June 20, 2006

Checking stack temperature is one of the simplest and least expensive methods to use to verify whether your boiler/burner unit is operating efficiently and within its rated capacity. All that is required is a thermometer. Smith Cast Iron Boilers is including a stack thermometer with every Series 28A and 28HE boiler. The stack thermometer is installed in a threaded opening located on the left side of the smoke-hood.

Normal expected high fire steady state operating temperatures on newly installed 28A or 28HE series boilers are noted on the chart shown below. Your new boiler should be operating near the recommended temperature.

Recomended Stack Temperature

Model 28A -  Nat Gas = 475F(+25),  #2 Oil = 500F (+25)

Model 28HE -  Nat Gas = 375F(+25),   #2 Oil = 425F (+25)

All Stack Temperatures are gross numbers

Now, before anyone gets too excited if they see an elevated temperature let’s discuss the details involved with stack temperature. A stack thermometer must be calibrated to guarantee accuracy. A calibrated gauge would show readings that will be close to the numbers provided on the above chart if the burner is properly adjusted and the boiler is new and clean. Gauges are sensitive to shock and vibration and may get out-of-calibration during shipping and handling operations.

Other elements that can effect stack temp are room temperature and excess air, which is indicated by the CO2 or O2 settings. The numbers represented on the chart above are the gross stack temperatures, since that is what the temp gauge reads. Gross stack temperature equals the actual stack reading plus the room temperature. We are assuming 75° F. room temperature. Any variance in room temperature will change the gross stack temp by the same amount. In addition, the excess air setting of the burner will effect the stack temperature. Many of today’s power burners can operate cleanly down to 15% excess air. A good burner technician knows how to improve your fuel economy by optimizing the burner combustion adjustments.

We advocate using the listed stack temperatures as a guide. If the unit is new and has just been started up and the temp gauge is reasonably accurate the stack temperature should be close to the recommended temperature. A temperature reading of 25° F. or more above the advised temperature could indicate a problem such as a misadjusted burner, or the burner might be firing above the boiler’s rated input. When the stack temperature is elevated it would be prudent to have a burner technician recheck the burner adjustment and firing rate. Overfiring the boiler and improper burner adjustments violates the boiler warranty.

Stack temperatures will rise over time as the unit gets dirty. This is due to scale and rust build-up in the heat exchanger. One method to utilize to verify if a boiler is getting dirty is to record the stack temperature when the unit is new or has just been cleaned and the burner is properly adjusted. When the temperature rises 100°F above the recorded (starting) number the unit needs to be serviced. A stack temperature that has increased 100° F is indicative of a dirty heat exchanger. This may originate from a smoky fire or condensing in the heat exchanger causing scale buildup but could be initiated by a host of other issues. No matter what the cause, it is time to have the unit serviced by a professional.

It is a common practice particularly with hot water boilers to under-fire them. Most boilers are oversized for their connected heating load during the majority of the heating season. This is because the boiler is sized for peak (coldest) demand, which is only reached a few times during the heating period. Because of the extra capacity the boiler has available during the "normal" winter temperatures the burner may short cycle numerous times in an hour. This syndrome is exacerbated when there are multiple boilers involved.

Even when modulating burners are used the units can short cycle. The ideal situation, especially if you have a low-hi-low or full modulation burner, is to have the burner operating in low fire most of the time. This keeps the on/off cycling to a minimum. Cycle rates in excess of 3-4 an hour are considered short cycling, which can have adverse effects on the boiler and burner, as well as diminishing efficiency and fuel savings. The way the building management control operates the burner/boiler also becomes a critical factor in preventing short cycling, but that is an issue for a future discussion. Under-firing the boiler is one way to minimize short cycling and increase fuel savings while not breaching the boiler warranty agreement.

Optimizing operating efficiency, minimizing fuel costs, and increasing fuel savings are subjects on everyone’s minds these days. Monitoring stack temperature is a simple and inexpensive way to assure a properly adjusted heating unit, reduce service calls, save fuel, and is a valuable added feature that also helps ensure boiler warranty coverage.

Tom Ferrante

Technical Service Manager

To provide adequate gas pressure to the gas unit,

size the gas piping as follows:

1. Find the cu. ft./hr. by using the following formula:

Cu. ft./hr. =Input

                 BTU

2. Refer to Table 1. Match “Length of Pipe in Feet”

with appropriate “Gas Input - Cu. Ft./Hr.” figure.

This figure can then be matched to the pipe size at

the top of the column.

Example:

It is determined that a 67 foot (20.4m) run of gas

pipe is required to connect a 75 MBTU gas unit

heater to a 1,000 BTU/cu ft. (0.29kW) natural gas

supply.

75,000 BTU/Hr

1,000 BTU/cu. ft.= 75 Cu. ft./hr.

