Modeling an Automatic Flow Control Valve

Modeling an Automatic Flow Control Valve

by Engineered Software, Inc.

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:

{Q = C_{v}\sqrt{ \Delta P}}

Where: Q = Flow (gpm)

             Cv = Cartridge flow coefficient (dependent on orifice area)

             ΔP = Differential pressure (psid) across cartridge

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 Sizing Valve in the branch and set the valve in the Property Grid 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 v12.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 sizing valve labeled "AFCV #" on the return side set to 5.33 gpm.  Each loop also has a sizing 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 D4 has a dP of 3.825 psid.  This represents the lowest pressure drop that any of the sizing 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).  The pressure drop at this valve is 54.09 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.

NOTE:  You will get an "Overcontrolled" message if you are using a sizing pump in series with sizing valves.  To overcome this, open one sizing valve to 100% open.  This loop will still get the 5.33 gpm design flow rate due to the Conservation of Mass law.  If possible, enter the manufacture's pump curve data in the model.  This will also solve the "Overcontrolled" message for this type of model.

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.
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 v12.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 Manufacturers



AFCV 1 v12.pipe