Asked & Answered - Variable Flow Hydronic Systems Cx
Quality, Consequences and the Construction Industrial Complex (part 51) - All IMHO:
A senior buildings services engineer recently asked me why his variable flow hydronic systems never delivered the energy savings he modelled and expected. My rambling answer follows.
Variable flow hydronic systems are ubiquitous in North America due to their perceived energy efficiency. However in my experience they rarely deliver on energy efficacy due to equipment and control valve over sizing plus a consistent disregard of how to Commission and optimize them. Variable flow hydronic systems are absolutely not, a start-up and walk away Commissioning activity.
A variable flow hydronic system is a complex group of interacting, interrelated elements forming a complex system. Commissioning them requires a holistic, whole system approach, so lets outline a logic process get this done. BTW; all logic processes start and finish with a logic diagram IMHO.
First things first, YES, you do have to proportionally balance a variable flow hydronic system. Why? Because a balanced system matters at times of maximum simultaneous demand.
Second things second, the DP sensor controlling the variable pump set must be installed about 2/3’s of the distance from the pump to the end of the pipework distribution network and NOT in the mechanical room.
The Commissioning process in summary is:
Installation and pre-commissioning tasks - whole system
- Install the whole system including the entire pipework distribution network, all pumps, power, VFD’s, controls plus other system interfaces.
- Quality control checks, pipework pressure testing, controls programming plus uploading of controls “code” and system start up.
- Complete and verify system dynamic flushing and chemical treatment.
- System, filled, vented & pressurized.
Dynamic commissioning tasks - whole system
- Start up and preliminary commissioning of the pump VFD’s.
- Controls point to point testing.
- Calculate and apply system diversity (if applicable).
- Complete proportional balancing with diversity applied at maximum simultaneous demand flow rate (System VMax).
- Test and determine the optimum (i.e. index circuit satisfied at lowest possible pump speed) DP set point with diversity applied.
- Measure and record the pressure drop across the system strainers (clean) and chiller at V Max.
- Test the system on minimum demand (V Min) and record the pressure at the system DP sensor plus the VFD speed.
- Test, determine, set and record pump VFD V Max speed, Vmin Speed and trip speed.
- Test and validate all controls sequences of operation i.e. morning start-up, Vmax, Vmin, night set back, temp reset on minimum demand, fire interlocks etc.
- Test and ensure, were applicable, that the primary flow is greater than the secondary flow at V Max and V Min and two mid points.
- Set parameters and alarms for "set points out of range” on the BAS.
- Complete all Functional Performance Tests (FPT’s) and demonstrate to the authority with jurisdiction.
So how many and who are the players required to commission a variable flow hydronic system?
- The main contractor / GC - to coordinate the specialist sub-contractors, provide access etc.
- The Commissioning engineer / technician / authority - please don’t call him or her an “agent”!
- The mechanical contractor - typically the installation is their responsibility.
- The TA&B specialist - to measure, test and determine empirically, the optimum system set points.
- The controls specialist - probably the most important player.
- Specialists from vendors such as the VFD, chiller, controls, cooling tower.
It should be noted that the empirical setting up of the VFD and DP sensor (TA&B, CxA and Controls people working together at the same time) are critical if there is any hope of achieving the system energy targets for the building. The set & operational control points that are required to be determined, set, locked in and recorded (with clean strainers) are as follows:
- Optimum set point @ DP sensor with diversity applied @ V Max
- VFD Hz @ V Max - Noted for record with clean strainers
- Pressure @ DP sensor with system @ V Min
- VFD Hz @ V Min - Noted for record with clean strainers
- Chiller temperature set point
- System water temperature set point @ maximum simultaneous demand
- System water temperature set point @ minimum simultaneous demand (set back, on low load if applicable)
- Time schedule
Call me old fashioned, but I believe building systems should be set-up for optimum operation to meet the design intent. This is the basic working assumption by building owners when they employe design and construction teams. Remember all energy models assume system optimization with no defects.
?Remember all building energy models assume system optimization with no defects. It is on buildings services design engineers to specify the level of systems (HTG & CHW) Commissioning outlined above.
How many systems do you know of that have been set to work to this level of testing and optimization?
Senior MEP Engineer at Carpenter Associates
9 年Cutting impellers where pumps are over-sized could be another reason for reduce efficiency. In my experience, cutting a pump impeller for balancing maximum required flow would have resulted in a greater efficiency loss than balancing with a VFD and taking the hit from the VFD loss. If the control scheme needs a VFD anyway, leaving the impeller alone is generally the best strategy with plenty of range left for control scenario adjustment. Regarding speed, 66% speed on the pump also gives 66% flow on the motor fan but only 29% of the motor heat. So 2.3 times the cooling air per horsepower. At 50% speed, 4 times the cooling air per horsepower. At some point, the cooling air turbulence goes away and that air film clings to, and insulates the motor but I've seen 10:1 turndown listed by motor manufacturers for variable torque applications (centrifugal pumps and fans) and don't hesitate to go down to 12Hz. Things like conveyors and positive displacement pumps are a different ball game.
CCP, LEED AP
9 年In my experience with variable primary chiller plants setting up of the bypass valve that decouples the chillers plant from the rest of the building is a challenging and crucial part of the process. The entire plant can fail if this is not set-up and tested thoroughly. The controls technician and chiller manufacture work together with the CxA to this set-up. When the system is staging up the bypass valve will modulate open to bypass more flow back to the online chillers so when the lag chillers isolation valve opens the flow through the operating chiller does not fall below the minimum flow requirement for chillers. For cooling towers, find out the minimum flow requirement so scaling is prevented. The controls technician and tower manufacture work together with the CxA to set this up. Water needs to flow through all areas of the tower cells evenly with no dry patches. Do you have TAB contractor do the equipment performance? I always make sure this is in the commissioning specifications and TAB specifications so contractually they must perform this task. This is also a crucial step so the owner has a true baseline of his equipment in the plant i.e. chillers and cooling towers. I have set up a variable primary plant that is optimized, saves energy, and it’s in a hospital. In the winter the plant will stage to the plate exchangers and the cooling towers are used to cool the buildings process loads and cooling requirements.