tetiary loop,vfd,bypass 2

[saeedplc]

Dear Experts,

I have some questions so could you please answer me.

1)What is function of pump in tertiary loop?
2)Is there any need to have a decouple line between secondary loop and tertiary loop or there is no need?
3)If there was VFD for secondary pump and a differential pressure transmitter in last component on the loop is it necessary
to have a bypass loop on the end of line or there is no need because pressure transmitter send signal to the VFD and the
secondary pump reduce it's speed and vice versa.

I will be appreciated to explain these because I am new in this field and above questions have occupied my mind a lot


Regards,

 

[saeedplc]

Your reply will be highly appreciated

 

[EnergyProfessional]

It depends on your system. A de-coupled loop will be added when an independent flowrate is required. Any of the separate loops can use a variable speed pump under certain conditions.

 

[saeedplc]

Thank you for the prompt reply

Could you please answer all questions as well as introduce me a good resource to find more details

 

[EnergyProfessional]

Too little information given. The decisions on type of loops, pumps and controls depends on equipment and system requirements. I don't even know if you have chiller or boiler system.

IMHO a variable primary flow is the best option and loops only get added if really necessary. In the old day with crude controls and equipment, more loops were needed. Now things are more advanced.

 

[saeedplc]

I mean those tertiary pumps that are used in the coils of AHUs.
Yes i mean in chiller systems on my third question

 

[EnergyProfessional]

They are used to maintain a minimum flow and if that is needed depends on the coil. So ask coil manufacturer about low flow performance. And they also may serve as a (bad) freeze protection. Again, if that is needed should be determined. For all I know you could be in Florida and never have freezing, or have glycol in the loop, or drain the system in winter, or use a freeze-stat... no information provided to make a decision.

I would design without such pumps when possible. More items = more things to go wrong.

KISS

 

[LittleInch]

Saeed. You need to draw the system so that we understand it and your issues.

1) you need a pump to pump water around the AHU loop. Otherwise there will be no flow.

2) difficult to say without seeing a diagram

3) see 2

Is not clear why there are three loops. Please explain.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[saeedplc]

Thank you so much for the replies
Please have a look at the diagram

 

[moideen]

IMHO, two types applications where tertiary system is helpful.one is head is not enough in the secondary loop for specific tertiary load. In this application common pipe is not required. The second application which the OP draws is the tertiary pump, common pipe or non-return flow and two-way valve. when the two-way valve is closed, the return water from tertiary recirculate through the common pipe. This system is applicable when the tertiary loop does not have the same temperature DeltaT of main loop. The two-way valve is controlled by maintaining the supply temperature as 1.2C above the supply temperature.

 

[saeedplc]

Thank you for your valuable reply
If i understand correct you mean the tertiary pump in above diagram has been used for increasing pressure and compensate the head to make even flow in the coil?
Could you please let me know is there any need for installing bypass valve at the end of the secondary loop? Differential pressure sensor is controlling speed of the secondary pump to maintain the pressure setpoint but i dont know if the pump will be warm in low speed because of not using flow in case of all picv valves are closed.

Thanks

 

[LittleInch]

In theory yes, a VFD and PD sensor might work, but depends on whether all your AHUs are the same distance from the header and have the same differential pressure when working.

It's not clear to me what and where the pressure is that the PIC valve is controlling?

Wouldn't a temperature differential control be better on the VFD?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[EnergyProfessional]

The tertiary pump enables a constant flow through the coil regardless of control valve position. the control valve basically is a mixing valve that adjusts fluid temperature to modulate coil capacity. In regular coils without the pump capacity is modulated by flow modulation by the valve.

Coils don't have such high pressure drop to require an added pump. the control valve pressure drop itself is high and not helped by the tertiary pump. they did this either for freeze protection and/or to keep flow rate in the coil turbulent. chances are you could convert this to a system without tertiary pump. it is apoor solution in search of a problem. the system pumps typically are large, robust and have redundancy. but your AHU will fail when that single small flimsy tertiary pump fails. IMHO, this just adds cost and unnecessary complexity that can fail.

 

[saeedplc]

The AHUs have different coil capacity and with different distance from header.
I turn off the pump and i saw the coil temperature start fluctuating and picv start opening and closing to bring the temperature to its setpoint. When the pump come into effect the coil temperature is stable and picv work normal.
I think the pump function is creating enough and even flow from the coil because the secondary line pressure is not enough to make even flow from the coil as well as creating enough pressure across picv ( minimum working pressure) in order for good operation of it.
However this is my analyze and need you to confirm it.

 

[EnergyProfessional]

If you turn off the pump, you still have flow through teh coil due to the check-valve in the leg. But the pump is a resistance to flow and the piping is sized for operation with pump. Some smaller pumps also have an additional check valve built in, which adds more pressure drop. Also may have a strainer, which could be dirty. If designed for operation without pump, it will work. But your system is designed for operation with pump. Unless you have spare capacity, it may diminish coil operation.

I say if the AHU meets setpoint with pump off, it will work. But research if that was for freeze protection.

 

[saeedplc]

Thank you for your reply

Does the check valve pass some flow even the picv is in low percent Am i right?
Because as the picv is in low percent(for example 5%) the pipe is somewhat block and pressure in entrance of the valve increase and can
cause the check valve open.

 

[EnergyProfessional]

Check valves can fail or get clogged up.... so verify proper operation. For example, the seat can have dirt or be damaged and cause some leak opposite the design direction. They don't have an eternal maintenance -free life. But if it is functional, it will block flow in one direction regardless of flowrate. they are designed to be closed when there is no flow and the flow needs to open it in the design direction. there are spring and gravity checkvalves. a gravity type needs to be installed in a typical way (so the valve doesn't' stay open from gravity). Google for the types and it will become very clear.

 

[saeedplc]

In the diagram that i sent earlier check valve come into effect when picv is closed and the motor is on.
Is the picv is opened for example only 2 percent this means the pipe is nearly blocked and the check valve can operate and circulate the water in coil. Am i wrong?

 

[EnergyProfessional]

Assuming proper checkvalve operation, the flowrate in the coil loop will be nearly constant regardless of valve position. If the valve opens, some less water will go through the checkvalve. If the valve is 100% open, no flow goes through the checkvalve. That is if the coil loop and pump are not vastly oversized and enough flow is allowed from the secondary loop.

That valve basically is a mixing valve in that loop and will modulate coil loop temperature, not the flow.

 

[saeedplc]

Hi

What happens if the pump is off and also the PICV valve is not activated(whiout any flow through it) and in this case the main line pressure increased a lot?
I believe in this case the coil most probably become defective.

Your opinion is very appreciated.