Automatic flow control valves (AFCV's) are used on HVAC hydronic systems to provide a fixed flow rate of chilled or hot water to various system loads.  The proper operation of an HVAC system depends upon a consistent distribution of flow to all loads in the system under design conditions as well as under partial load conditions.  AFCV's are unique in that they will provide a fixed rate of flow even when the differential pressure (dP) across the valve is varied.  As long as the dP is within the working range of the AFCV, then the valve will typically supply a fixed flow rate to within ± 5% of the design flow.

AFCV Design:

The heart of most automatic flow control valves is a stainless steel cartridge.  The cartridge consists of a stainless steel spring-loaded piston with precisely cut orifices, and a stainless steel housing as shown in Figure 1.  



Figure 1. AFCV Cartridge - Image courtesy of Griswold Controls

When upstream pressures are at the low end of the AFCV's range, then the piston protrudes more into the flow of liquid, thus exposing more orifice area and generating a higher flow coefficient (Cv).  Conversely, when upstream pressures are at the high end of the AFCV's range, the piston is compressed more into the housing unit, thus exposing less of the orifices and generating a lower Cv as shown in Figure 2 below.  The flow equation is as follows:





Figure 2. AFCV Funtionality - Image courtesy of Griswold Controls

The spring loaded piston allows the cartridge Cv to vary inversely with the dP.  The result is a constant flow rate under varying loads.  When differential pressures are outside the range of the AFCV, then it acts as a fixed orifice with the flow rate dependent upon the out-of-range dP.

In many HVAC systems, each coil load may have an automatic flow control valve paired in series with an automatic temperature control valve (ATC).  Under design conditions, the AFCV will act as the flow limiter in each branch of the system.  Under abnormal conditions such as start-up, the ATC may be actuating to bring the process under thermal control, and thus the ATC would act as the flow limiting device.  In this case, the ATC would bear most of the pressure drop, and the AFCV might have a dP below its range.  This is one case where the resulting flow might not be equal to design flow.

An AFCV is set at the factory to the desired design flow rate.  When you are ready to spec an AFCV, you must supply the company with the design flow rate, and the range of differential pressures.  You can get this information from PIPE-FLO by modeling the system as described below.


Calculating the AFCV Requirements with PIPE-FLO:

An AFCV is easy to model in PIPE-FLO.  Simply insert a flow control valve (FCV) in the branch and set the valve to an "Automatic set value" equal to the design flow rate.  PIPE-FLO will calculate the required differential pressure across the valve needed for sizing an AFCV.  To demonstrate this, open the PIPE-FLO project titled AFCV-1.pipe.  The left half of the system is shown below in Figure 3.



Figure 3. PIPE-FLO System with AFCV's

This represents a small direct-return cooling water system with 4 banks of coils, and 5 coils in each bank.  Each coil has a design flow requirement of 5.33 gpm so each loop has a flow control valve labeled "AFCV #{###}" on the return side set to 5.33 gpm.  Each loop also has a flow control valve labeled "ATC #{###}" on the return side set to fully open which represents the automatic temperature control valve.  Under design conditions, the AFCV's will be the flow limiting devices.  Design specifications require that there will be a minimum of 2 banks active at all times.  When you calculate the Design Case lineup which has all 4 banks active, note that the most hydraulically remote control valve (AFCV D{005}) has a dP of 4.463 psid.  This represents the lowest pressure drop that any of the flow control valves will ever see.  Now, change the lineup to the partial load lineup titled "C&D Loads Isolated".  With the closest two banks of coils open, this represents the highest pressure that any of the control valves will see.  Note the pressure drop across the closest valve (AFCV A{001}).  The pressure drop at this valve is 54.84 psid.  So we have a dP range of roughly 4 psid to 55 psid.  This, along with the design flow rate of 5.33 gpm and the desired valve size (½ inch because of the ½ inch line size) is the information you need to supply the manufacturer to size an AFCV.


Sizing and Selecting AFCV's:

With the information generated by PIPE-FLO, we can size and select automatic flow control valves to meet our system requirements.  One example of a manufacturer which has many Automatic Flow Control Valve solutions is Griswold Controls.  Click on the following link to open up the web page for Griswold Controls.

Griswold_Controls

Follow the link to Products >> HVAC Products >> Automatic Balancing Valves.  Scroll down and open the link for the ½" -3" Isolator™R valves.  From the pdf file, note that a ½ inch model IR14 has a dP range of 4 to 57 psid, and is available with a flow setting of 5.33 gpm.  This valve would certainly fit our application so we will use it in our PIPE-FLO project.


Modeling AFCV's  with PIPE-FLO:

Return to the PIPE-FLO project AFCV-1.pipe.  Since all of the AFCV valves are modeled with an Automatic set value, then there is nothing further that needs to be done.  When you change lineups, you will see that the AFCVs' differential pressure will change, but the flow rates will remain the same.  One thing that you can do is add the "Allowable dP range" to the valves in the model.  When you double-click on one the AFCV's, type in From 4 psi To 57 psi in the allowable dP range section.  Then, if you ever calculate an operating scenario which causes the dP's to be outside of this range, the program will give you a warning indicating so.


AFCV's  vs. Manual Balancing Valves:

Now open the PIPE-FLO project titled AFCV-2.pipe.  The top half of this project is our system using AFCV's to supply the required flow to the coils.  The bottom half is the same system balanced with manual balancing valves.  The left half of the system with manual balance valves is shown below in Figure 4.

Figure 4.  PIPE-FLO System with Manual Balance Valves

Notice that the manual balance valves are represented with a tee instead of a cap.  There are many advantages to using AFCV's over manual balancing valves including the following:

• Probably the biggest reason is that the AFCV's will supply a constant supply of water to each and every load, even under partial load conditions.  Manual balancing valves will experience overflow conditions on open coils when part of the system is shut down.  Calculate the PIPE-FLO system and note the balanced flows through all of the coils in the Design Case scenario.  Now open one of the partial load lineups such as A&B Loads Isolated.  Note the overflows in the coils in the manually balanced system.
• Far fewer AFCV's are required to balance a system than the same system with manual balancing valves.  As indicated by the PIPE-FLO model, the system with AFCV's requires only 20 valves, whereas the manually balanced system requires 27 valves.  The manually balanced system must incorporate partner balance valves in the main lines, the risers, and in the branches in order to balance the system.
• Manual balancing valves must be set using the services of a balancing contractor.
• Manual balancing valves require more pumping power.  Notice the lineup with A&B Loads Isolated that the manually balanced system is pumping 63.49 gpm when only 53.3 is required.  That's about 19% more flow than is required.  Also, look at the graphs for each of the pumps.  The pump in the AFCV system is using 5.38 bhp, and the pump in the manually balanced system is using 5.75 bhp.  This is a 7% increase in horsepower and will result in higher pumping costs.
• AFCV costs are fairly competitive with manual balance valves.  In particular, when you look at valves which are 3 inches or larger, the AFCV valve costs are closely comparable to the manual balance valve costs.
• To maintain accuracy with manual balance valves, there must be a minimum length of unrestricted straight pipe both upstream and downstream of the valve.  AFCV's can be plumbed anywhere in a system without regard to proximity to pipe bends and fittings, or horizontal or vertical orientation.

AFCV Manufacturers:

Griswold_Controls
Flow_Design_Inc
Hays_Fluid_Controls:
Nexus_Valve