Using Table 1, a 3/4 inch pipe is needed.

“Boiler Thermal Shock” can be loosely defined as sudden thermal change that occurs within the boiler causing rapid and uneven expansion and contraction of a boiler’s structure.  The problem of thermally induced stress has been apparent for many years, particularly in hot water heating systems.  Several conditions can contribute to boiler stress and eventual cracking. All involve introducing excessively low temperature water or cool water at high flow rates into a hot boiler.  The term “shock” suggests a sudden impact type failure, which in the vast majority of cases is far from what actually happens.  Most failures of this type occur over a period of time, sometimes materializing within as short a time frame as a few weeks, but occasionally a considerably longer time period ensues before damage is detected. 

Failures are typically of the iron or metal fatigue type and are caused by thermally induced stress cycling of the boiler structure.  Thermally induced cyclic stresses are due to the resistance of the boiler structure to the movement caused by the thermal expansions and contractions within the boiler.  The stresses occur during every firing cycle of the burner, a cycle defined as burner on, burner off.  Failures of this type appear as leaks from sectional cracking. Malfunctions like this are not catastrophic in nature but are serious in terms of downtime and repair costs.  Equipment manufacturers and system designers have devised many approaches over the years to combat this problem.  Some have been successful and some have demonstrated a lack of understanding of the nature of these failures. 

For their part, manufacturers of cast boilers have learned to pay attention to the shape of the boiler casting.  Sharp radius corners and abrupt changes in the thickness of cast metal can amplify stresses encountered during operation.  Some advancements such as the development of the wet base designs in the late 1930s seemed to dictate that these shapes and creative approaches were needed to regain the stress relief of the oval section design found in older designs such as Smith’s Mills boiler. Fortunately, the development of finite element analysis and other computational tools has allowed manufacturers to adjust designs without the trial and error type of development that characterized the 19^th century design process.

Manufacturers of cast boilers can also limit the effects of thermal stress by varying their pouring process.  The metal mix, addition of trace elements, pour temperature, and cooling times all play a role in producing cast sections that stand up better to challenges in boiler system recognition. It is important to point out that there are 100-year-old cast iron boilers still operating.  This undeniable fact indicates that while advances in manufacturing are quite important, what a system demands of a boiler has a lot to do with its longevity.

A rule of system design is that boiler supply water temperature and the system return water temperature shall not exceed a 40° F. delta T. This figure is commonly reduced to 20° F. delta T for steel boilers.  Boiler protection is frequently ignored when system design is focused heavily on saving energy.  Systems incorporating night setback and/or weekend shutdown are designed to save energy by reducing space temperature set points, thus reducing the building’s heating requirement.  Turning down or shutting down the building’s temperature causes the system to become ambient.  When the building moves to an occupied mode all the zones at the terminal unit open. The pumps are then enabled, and the ambient system water is injected into a hot boiler.   Data from both laboratory stress analysis and field test programs indicate that failure is often the result of an ongoing conflict between the energy conservation requirements imposed on the control system designer and the requirements of the boiler manufacturer to protect the structural integrity of the heating equipment.

A common scenario has the Building Automation System (BAS) control bring the boilers up to operating temperature after a night or weekend setback. Systems incorporating night setback and/or weekend shutdown are designed to save energy by turning down or shutting off the building s temperature.  This, however, causes problems when all the zone valves and pumps come back on, delivering room temperature water to a hot boiler.  Once the set point has been reached the BAS calls for all the zones to be energized simultaneously.  The large reservoir of cold (ambient) water in the system is immediately brought back to the boiler at a high flow rate, resulting in a temperature differential greater than the boiler can endure.

Heating systems that have boilers maintaining temperature without flow are susceptible to thermal shock by sudden changes in flow due to pump operation. Controlling the load imposed on the boiler can prevent waterside thermal shock.  Boiler load is a function of flow rate and temperature difference. One of the most effective methods known to prevent thermal shock is to create a boiler loop separate from the system and pump it with its own circulator. Since the flow rate is constant, the temperature difference across the boiler becomes the measurement of the boiler s load, and if the boiler is maintaining temperature, the return water s temperature will determine the boiler load. Control against "boiler shock" involves control of the incoming cold water flow rate so that the boiler s temperature is changed slowly.

Additionally, a common cause of thermal shock is a system that incorporates outdoor reset with 3-way valves while the boiler maintains temperature. The boiler is set at 180° F, but based on outdoor temperature the system may require only 100° F. (60° F. outside air).  The return temperature can be as low as 90° F., which can cause a 90° F. differential across the boiler.  Most cast iron boiler manufacturers would like to see no more than a 40° F. temperature difference between the boiler s return and leaving temperature.

