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20 points every engineer and contractor should know about control valve and balancing theory - sample slides.  For additional support visit our client service page.

Our integrated design program has over 2100 slides illustrating architectural, interior design and HVAC engineering principles which contribute to indoor environmental quality and energy allocation for conditioning the occupants and building.

The following course materials on control valve and balancing theory are samples from the lecture and based on a Steven Covey principle of "Begin with the End in Mind". They are a very small but important sample of the Covey principle and are provided here to give you an idea of what kind of materials we'll be discussing during the program.

The course is also registered with AIA and participants can earn up to 21 Learning Units.

For more sample slides visit our list of training modules.

Figure 1: One of the "meat and potatoes" topics in our three day integrated design program addresses control valve and balancing theory. We'll get into all the gory details of valve authority and characteristics of heat terminal units and valves...all very fun stuff!

Figure 2: One thing we're known for is our in-depth research and the basis for this lecture is grounded in four of our favourite resources spearheaded by Petitjean's Total Hydronic Balancing. If these books are not in your library we suggest you obtain them for study and day to day design practice.

Figure 3: We get that this content on control valve theory is not for all our readers. So you may ask, why post it? For two reasons, the first is to illustrate some of the content in our three day integrated design course which is for design professionals and two to illustrate to non professionals that there is no such thing as "control valve theory for dummies" although there are many in the latter category who choose to disregard the theory resulting in unintended consequences such as occupant discomfort and system inefficiencies. Shown above is the standard control loop – you’ll find a schematic similar to this in just about every decent manual on controls. It points out all the control system “players” and the “positions” they play and the rules to which they are to play by…in this set of sample slides we are going to look only at the control valve – in the full program we cover all the other components.

Figure 4: We engrain the above statement in our students heads because even though we call fluid based HVAC systems “hydronic”, they in fact are “hydraulic” systems where control over pressure trumps all other considerations. You can play around with temperature all you like but if the system hydraulics are messed up, no amount of temperature control can completely compensate for the negative impacts of pressure oscillations created by valves opening and closing and/or circulators turning on and off or ramping up and down.

Figure 5: In every fluid based system we’re going to have devices which convert electrical energy into kinetic energy and those friction devices which will work against the kinetic energy. Circulators are the energy convertors taking electric energy and converting it to kinetic energy (moving water) or what we call the dynamic head pressure of the system. Pipe, valves and fittings are those devices which work against the kinetic energy through friction or what is called resistance measured in units of 'feet of head' or kPa. Electrical energy that is not converted to kinetic energy becomes heat, vibration and sound. When devices like control valves close (think of your thumb over a garden hose, Figure 4), they disturb the system hydraulics which can translate into poor motor efficiency ergo valves should not be viewed as something which opens to give you heating or cooling. We explain why in the course but suffice to say, when the hydraulics of a system are not evaluated the result is a lot of wasted energy and that inefficiency lasts the entire life of the system. This is not a trivial matter because the destruction of efficiency is ultimately paid for by the customer…and sadly 95% of all customers don’t even know to ask for nor interpret an equipment efficiency evaluation.

Figure 6: We like this table because it is representative of all tables produced by control valve manufacturers; furthermore it contains valuable information useful to the control technician but is regrettably ignored by those who frequently sell, buy, install and service control valves. Sounds strange right? Well there is no academic requirement in industry for studying control valve theory so it gets ignored. So let me ask you this…why would a manufacture offer a 1/2" (13mm) valve but with six different Cv’s, rangeabilities and flow characteristics if it were not important? Hold onto that thought and head to Figure 7 below.

Figure 7: Illustrated above is the flow characteristics for three styles of control valves (see descriptions in Figure 6). It tells us in general terms that a quick opening valve (#3) typical of most on/off “zone valves” would flow approximately 70% of its full flow at an valve opening of approximately 30%; likewise a linear valve typical of most modulating radiator style valves would flow approximately 50% of its full flow at a valve opening of approximately 50%; and the equal percentage valve (logarithmic) would flow 30% at approximately 70% of its full opening. So the question is - where would you use one style of valve over another…well have a look at Figure 8 below.
 

Figure 8: Presented above are various heating devices or what is called in industry, “heat terminal units (HTU’s)”.  HTU’s and control valves are like people in that they have different personalities. Most assume when it comes to heat transfer that 50% output comes from 50% flow. Although this is possible it can only happen by design. Let me explain...a fan/coil (upper right hand corner) has a “postal” personality in that it’s output at 30% flow approaches 70% based on approximately 180F (82C) supply's and approximately a 10F (5.6C) differential (see Figure 9 and Figure 11). Conversely as you move away from the forced convective light weight HTU’s at high temperatures to the more heavy mass systems such as radiant slabs using lower temperatures and wider differential temperatures you have a more linear performance. What’s the significance? As noted above on control valve characteristics, a quick opening on/off valve typical of most “zone valves” can also be characterized as ’postal’ because it has no modulating traits, i.e. it is fully open or fully closed and its design is such that it has ‘low fidelity’. Such a coarse control device is the worst thing you can marry to a forced convective device like a fan coil, panel radiator, chilled beam or fin/tube baseboard because its like marrying two crazy people together. It’s not possible to achieve 50% output at 50% flow because together these devices have no "mid range". To stabilize the output of HTU's shown towards the right hand side of the illustration, marry them to valves with an inverse personality, something with “high fidelity” called an equal percentage, logarithmic or split characteristic valve; or you could design your system around a lower supply temperature with an increase in the differential and use a linear characteristic valve. This control valve principle is illustrated below in Figure 9.

