Turn Up the Heat in a Rooftop Heating Unit

By Chait Johar, Project Engineer, AAON Inc., Tulsa, USA

In designing a new rooftop heating unit, AAON engineers needed to deliver higher airflow while maintaining the same footprint as an earlier design. The team used ANSYS computational fluid dynamics (CFD) software to calculate airflow through the heat exchanger of the unit and iterate to a design that meets energy efficiency, airflow and heat transfer requirements. The use of simulation in this project saved 60 to 80 hours of physical lab work compared to traditional design methods.

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Turn Up the Heat in a Rooftop Heating Unit

simulation for HVAC 

AAON rooftop packaged units incorporate a tubular heat exchanger and air handling system for efficient heating of commercial and industrial buildings. To design the tubular heat exchangers used in rooftop heating units, engineers must maximize the transfer of heat from the hot gas flowing through the tubes to the air flowing through the cabinet. This delivers a high level of energy efficiency. The unit must also withstand the service conditions of the environment while minimizing cost and size.

In the past, the design process involved building prototypes and performing physical measurements, such as testing the amount of heat that is transferred to the air flowing through the unit. Over the past few years, AAON has transitioned to using upfront simulation to optimize airflow and heat transfer prior to building physical prototypes. Simulation takes less time than building and testing a prototype, and it provides more complete diagnostic information, enabling engineers to iterate to an optimized design at a faster pace. AAON engineers achieved physical space, air handling and thermal efficiency goals while saving 60 to 80 hours of manual lab work.

AAON RQ
AAON RQ series rooftop units are engineered for performance, flexibility and serviceability.
heating unit
Unit skeleton with only the heat exchanger tubes (8 tubes) and the fan

Heat Exchanger Design Challenge

In designing the new rooftop unit, engineers had to increase the air handling capacity while maintaining demanding levels of efficiency and the same footprint as the previous-generation product. The hot gas enters the heat exchanger, is divided into internal tubes and is then released out of the unit. Fresh air is brought in for the next combustion cycle to supply oxygen. The shell of the heat exchanger guides air driven by a fan over the tubes. Traditional design methods rely upon handbook formulas and engineering judgment that typically focus on the surface area through which heat is transferred by convection between the hot gas and cooler air flowing through the cabinet. The heat transfer capability and efficiency of the unit are largely dependent upon the flow of air through the heat exchanger tubes: The airflow should be uniformly distributed around the tubes carrying hot gas.

A key limitation of build-and-test methods is that they generally do not account for the flow geometry and thus must make assumptions for the distribution of flow through the device. Due to these inherent inaccuracies in the traditional design process, soon after an initial concept design is generated, the lab builds a prototype and puts it through its paces. This process takes about eight working days. At this stage, the results are rarely good enough to meet product requirements, so the engineering team embarks on an iterative process of rebuilding and retesting the prototype. Engineers place thermocouples on the exterior of the tubes, which provide an accurate measure of the thermal performance of the prototype. But it is not practical to accurately measure the airflow around the tubes, so these tests provide very little diagnostic information on how flow patterns are impacting thermal performance.

centrifugal fan simulation

The axial, radial and tangential vector components of the centrifugal fans were determined by physical testing.

"Simulation takes less time than building and testing a prototype and provides more complete diagnostic information, enabling engineers to iterate to an optimized design at a faster pace."

Simulation Now Drives Design Process

Over the past few years, AAON has transitioned to a new approach in which engineers use simulation to evaluate more design iterations in less time. Simulation provides more diagnostic information and iterates quickly to an optimized design. In designing the new RQA-B rooftop unit, engineers needed to move more heat and more air through a unit that occupies the same footprint but is taller than existing units. The new unit had to achieve an energy efficiency of 81 percent to be certified in all the regions in which it is being marketed.

Engineers created an initial design iteration and modeled the unit in ANSYS CFX computational fluid dynamics software. A master’s student at Montana State University–Bozeman used physical testing to determine the axial, radial and tangential vector components of the airflow generated by different centrifugal fan sizes and speeds as a function of distance along the fan’s axis of rotation. These values were used as boundary conditions in the CFD model. The wall function approach was applied to model the boundary layer profile with a reduced cell count. Inflation layers were employed in the fluid domain near the tubes to provide a sufficiently fine mesh to accurately capture this region, where the flow experiences rapid changes in velocity, pressure and temperature. The placement of the first node in the mesh at the end of the tubes is particularly important. A nondimensional distance based on local cell fluid velocity called y+ ensures that simulation accuracy in this area is acceptable. With the k-epsilon turbulence model that was used in this case, a y+ value of less than 100 is recommended. AAON engineers tweaked the mesh to keep y+ below 100.

heating unit simulation
Initial run showing air leaving the fan and exiting the cabinet. Much of the air did not pass through the heat exchanger tubes.
heating unit simulation
Baffles were added to divert the airflow and improve flow distribution.
heating unit simulation
Streamlines depict the temperature changes through the cabinet.
heating unit simulation
Temperature on a cross section of the unit shows air temperature rising downstream of the cabinet.

AAON Reduces Physical Prototyping

The simulation results showed that the energy efficiency of the initial design was well below the required levels. AAON engineers also built a prototype of the initial design and used it to validate the simulation results. Looking at the flow and temperature distribution in the cabinet and tubes, AAON engineers could see that a significant proportion of the air flowed past the heat exchanger tubes into the outlet without ever coming into contact with them. Based on these results, AAON engineers added baffles to the cabinet to redirect flow that had been bypassing the tubes. Using simulation, they were able to digitally explore different baffle positions and geometries, along with different positions of the tubes relative to the sheet.

Each simulation run took 6 to 8 hours on a single core, so AAON engineers set up multiple runs when they left work in the evening. They are now using a computer with 4 cores, which has reduced the solution time to 1.5 to 2 hours. Guided by the flow simulation results, engineers rapidly iterated to a design that more efficiently routes air through the cabinet. The average air velocity through the heat exchanger tube faces increased by almost 25 percent, and the temperature into the outlet went up several degrees with the same flow rate through the cabinet. Engineers built a prototype of the optimized design, and the results closely matched the simulation results, providing an efficiency of 82 percent. AAON is currently ramping up production of the new rooftop unit and preparing to bring it to market. The AAON test lab manager estimated that, in this one application, simulation saved 60 to 80 hours of physical lab work, representing a substantial cost savings. Simulation also generated incremental revenues by bringing the product to market earlier than would have been possible using the build-and-test method.

CFD velocity simulation
Flow velocity plot for initial design shows much of the air is flowing through the cabinet perimeter.
CFD velocity simulation
Flow velocity plot for final design with baffles added shows much more air is flowing through the tubes.

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