Boilers: Types, applications, and efficiencies
Primary/secondary boiler piping systems: Decoupling the boiler piping flow loop from the building flow loop has become more common in the past several years. The concept of primary/secondary pumping has been around since the 1950s. It has been primarily used in chilled water systems. The advantages include the ability to have non-equal flow in two loops that are connected by a common “decoupler” section of piping (see Figure 3). This is a similar arrangement to a primary-secondary chilled water piping system. The primary loop allows for constant flow through the operating boiler while allowing the secondary building loop to act semi-independently of the primary loop. The helps avoid many of the flow issues with primary-only piping arrangements (such as low flow, flash steam in tubes, and increased fouling of the tubes due to low tube velocities).
All of the hot water boilers listed in Table 1 can be applied successfully in a primary-secondary piping arrangement. Special care, as previously stated, must be used when applying cast iron, straight water tube, or fire tube boilers to ensure the maximum water temperature difference (between supply water and return water to and from the boiler) is not exceeded throughout the boilers’ operating range.
Caption: All efficiencies are typical and based on natural gas as the fuel. The efficiency ranges are typical based on a multiple manufacturers published data.
Maximizing efficiency in fuel-fired hot water boiler systems is dependent upon a few key design points. These points, for consideration by the system design engineer/designer, are as follows:
- Accurate heating load for the building based upon conduction loss without credit for lighting, people, and equipment.
- Determining the minimum heating load for the system during the “shoulder” months (late winter through spring, late fall) for the particular location. This can be accomplished by modifying the computer based heating load calculation to provide a typical energy model for the building or by specifying the month to determine the lowest heating requirement. This is relatively easy to accomplish using Trane TRACE 700, Carrier HAP, or similar software that allows for typical-year or full-year energy modeling.
- Selection of high-efficiency or ultra-high-efficiency boilers to match the full range of the calculated heating load. In many cases, the use of more than two boilers is suggested to allow for a good range of boiler “turn-down” capacity to meet the instantaneous heating load of the building. An example is to use three boilers each at one-third of the required system capacity. This will allow for the traditional two-thirds capacity “reserve” or back-up while allowing for greater controllability of the heating water system. Additional boiler(s) may be required for full redundancy to meet the building owner’s requirements for N+1 or greater reliability.
- Consider the modular boiler approach (noncondensing type or condensing type) with lower boiler mass and minimum of 5:1 turn-down ratio.
- Use nontraditional controls to manage staging of multiple boilers to match the instantaneous heating load. Boiler staging should be accomplished by trending and matching the supply water temperature required in the secondary loop at any given time.
- Monitoring heating coil control valve position versus required supply air temperature. Use this information to determine the secondary loop supply water temperature setpoint.
- Enable and disable each boiler based on the building energy management system calculation of the required system heating load. Once enabled, allow each boiler to fire and modulate on its own controls. Many packaged or modular boiler manufacturers have optimization hardware/software to maximize the efficiency of their equipment and that can be provided with energy management system interfaces or nonproprietary communications protocols such as BACnet. This writer suggests the use of optimization programming provided by the boiler manufacturer in multiple boiler design installations with the use of “rolling” sequence control (1,2,3,4; 2,3,4,1; 3,4,1,2; etc.) to maximize/exercise the use of each boiler in the system. If this control/programming is not available from the boiler manufacturer, it can be readily accomplished by the temperature control contractor.
- Use flow meters and temperature difference in the secondary supply and return main piping to calculate the current boiler capacity required by the heating system. This is a simple yet extremely effective method of matching the boilers to the instantaneous building heating demands.
- Provide automatic boiler isolation valves to allow non-required boilers to be isolated from the primary boiler loop. This works only for boilers capable of withstanding thermal shock. Do not use this on cast iron, straight water tube, or fire tube boilers without using the previously mentioned thermal shock prevention techniques. (Individual boiler circulation pumps with blending valve to allow for slow warm-up.)
- Provide boiler stack dampers to reduce heat loss of non-firing boilers.
- Use individual primary pumps dedicated to each boiler. The use of automatic balancing valves may help ensure proper flow through each boiler over the entire load operating range of the system.
- Use variable flow secondary system to reduce system pumping requirements.