Doubt with Nomenclature

[IBechir]

Greetings

Is the attached control scheme (excerpt from handbook, see section 9.10) also known as pressure independent and auto balancing control valve (PICV) or am I mistaken?

Much obliged for any information.

 

[EnergyProfessional]

Not sure I understand correctly. but is the valve just one of the branches for the VFD pump? if it was just one pump per HX, you could just skip the valve and control pump speed based on temp.

In general I'm a bit curious if controlling the flow on one side of the HX by the temp on the other side is good. If the Building pump fails, the other pump will just speed up without accomplishing anything. And is the building loop VFD, or CV?

The valve could be PICV (if you have multiple branches)

 

[IBechir]

The 'building loop' is VFD. However, the HEX is in an intermediate floor to lower the pressure requirements. So the building has two loops: for lower (let's call it primary) and upper floors (secondary). This HEX in the intermediate floor is part of the load of the primary loop; it is the critical load in the primary loop. According to the same handbook, it is advised to have a diiferential pressure tranmitter across the critical load. This differential pressure, the supply temperature to the secondary loop ('building loop') and the control valve position are the inputs to control the primary pumps. I am not sure of te details of this adaptive control scheme.

You raise a good point about the secondary pumps speeding up without accomplishing anything if there is something wrong in the primary side. Did you use a control scheme that also uses the temperature in the primary side, as input, to prevent such a scenario?

May you please elaborate on what you mean by 'could be PICV'?

 

[EnergyProfessional]

Regarding critical, both loops are critical for operation. Is the HX served by a pump that only serves the HX, or si the HX jsut one branch device? If it is a branch device, you would control it with a valve. I think that needs to be clarified first. this is more a hydraulic problem and we need to know that.

Good thng about HX is, they also work with little flow (with less performance), not like chillers that may stop working entirely.

 

[IBechir]

Critical load not critical loop; large load/coil.

The HX is on one branch served by the lower loop.

What I don't understand is what is the problem with the first scheme, the one without the control valve posistion indicator (see attachement in first post). Why would there be great losses at the valve when its closed? The pumps have ramped down haven't they? So why?

 

[EnergyProfessional]

Since the HX on primary side is one of many devices, I recommend using a PICV (some people here argue a properly sized control valve w/o balancing valve will suffice). Controlling this valve based on the secondary leaving temp (as they show in the book). For secondary side you can just use normal VSD based on building demand.

Care must be take in temperature setpoints, if you have a higher SP than what the supply water temp is (or colder in case of cooling), then the valve will just open up without accomplishing anything. So if you supply 140°F water, having secondary loop SP at 180°f would be bad.

The book shows the dT pressure senor controlling the VSD by the HX. this isn't necessarily true. the dP sensor should be either at the largest pressure drop, or somewhere in the system. but the dP sensor shoudl not be "behind" a control valve as this will screw it all up. the dP sensor needs to be connected to mauin pipe. Becasue if your control valve clsoes, it will read dP=0 and the VSD will just ramp up.

As always, I also recommend static pressure reset (based on valve position) and if you have control over supply water, temperature reset based on the same. Unless you get district water which coems as it comes.

 

[IBechir]

Yes, the dP sensor is shown downstream the control valve. It seems like a mistake.

I understand what you're saying about using a PICV and controlling this valve based on secondary leaving temperature.

I read the posts in the other thread Differential Pressure Transmitter in Chilled water loop and you said: 'If you implement static pressure reset based on valve positions, the dP setpoint will be increased when a coil doesn't get enough flow (is 100% for long).' I think this is what the author of the book is communicating. Could you please, please, please recommend an article that explains this control method, or elaborate this point more, because I don't understand it.

 

[EnergyProfessional]

I don't have an article, but we implemented that in our systems and it works great. the BAS basically defines a call for more heating: when 5 (adjs.) valves are 99% (adjs.) open with 5% (adjs.) hysteresis.
when call for more heating, supply temp (up to 10°F adjs.) and pressure setpoint get increased, when no more heating demand, they get decreased.

the temp is based on OAT (you have the slope) and based on valve position we deviate from the OAT-based SP.

so in an example if you OAT based temp SP is 140°F, the actual setpoint will go from 130-150°F depending on actual demand. Same with pressure SP, it will be between 7 and 12 psi depending on actual demand (determined by valve position). We do the same with VAV boxes based on damper position.

 

[IBechir]

Thank you for sharing your experience and for your patience.