# Numerical Modeling and Optimization of an Air Handling Unit

^{1}

^{2}

^{*}

## Abstract

**:**

## 1. Introduction

^{−3}s

^{−1}. Using data from nearly 1000 audited fans used in HVAC systems, including AHUs, in Sweden, an average measured value of 1.5 kW m

^{−3}s

^{−1}was obtained. Apparently, this scenario was found in other countries too. Nowadays, in the more demanding projects in terms of energy efficiency requirements, the requested specific fan power for an AHU, for instance, can reach 0.1 kW m

^{−3}s

^{−1}. By analyzing the contract forms used by Swedish builders and consultants’ design practices, the two most important causes were identified. First, the lack of energy performance specifications by the builders due to the nonexistence of economic incentives and knowledge. Then, the builder has a first-cost minimization incentive, which combined with the first cause ends with the installation of HVAC systems with low initial cost rather than systems with an optimized life-cycle cost.

^{3}/h. For this purpose, a numerical model using the ANSYS Fluent software was developed. Furthermore, to improve the performance of the AHU, an analysis of other constructive solutions are evaluated to decrease the pressure drop in future products and, consequently, decrease their specific fan power.

## 2. Air Handling Unit

#### 2.1. Description of the Air Handling Unit

^{3}/h both in insufflation and extraction was required, to handle the air inside of the corresponding space. The AHU presented is equipped with a heat wheel, water heating, and cooling coils, mixing box, filters, fans, dampers, and a condensed tray. This is a very standard layout for a common AHU. Beyond the mentioned equipment, the AHU has panels, doors, top coverings, chassis, among others. Figure 1 shows the AHU under analysis.

#### 2.2. On-Site Measurements

## 3. Numerical Model

#### 3.1. Mathematical Model and Fan Modeling

^{−3}for continuity and 1× 10

^{−4}for energy, momentum, and turbulence equations. The pressure-velocity coupling is solved with the Semi-Implicit Method for Pressure Linked Equations (SIMPLE) algorithm, using the standard initialization. The SIMPLE algorithm provides more efficient and robust single-phase flows. The pressure is modeled with a pressure staggering option (PRESTO scheme). The contribution of the gravity acceleration was implemented in a downwards direction.

#### 3.2. Computational Domain, Mesh and Boundary Conditions

^{3}/h and, therefore, to reduce the computation time and effort, only 560,784 hexahedral elements are necessary.

^{3}.

## 4. Results and Discussion

#### 4.1. Validation of the Numerical Model to Nominal Operating Conditions

#### 4.2. Improvement of the Air Handling Unit Performance

## 5. Conclusions

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## Nomenclature

k | factor for each turbomachine, - |

p | pressure, Pa |

Q | Air flow rate, m^{3}/s |

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**Figure 6.**Velocity field inside the AHU: (

**a**) velocity contour and velocity vectors at medium height and (

**b**) velocity streamlines inside the AHU.

**Figure 11.**Velocity field inside the AHU with the FCU optimized: (

**a**) velocity contour and velocity vectors at medium height and (

**b**) velocity streamlines inside the AHU.

Parameter | Supply Fan | Exhaust Fan |
---|---|---|

Air volume flow rate (m^{3}/h) | 3850 | 3850 |

Pressure rise (Pa) | 729 | 308 |

Fan static efficiency (%) | 61 | 50.6 |

Working angular velocity (rpm) | 2342 | 1941 |

Nominal angular velocity (rpm) | 2600 | 2140 |

Electrical power input (kW) | 1.28 | 0.65 |

Nominal electric power input (kW) | 1.7 | 1 |

Local | Pressure (Pa) |
---|---|

Fan inlet | −212.40 ± 1.42 |

Fan outlet | −1143.80 ± 8.80 |

Bag filter inlet | 238.60 ± 7.83 |

Bag filter outlet | 78.40 ± 3.68 |

Parameter | Value |
---|---|

Air volume flow rate (m^{3}/h) | 4534 |

Pressure rise (Pa) | 451 |

Fan static efficiency (%) | 55 |

Working angular velocity (rpm) | 2227 |

Electrical power input (kW) | 1.03 |

Location | Boundary Condition | Parameter | Value |
---|---|---|---|

Inlet | Pressure inlet | Gauge total pressure (Pa) | −212 |

Turbulent intensity (%) | 3.8 | ||

Hydraulic diameter (mm) | 814.9 | ||

Outlet | Pressure outlet | Gauge pressure (Pa) | 78 |

Back flow turbulent intensity (%) | 3 | ||

Hydraulic diameter (mm) | 814.9 | ||

Fan wall | Wall | Stationary wall—no slip | - |

Bag filter frame | Wall | Stationary wall—no slip | - |

Parameter | Experimental Results | CFD Results | Error |
---|---|---|---|

Air flow rate (m^{3}/h) | 4534 | 3971 | 12.4 |

Fan differential pressure (Pa) | −938 | −1334 | 42.2 |

Fan static pressure increase (Pa) | 451 | 548 | 21.5 |

Bag filter inlet pressure (Pa) | 239 | 276 | 15.5 |

Bag filter outlet pressure (Pa) | 78 | 82 | 5.1 |

Bag filter pressure drop (Pa) | 157 | 194 | 23.6 |

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**MDPI and ACS Style**

Lopes, J.; Silva, J.; Teixeira, S.; Teixeira, J. Numerical Modeling and Optimization of an Air Handling Unit. *Energies* **2021**, *14*, 68.
https://doi.org/10.3390/en14010068

**AMA Style**

Lopes J, Silva J, Teixeira S, Teixeira J. Numerical Modeling and Optimization of an Air Handling Unit. *Energies*. 2021; 14(1):68.
https://doi.org/10.3390/en14010068

**Chicago/Turabian Style**

Lopes, José, João Silva, Senhorinha Teixeira, and José Teixeira. 2021. "Numerical Modeling and Optimization of an Air Handling Unit" *Energies* 14, no. 1: 68.
https://doi.org/10.3390/en14010068