By installing the 3-way valve in the boiler loop, the outdoor reset can control the amount of hot water that is introduced into the system based upon a reset schedule. More importantly, the reset controller can measure the return temperature entering the boiler. If water temperature becomes too low for the boiler manufacturer s recommendations the 3-way valve will close off the system loop. Hot water from the boiler will then be pumped right back into the return, raising the water temperature entering the boiler. The 3-way valve and controller will float back and forth, resetting the supply water to the system while protecting the boiler from cold water.

Dual temperature changeover systems can experience boiler problems when the system tries to change over from a cooling demand to heating. The piping system and terminal units are filled with water at temperatures of 50 - 60° F; however, the boiler may contain 180° F. water.

Heat pump loop systems typically require some form of supplementary heat to maintain supply water loop temperatures when the outdoor temperature approaches design conditions.  Boilers are the common source for this additional heat, but design loop temperatures are as low as 70 - 85° F., while non-condensing commercial cast iron boilers do not operate below 140° F.

A dependable method for interconnecting this boiler loop with the system loop is through primary/secondary pumping techniques.  By keeping the supply and return tees close together the pressure drops in the common piping are kept to a minimum. This allows different size pumps to coexist in the system without affecting each other as well as preventing ghost flows from occurring from one loop into the other.

BAS or Building Automation Systems are becoming the norm in most new construction or building renovation design.  These systems control HVAC operation and equipment. The benefits of BAS include simplified wiring and piping in addition to energy-efficient operation, flexibility in meeting diverse building use, and increased occupant comfort. These benefits cannot be achieved unless the BAS and the personnel operating them properly understand the heating equipment and its controls.  All phases and modes of operation must be anticipated prior to putting the heating system on line. 

The capabilities and operating limitations of the equipment, regardless of the material of construction, must be understood and accounted for to avoid risks of equipment damage.  Cast iron is a very convenient material to use in heating equipment.  It offers longevity and excellent heat transfer characteristics at a reasonable price.  However, it is a brittle material and cannot withstand significant abrupt temperature changes.  Steel boilers are built from different grades of material. Welds expand and contract at rates different from the metal they join.  Copper boilers, particularly operating with hard water conditions, must operate with minimum and maximum flow rates for long life. 

BAS management personnel seem to understand the philosophy and operational sequence for loading and unloading chillers.  However, when it comes to heating equipment, these units are treated as on/off appliances. For example, a boiler with an input rate of 4000 MBH is similar to a chiller rated 333 tons.  No control program would allow the enabling and disabling of a 333-ton chiller to be controlled on/off.  The chiller must be staged on and staged off.  This is mandatory for boilers as well to insure safe and efficient operating equipment.

Following are some typical findings from projects where thermal stress has been suspected:

1.         All systems had a reset schedule for the building system temperature based on the outdoors-ambient temperature. This resulted in boiler return temperatures that were lower than manufacturers recommendations.  Return temperatures as low as 90 degrees F. were tolerated in mild weather.

2.         Boilers were oversized for the load, especially during the mild weather of spring and fall. This resulted in excess cyclic operation of the boiler, one of the key ingredients for a fatigue failure.

3.         Burners’ operating and modulating controls were improperly set in relation to one another, resulting in high burner cycling rates to high firing rates. Operation of the same boiler at the same outdoor temperature when the burner controls had been properly set showed that cyclic operation had been eliminated for the outdoor temperature in question. Burner firing rate, and hence stress level, was also reduced considerably.  In this particular case, no further failures were experienced.

4.         Electrical load shedding is a factor to be considered in systems where air handling units are shut down for many hours, allowing large volumes of water to cool down. On system restart, large volumes of relatively cold water can enter the boiler in a short period of time unless the proper preventive measures are taken.

Protective Measures against Thermally Induced Stress Cycling (Thermal Shock)

It may not always be possible to design a boiler structure within the confines of boiler design codes and at the same time encompass thermally induced operating stresses that fall below the fatigue limit of the materials of construction.  This is especially true when one considers that heating system operating parameters controlled by the BAS are most often unknown to the boiler manufacturer. It then becomes necessary for the boiler manufacturer to set guidelines for the system-and-controls designers to use, in order to minimize these effects.

Typically these guidelines may include:

1.         Maximum Delta T across the boiler.

2.         A minimum water return temperature to the boiler.

3.         A minimum water flow rate through the boiler.

4.         Recommendations as to how to set burner controls to maximize the boiler shell temperature for a given operating pressure and minimize the number of operating cycles and the burner firing rate for a given load condition.

In designing a system, there are two key points to keep in mind.  First, prolonged periods of low firing rates are preferred to a series of on-off cycles to high firing rates. Secondly, an attempt should be made to isolate the boiler as much as possible from system temperature changes.

