Canadian Consulting Engineer

Design FOR HYDRONICS

While hydronic systems may seem to be relatively simple to design, there are technical challenges with the controls that can cause them to operate under par.

August 1, 2008   By Mark Hegberg ITT R&CW Bell & Gossett

While hydronic systems may seem to be relatively simple to design, there are technical challenges with the controls that can cause them to operate under par.

Today in North America the rooftop, self-contained air-handling unit is standard on most buildings. Hydronic systems tend to be used only in large buildings or campus type applications.

Hydronic systems are viewed as fairly basic systems and most HVAC system designers believe that they can design them. Still, based on the relatively small number of hydronic installations, it’s likely that many designers do not have that much experience in these systems. Many will resort to design by rule of thumb, which sets up the potential for conflicts later.

Don’t rely on the technician

Often, “test and balance” or TAB is used after the system has been installed as the “philosophical” design method to achieve performance. This approach gives immense responsibility to the TAB field technician. Those technicians may not have the experience to implement the conceived design properly, and they generally do not have the benefit of the design calculations from the designer.

Getting performance from the hydronic system as “designed” requires a new paradigm of contract communications. There are established lines of communication from the designer to the general contractor. Similarly, there are established communication paths between the general contractor and its primary sub-contractors, and then from the subs to the specialty sub-contractors. Often TAB, temperature control and building automation will fall under the contract of the mechanical sub-contractor. As in the old child’s game of whispering from one ear to another, these several levels of separation mean that the message at the beginning of the line may become completely different at the end.

It is important to “close the loop” between the specialty sub-contractors and the design engineer of a hydronic system so that there will be no misunderstandings of the required functional performance and the selection of the components that go into that performance. While “commissioning agents” are now supposed to be this link throughout the design and construction process, we need to ask questions. Is there a commissioning agent? Has he been there from day one? Is he knowledgeable in hydronic systems?

Establishing an early dialogue with the specialty sub-contractors for a project is invaluable. Discuss the selection of control valves and processes, and the pipe head loss calculations. Doing detailed calculations for the system at this early stage may shift about 10%-20% of work typically done later in a project by a contractor to the beginning of the project. That’s a small price to pay when long-term performance is desired.

There are numerous standards that exist for hydronic systems. The valves used to balance and control the system flow rate may be tested against the Instrument Society of America ISA/ANSI Standard 75. The ISA provides a wealth of knowledge on control processes and applications. Granted, the ISA focuses on “process” controls, but the concepts are still appropriate.

Relying on standards

Pumps for the hydronic system are covered in Hydraulic Institute Standards Series 1 and 9, which cover most HVAC applications. The series documents the standard methods for establishing pump curves and pump installation. ISO standard 5167 establishes methods of measuring fluid flow using differential pressure measuring devices. These tools provide the opportunity to make repeatable and verifiable measurements of the hydronic system. Better yet, they can provide field and laboratory correlation.

Translating all of these standards into the appropriate design elements and then into field measurement and performance requires an artful interpretation and practical field experience to achieve high performance results. Always remember, the basics of the hydronic system will help you overcome most potential problems.

As a starting point for verifying a design, ASHRAE members can benefit from their research contributions by downloading (for free) research project RP-827. Its goal was to develop the methodology to make in-situ measurements of chillers pumps and fans. As with all research though, use common sense. Recognize that some control disturbances and interactions will show themselves only in the field.

Potential glitches with variable speed pumps

The control of pumps in variable speed applications is one interaction that has to be considered in situ. The “control area” has been the focus of considerable discussion within the hydronics community for many years. Essentially, valves are positioned in an open or closed mode, and the pump head and resulting system flow are calculated based on the resulting controlled reaction. Each combination of flow paths yields a discrete pump curve and system curve intersection point. When these points are connected graphically, a “control area” results that outlines the many potential operating points of the pump based on how the control valves respond to their individual temperature controllers. It would be wonderful if actual operation followed the “system curve” as traditionally known. However, in reality there are many non-linked control processes that influence others and create flow disturbances.

Calculating controlled effects such as the control area is easier done in design, rather than when one is trying to correct a potentially catastrophic problem later. Understanding the balancing method (yes, balancing is required), and the method of diversity application is highly important.

