that way, IE. to allow a certain amount of reheat coil intensity depending on system control?

 

[GoldDigger]

Thank you. Is it possible they put these reheat coils on VFDs bc the HVAC system is designed that way, IE. to allow a certain amount of reheat coil intensity depending on system control?

It is certainly possible that there is an "off the shelf" configuration for their management system to control a particular brand of VFD while the use of a less expensive SCR product might require more costs for hardware and software development and system programming.

 

[Designer69]

My current hypothesis is that in a Variable Air Volume system, VFDs are not generally provided. In a Constant Volume system VFDs are generally provided.


Any truth at all to my hypothesis?

 

[kda3310]

And I was wondering why one needs electric reheat when you have a boiler.

I'd have lots of questions too.

We have reheat coils on our electric static plates that catches particles out of the smoke before it leaves the boiler. If we have to shut down for any reason in the winter time especially we do not want these plates to get to the outside ambient temperature. So the reheat coils keeps these plates warm until the fire is restarted. Going from 300° to 32° too fast could warp these plates or vice versa. It is better for us to keep these plates at 90 to 100 degrees until we restart unless we have to go into the ESP to work on it. If we have to go in to the ESP to work on it it is easier to warm it back up with the reheat coils then to pour the fire to it warping the plates.

 

[Jraef]

...
Also, if the HVAC system is designed without throttle valves operation then the electrical engineer has no choice but to specify VFDs anyway right? (Is this a true statement?)...

Probably not. The decision to use VFDs vs VAV boxes and valves was likely made by the ME that designed the overall HVAC system, the EE would just be given an equipment list stating that those loads needed VFDs and he takes the responsibility for the prober electrical design to implement them. I’ve also seen where an ME will do it all without involving an EE, depending on local rules as to who can stamp drawings.

Often times that decision on using VFDs is tied to having to meet energy efficiency requirements for the entire facility. Here in California for example we have what’s called “Title 24”, a State code requiring minimum energy efficiency standards for all building designs. Elsewhere even if there is no official code requirement, some people may be going for what’s called LEED (Leadership in Energy and Environmental Design) Certification, which helps attract tenants that need to show their civic responsibility in this way.

 

[petersonra]

If they understand VFDs surely they would no difficulty in understanding a very much simpler SCR piece of kit?

it is not as if they could not learn but if you have one odd ball piece of equipment in a plant and 10 technicians that have to be trained on it if you use it ....

 

[Besoeker]

Also are electric reheat coils. These must be a needed part of the overall HVAC system? (Also on VFDs)

Yes, a VFD for that application makes no sense to me.

 

[topgone]

Yes, a VFD for that application makes no sense to me.

A fixed load being fed with a "variable" frequency drive seems a mismatch for me too!:lol:

 

[db236591]

My current hypothesis is that in a Variable Air Volume system, VFDs are not generally provided. In a Constant Volume system VFDs are generally provided.


Any truth at all to my hypothesis?

Used to be fairly true. Now with newer system designs, VAV systems are required to slow their main fans to use the minimum airflow required to keep a space at setpoint. The logic (very simplified) looks like this:

Reduce Static Pressure Setpoint (controlled by VFD fan) until atleast 1 damper box is full open
Power Input to VFD reduced by theCube of CFM ReductionSpeed Reduced 1/2, Power to 1/8.

This document is a pretty good summary on air systems, both constant volume and VAV. The static pressure reset stuff starts on page 50.

http://illinoisashrae.org/images/meeting/030713/intro_constant_volume_vav_systems_moccio.pdf

I would be surprised if a newly designed air system that was big enough to have a boiler and chiller didn't have dampers. I don't understand the VFD reheat coils though, that's new to me.