VFD on Existing 2-Pipe Fan Coil System

[mechth]

One of our retrofit project is an apartment. It has a 2-pipe fan coil system. There are 2-way control valve for each fan coil unit. In winter, the supply water temperature is reset depending on outdoor air temperature. In summer, the water temperature is constant and the control valve will regulate the flow rate for each fan coil.

My questions are:

1. Can we put a VFD on the existing recirculation pump? If so I believe we should add a pressure relief valve in the end of the loop since all the suites are in the perimeter area and the load is not relatively constant.

2. I think the VFD will reduce the energy consumption of the pump motor in summer (3 months). But in winter (9 months), since the system is designed as a constant-flow system with temperature reset, the savings may be very limited.

If someone who has done a similar project could give me some suggestions, I greatly appreciate that.

 

[quark]

The beauty of energy conservation projects is they always have two logics. One works for who believe it and the other for non believers.

1. You can add a VFD to your existing pump if your motor suits to it (or it should be inverter duty motor, that is what electical engineers say). No pressure relief valve is required when you use variable speed pumping system. Actually these pumps operate on a fixed differential pressure across the supply and return headers.

Believers say

2. If your 3 month flow control savings justify your payback period, go for it. Further, what is the duration of your temperature reset during winter? Is it one day? How do you take care when heat load changes within a day? How constant flow is maintained throughout when you have employed 2 way valves?

The possibility of running your pump near BEP is good when using VFDs.

Also, you will be rewarded when global warming is realized(I mean really)

any non believers out there?

Regards,

 

[mechth]

I mean the flow is relatively constant since there is temperature reset in winter. If the pump curve is flat, the flow rate will be relatively constant.

In summer, when the pressure loss increases much more due to the 2-way control valves on the fan coils, the pump operating point may go left further and the efficiency would be low.

 

[HVAC68]

Trust you are talking about a primary-secondary system. If yes, I agree with quark.

If it's a "primary only" system, then you may have problems with the minimum water flow required through the chiller.

HVAC68

 

[cme]

How many GPM and HP is the pump? It's 3-phase, right?

 

[mechth]

Sorry for the delay. I was occupied by something else.

The thermostat of each fan controls a three-way mixing valve. Fan speed is controlled by the residents. This is an old system. There is no secondary pump. Circulation pump for heating and cooling is 428 GPM, 15 HP (one operation with one stand-by). Since both the boilers and chiller require a minimum flow, I feel that VFD on the pump motor is not practical unless we change the piping configuration.

 

[HVAC68]

Your first post indicates a two-way valve and the last one says a 3-way valve !!!

HVAC68

 

[mechth]

sorry, my mistake. it's an old building and the drawings are not complete.

 

[ChrisConley]

So.... what is it then? Two-way valves or three way valves?

 

[Tvrm]

15 Hp pump? The actual water hp is probably in the range of 10 to 12 (assuming the pump is powered thru the curve). At this hp it is not worth time or energy to convert to VFD. The amount of saving will probably be minor, the potential for new problems to arise (the hydra effect) are more than likely. Look at other items to improve, like pipe insulation or plant efficiency. We generally don't put VFD on anything 15 hp or less.