These targets can be achieved in two steps:

1.         Correctly setting the burner operating control in relationship to the boiler operating pressure and limiting the temperature of the water returning to the boiler below which the burner would be held at low fire to reduce stress levels. It should be noted that in some systems, design or operating parameters might preclude the use of low-fire hold devices, whether they are time delays or aquastats.  This is due to the fact that the system will be unable to attain its design operating temperature without the burner being allowed to achieve higher firing rates. In this case a manually supervised start-up should be employed.

2.         The common method of minimizing the effect of heating system temperature changes on boiler bulk water temperature is to use a two-loop system. One loop of the two-loop system is the boiler loop, which is operated at a constant temperature set in accordance with the guidelines referenced above.

Typically, this loop would have a minimum flow requirement imposed by the boiler manufacturer. (In the absence of a detailed knowledge of system operating parameters, a rule of thumb is 0.5 to 1.0 GPM per BHP, depending upon the boiler manufacturer.) This sabotages any tendency for temperature stratification within the boiler, as well as attempting to achieve the desired level of shell temperature.

The second loop is the building system(s) loop in which the temperature will vary in response to an outdoor temperature reset schedule, and in which flow rate may vary to meet energy conservation criteria. The interface between these two loops is usually either a three- or four-way valve, to which the building system temperature reset schedule is applied. This allows sufficient water from the hot boiler loop to blend with the cooler water returning from the building heating system to achieve the temperature requirements prevailing at any given time.  Obviously, many system configurations will meet this basic concept.

Reducing the number of stress cycles is achieved first by correctly setting the burner operating and modulating controls in relation to one another. This means that the modulating control must not send the burner to the high fire position immediately after the operating control has initiated the firing sequence and the main flame is established.

When a BAS system is used to sequence the boilers and directly reset water temperature in relation to outdoor air temperature the three-way valve that can be a source of thermal stress may be eliminated.

With the BAS providing direct hot water reset the control designer needs to remember that he must not only reset the “enable” point of the boiler but also should reset the firing rate control to a set point slightly below the “enable” point.  This step will prevent the burner from immediately driving to High Fire and the short-cycling and undue thermal stress consequence.

The second method of reducing the number of stress cycles is to carefully select boiler size and burner turndown rates (high-fire fuel flow/low-fire fuel flow) for a given boiler room. The heating season load profile must be matched, paying particular attention to the low-load segments in the spring and fall, as it is in these periods when there will be an inclination for the boiler/burner to cycle on and off more frequently. Selecting the number and size of boilers to suit a seasonal load profile may mean that the boilers will not all be of equal size, but rather a smaller boiler will be used in the spring and fall with larger boilers handling the high load of the winter months. Alternatively, a large number of smaller boilers may be used with a lead/lag control system (the modular approach). Properly selecting boiler sizes for a seasonal load profile has the added benefit of energy savings in terms of reduced heat loss from the boiler shell. It is always possible that financial constraints will preclude the ideal size selection of boilers and their numbers for a given installation. However, if this is kept in mind as a goal a better system will result.

Conclusion

Failures caused by what is commonly called thermal shock are actually fatigue malfunctions originated by thermally induced stress cycling.  They are not an indication of boiler design or manufacturing deficiencies, as has been inferred on occasion, but are rather due to the manner in which the heating system has been designed, controlled or operated. In some cases, a lack of knowledge may be an explanation, while in others financial constraints may play a part.  Remember that a boiler is not a light bulb to be turned on and off in an effort to conserve energy.  It is an engine that when warmed up and driven first at moderate speeds will provide both economy and long life.

ANDREW L. WOLF AND THOMAS E. NEILL

APPLICATION ENGINEERING/TECHNICAL SERVICES

Enter CD into Computer

Make sure Computer is connected to the Internet
 

Enter your SalesAssistant User Name and Password.

If you do not have an account go to www.mesteksa.com

If you are a new user click link for new users, otherwise contact an administrator if you are having problems getting access to the business units you require.

Make sure you select the business units that are applicable. 

When you receive your username and password continue with the installation.  Click the next button and accept the terms of agreement to install Specifiers.  Preview which programs you want to install.  These are based on your registration in SalesAssistant.  If you are missing the Specifiers you want to install hit cancel and confirm your registration in SalesAssistant by contacting an administrator.  www.mesteksa.com

If everything is ready to be installed click the continue button and then click the install button.

The mechanics behind the installation process:

This process installs a local copy on your machine and goes out to a web service to check your registration state for certain programs and grants access for installation based on these values.  These are set by your registration in SalesAssistant.

The SalesAssistant is our premier ASP, (Application Service Provider) extranet application.  It is an e-commerce tool that provides our outside sales force with an information bridge to Mestek, Inc.   The SalesAssistant is a secure web space where Mestek can communicate confidential information to individual or groups of reps/customers depending on the need.

The installation process will install the SalesAgent, which is the front-end order entry vehicle for the Specifiers.  It will also install Business Unit specific Specifiers based on your registration state.