# Numerical Study of Hydrocarbon Charge Reduction Methods in HVAC Heat Exchangers

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## Abstract

**:**

## 1. Introduction

## 2. Tested Systems

#### 2.1. Heat Pump Architectures

#### 2.2. System A—Fin-and-Tube Heat Exchanger

#### 2.3. System B—Brazed Plate Heat Exchangers

## 3. Simulation Method

#### 3.1. HXSim

#### 3.2. IMST-ART

## 4. Results and Discussion

#### 4.1. Fin-and-Tube Heat Exchanger Designs

#### 4.2. System Comparison

#### 4.3. Charge

#### 4.4. System Effectiveness

## 5. Conclusions

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

## Nomenclature

Greek | |

$\beta $ | Spiral angle |

${\delta}_{30}$ | Percentage of predicted values with less than 30% error |

$\eta $ | Efficiency |

$\gamma $ | Fin angle |

Roman | |

BPHE | Brazed plate heat exchangers [-] |

COP | Coefficient of Performance [-] |

${d}_{i}$ | Fin tip diameter for MF tubes, internal diameter for smooth tube [mm] |

${d}_{o}$ | Outer diameter [mm] |

EER | Energy Efficiency Ratio [-] |

h | Enthalpy [kJ kg${}^{-1}$] |

HTC | Heat Transfer Coefficient [kWm${}^{-2}$ K${}^{-1}$] |

${l}_{f}$ | Fin height [mm] |

m | Refrigerant mass [g] |

n | Number of fins [-] |

p | Pressure [Pa] |

${P}_{r}$ | Refrigerants side heat exchange perimeter [m] |

Q | Capacity [kW] |

q | Heat flux [kWm${}^{-2}$] |

${R}_{x}$ | Heat exchange area ratio of a MF tube to a smooth tube [-] |

SM | Smooth tube [-] |

T | Temperature [°C] |

${t}_{w}$ | Wall thickness [mm] |

${U}_{r}$ | Refrigerants overall heat transfer coefficient [kWm${}^{-2}$ K${}^{-1}$] |

W | Power [W] |

x | Vapor quality [-] |

z | Length of tube simulated [m] |

Subscripts | |

$sat$ | Saturated condition |

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**Figure 1.**Overview of the tested systems. System A: direct system with fin-and-tube heat exchangers, System B: indirect system with plate heat exchangers and intermediate brine and water loops.

**Figure 4.**Brazed plate heat exchanger Geometry: (

**a**) General parameters (

**b**) Plate design of a sinodial herringbone-type plate (

**c**) Side view of a symmetrical pattern (

**d**) Side view of an asymmetrical pattern.

**Figure 5.**Heat transfer coefficients based on correlation of two-phase flow (x < 1) and single phase vapour flow (x > 1) (blue) and smoothing function at dry-out region (0.8 < x < 1.1) for HXSim program.

**Figure 9.**Charge distribution for all tested heat exchangers, numbers on top of figure show the charge for channel/core and port/header.

**Figure 10.**Total inner volume of the heat exchangers, numbers on top of figure show the volume for channel/core and port/header.

**Figure 11.**Total charge of the heat exchangers, numbers on top of figure shows the charge for inside/outside heat exchangers.

**Figure 12.**Coefficient of performance (COP for winter) and energy efficiency ratio (EER for summer) in different configurations.

**Table 1.**Parameters set based on standard DIN EN 14511 and DIN EN 14825 for summer and winter conditions.

Thermal Capacity [kW] | Ambient Dry-Bulb Temperature (Wet-Bulb Temperature) [°C] | Inside Dry-Bulb Temperature (Wet-Bulb Temperature) (System A) [°C] | Water Temperature (System B) [°C] | |
---|---|---|---|---|

Summer | 5 | 35 (24) | 27 (19) | 23/18 |

Winter | 8 | −7 (−8) | 20 (max. 15) | 30/35 |

Unit | Smooth Tube | MF1 | MF2 | |
---|---|---|---|---|

Outer diameter (${d}_{o}$) | $\mathrm{m}\mathrm{m}$ | 5 | 5 | 5 |

Internal diameter ${}^{a}$ (${d}_{i}$) | $\mathrm{m}\mathrm{m}$ | 4.1 | 4.32 | 4.26 |