Figure 9: During our control valve lecture we’ll explain one of the most basic principles in control valve theory which addresses conditions necessary for linear output, i.e. 50% flow delivering 50% output. As noted above it doesn’t happen by accident it has to be designed into the system including the correct selection of actuator, valve, and heat terminal unit. Shown above is the net result for a circuit when all the right parts have been assembled. Note: the circuit characteristics (upper right graph)...the fine dashed line represents the heat terminal characteristics and the course dashed line represents the equal percentage valve with the solid line representing the linear performance as a result of marring the two characteristics together.
 

Figure 10:  One of the key reasons for selecting modulating valves with high fidelity for high performance heat terminal units has to do with flow requirements across the HTU during the season. As you can see for this data set, 4874 hours of the 5888 operating hours run at less than 60% load. Another way of saying this is only 17% of the operational time is the system ever running at 70% load or more. Now image what would happen if you specify an on/off zone valve on a forced convection HTU…the valve fully opens in under 45 seconds and the coil reaches maximum output also in seconds and this occurs even though for most of the year the full output of the coil is not required...and now you know why some systems cycle.

Figure 11:  Here is another way of looking at this concept….the x axis is flow and the y axis is output. The curvatures represent the characteristic of the heating coil under different differential temperatures. Due to the climate based load, 80% of the year the coil could provide sufficient output with just 20% of the flow but 20% flow is not possible with on/off zone valves – you must in this case install modulating control valves with equal percentage or logarithmic characteristics. It’s a bit more complicated than that but you get the idea. We’ll explain it in greater detail during the course.

Figure 12:  Here you can see the room temperature results as a function of over and under heating due to incorrect flows. As noted earlier, when we control the pressure , we control the flow; and when we have control over the flow we have authority over the system. So the principle in selecting control valves has to do with pressure and more specifically how much pressure drop across the control valve in relation to the pressure drop of the circuit it must control. This relationship is called the control valve authority and in selecting the Cv (flow coefficient) we should try to have between 30% to 50% of the circuit resistance within the valve.

Figure 13: Establishing the pressure drop across a control valve in relation to the circuit is quite simple in simple systems but in large multi zone system the interactions can be quite complex. These complexities can be solved with various pressure balance components which control the differential pressure across risers, branches and circuits. We’ll look at the traditional components as well as the pressure independent control valves for circuits.

Figure 14:  As referenced above, the pressure drop across the control does not happen by accident rather it is carefully designed into the system. Shown here are the mathematical principles which should be followed when specifying the valve Cv as well as its characteristics and rangeability.

Figure 15:  There are many types of valves in hydronic systems each with their own purpose. Frequently in multi temperature systems, mixing is required to lower temperatures to design conditions. Rotary or shoe style 3 and 4 way types are useful as “master” control valves for tempering purposes, but unlike the single or double seated valves, the rotary or show types should not be used for final control over the individual circuits. We’ll explain this in our course.

Figure 16:  During the course we’ll address many aspects of thermal dynamics and fluid flow including the calculation procedure for mixing two or more fluid streams. We’ll spend considerable time discussing fluid characteristics, velocities and pressure drops and their relationship to heat transfer and control.

Figure 17:  It’s shame that so much effort goes into calculating loads, flows, pressure drops and then when it comes time to assemble the system people skimp on the balancing devices…translation: without the right components there is no way of verifying in the field the calculations done during the design stage. As expert Robert Petitjean would say, “why do the calculation if you are not prepared to validate the results in the field?” We’re with Robert…if you are going to take the effort to do the math - then make the effort to install the necessary adjustable devices to validate your calculations.

Figures 18:  I took this photo of the geothermal loops for the Manitoba Hydro Building…make note of the individual balancing valves on each circuit (just below the gauges) …with such installations its easy to make readings of the flow to validate whether the ground loops system is functioning “hydraulically” as intended.

Figure 19: …and finally our ode to old wise HVAC engineers who get that satisfied occupants in good buildings with good mechanical systems doesn't happen by accident…it occurs because of an understanding of integrated principles which is the DNA for our three day program.

Figure 20:  Yep...this is one of those topics that'll take a couple of hours to teach followed by a well deserved refreshment!

So there you have it, a few sample slides from our control valve lecturer...just a hors d'oeuvre from our library of over 2100 slides addressing a small but important element of integrated design and radiant based HVAC systems. In the program we will get into this and a whole lot more? How much more? Well just follow the links to the other parts of our website and you’ll get a feel for the scope of materials that we’ll be covering.

See you soon.

Robert Bean, R.E.T., P.L.(Eng.)
Registered Engineering Technologist - Building construction (ASET #8167)
Professional Licensee (Engineering) - HVAC (APEGA #105894)
Building Sciences / Industry Development
ASHRAE Committees: T.C.61. (CM), T.C.6.5 (VM), T.C. 7.04 (VM), SSPC 55 (VM)
ASHRAE SSPC 55 - User Manual Task Leader

Note: The author participates on several ASHRAE and other industry related committees but be advised the materials and comments presented do not necessarily represent the views of these societies, only the president of the society or nominated representative may speak on behalf of the organization.

For further studies on this topic visit:

ACCA 2012 Hydronics Roundtable: Radiant Cooling Presentation
 


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