Recently I had discussions with peers about projects where these types of consideration were not given due attention. In these cases, “diversity factors” were applied to chilled water system designs; pipes and pumps were sized for the “diverse flow,” with load calculations to “back them up.” The projects didn’t work, and there was an immediate reaction to blame the balancer, or to look to the pump, when in reality the simple nature of “simple” control went unaccounted for.

What happened in these cases is that when it became too hot in a space the controller opened a valve. If the building environment gets out of control, eventually all of the valves will open — regardless of what the load calculation says! At that point, there is a tendency to assume that a “balanced system” means that the water will automatically go where it was intended. No such luck! The system will act as the hydraulics dictate.

Check-offs for these situations call for examining the following: the need for variable speed pumping over constant speed; the strategic sizing of pipe vis–vis friction loss placement for balance; proportional (manual) versus flow limiting (automatic) balancing; proportional vs. binary temperature control valve flow control. The list can go on and on, but the potential problem is created by the shift in design paradigms that the industry has embraced during the past 20 years.

Formerly, pumps were selected using “flat” pump curves. Establishing “controllability” was based on keeping the system operating variables constant as much as possible.

When energy concerns took over the design mindset, with the availability of inexpensive variable speed drives an assumption became “drives save money.” This can be true. However, the drive can also cause large deviations of the pressure drop across the control valve. This makes the dynamics of the temperature control process unpredictable. We also presume that temperature control valves are proportioning, let’s say incrementally positioned, so as to match the coil. That too is an assumption that requires challenging.

Inexpensive two-position control valves also can cause operational inequities. It is not uncommon to examine a cooling load profile and see that 80% of the operation hours will be at 30% of the design load or less.

If the temperature control valves were truly
proportioning and pumped with a controlled differential pressure of 20% of the design head, most of the pump operation hours (80%) could be at less than 3% of the pump design horsepower. Do we achieve this performance as an industry? Anecdotally, it would appear that the answer is no. The reasons are several. Over-application of the binary temperature control valve with a simple “on-off” thermostat is one contributing factor. It will eventually reach a state where every single control valve is open at the same time, forcing the hydronic system to go from no-flow to full-flow, albeit over a varying time period.

It is the control dynamics and disturbances, both contemplated and not contemplated, that govern system performance. When this dynamic is fully recognized, it becomes apparent why the pressure independent control valve is gaining in popularity. The anecdotal reason often cited for this type of control valve is that it “maintains system differential temperature.” That’s not true! However, the addition of a differential pressure regulator in series with the temperature control valve does allow for one control process to deal with the variable differential pressure being applied by the pump in reaction to changes in flow, while the temperature control process deals with the flow rate.

The coil needs to be carefully selected in order to produce the desired sensible and latent performance at part load conditions. The application of binary flow control valves comes in part from the belief that just because there is full flow there is full latent ability provided by the coil. While true, there are potential unintended consequences of increased pump and chiller energy consumption.

The above outlines some subtle points for achieving the efficient long term operation of a hydronic system, but there are still traditional design issues such as ensuring the proper installation of equipment, air and pressure management, gravity flow, etc.

Hydronic systems provide a fantastic opportunity to provide exceptional comfort. As building codes adapt to new materials and methods, we should see more economical installations.

Mark Hegberg is Manager of Engineered Specialties for ITT R&CW in Morton Grove, Illinois. He has chaired ASHRAE Technical Committee 6.1 Hydronic Systems Design and 7.7 Test and Balance, and has a BSME degree from Valaparaiso University.

Editor’s Note: the Canadian Hydronics Council (www.ultimatecomfort.ca) began working on an installation code a decade ago, which evolved into the official Canadian Standards Association CSA B214-07 Installation Code for Hydronic Heating Systems. This code will be acknowledged in the 2010 edition of the National Building Code of Canada.

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KEY ASPECTS TO DESIGN

Laser align pumps.

Install some “high quality” flow meters with the accompanying length of straight pipe.

Use good balancing design techniques on the piping system.

Don’t try to put too much in a confined area.

Don’t use the variable speed drive if you use rules of thumb as your only design calculation.

Spend some cash: recognize that sometimes you have to spend a little extra to get to the level of quality that will make a long term difference.


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