Wall thickness ${}^{b}$ (${t}_{w}$) | $\mathrm{m}\mathrm{m}$ | 0.45 | 0.22 | 0.22 |

Actual cross sectional area | ${\mathrm{m}\mathrm{m}}^{2}$ | 13.2 | 15.7 | 14.8 |

Effective diameter ${}^{c}$ | $\mathrm{m}\mathrm{m}$ | - | 4.47 | 4.34 |

Fin height (${l}_{f}$) | $\mathrm{m}\mathrm{m}$ | - | 0.12 | 0.15 |

Fin number (n) | [-] | - | 35 | 56 |

Fin angle ($\gamma $) | ${}^{\xb0}$ | - | 35 | 15 |

Spiral angle ($\beta $) | ${}^{\xb0}$ | - | 15 | 37 |

Heat exchange area ratio (${R}_{x}$) | [-] | 1 | 1.51 | 2.63 |

Air Side Fin Pitch [mm] | Vertical Tube Pitch [mm] | Fin Thickness [mm] | Air Face Velocity [m s${}^{-1}$] | |
---|---|---|---|---|

Inside Unit | 2 | 50 | 0.075 | 2 |

Outside Unit | 3.2 | 50 | 0.09 | 5 |

Unit | BPHE1 | BPHE2 | BPHE3 | |
---|---|---|---|---|

Number of Plates | - | 40 | 36 | 40 |

Height | mm | 471 | 324 | 328 |

Width | mm | 81 | 94 | 90 |

Depth per plate | mm | 2.3 | 1.46 | 0.95 |

Inner volume (Refrigrant) | L | 1.0 | 0.53 | 0.33 |

Inner Volume (Secondary Fluid) | L | 1.1 | 0.54 | 0.34 |

Heat transfer area | ${\mathrm{m}}^{2}$ | 1.50 | 0.95 | 0.78 |

Port Diameter | $\mathrm{m}\mathrm{m}$ | 20 | 27 | 25 |

**Table 5.**Correlations used for prediction of two phase flow characteristics, values in parenthesis present the $\delta $ 30% for both tubes.

Evaporation ($\mathit{\delta}$ 30%) | Condensation ($\mathit{\delta}$ 30%) | ||
---|---|---|---|

HTC | Smooth | Liu and Winterton [21] (100) | Dorao and Fernandino [22] (100) |

MF | Rollmann and Spindler [23] (MF1 = 100, MF2 = 66.7) | Cavallini et al. [24] (MF1 = 100, MF2 = 5.4) | |

$\Delta $P | Smooth | Xu and Fang [25] (100) | Xu and Fang [25] (100) |

MF | Diani et al. [26] (100) | Diani et al. [26] (MF1 = 98, MF2 = 100) |

Tube Type | Unit Location | Parallel Circuits (Rows) | Passes in Each Circuit | Total Heat Exchanger Tube Length [m] |
---|---|---|---|---|

Smooth (SM) | Inside unit | 8 | 4 | 33.6 |

Outside Unit | 18 | 4 | 46.8 | |

MF | Inside unit | 9 | 2 | 17.1 |

Outside Unit | 19 | 2 | 22.8 |

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**MDPI and ACS Style**

Allymehr, E.; Skaugen, G.; Will, T.; Pardiñas, Á.Á.; Eikevik, T.M.; Hafner, A.; Schnabel, L.
Numerical Study of Hydrocarbon Charge Reduction Methods in HVAC Heat Exchangers. *Energies* **2021**, *14*, 4480.
https://doi.org/10.3390/en14154480

**AMA Style**

Allymehr E, Skaugen G, Will T, Pardiñas ÁÁ, Eikevik TM, Hafner A, Schnabel L.
Numerical Study of Hydrocarbon Charge Reduction Methods in HVAC Heat Exchangers. *Energies*. 2021; 14(15):4480.
https://doi.org/10.3390/en14154480

**Chicago/Turabian Style**

Allymehr, Ehsan, Geir Skaugen, Torsten Will, Ángel Álvarez Pardiñas, Trygve Magne Eikevik, Armin Hafner, and Lena Schnabel.
2021. "Numerical Study of Hydrocarbon Charge Reduction Methods in HVAC Heat Exchangers" *Energies* 14, no. 15: 4480.
https://doi.org/10.3390/en14154480