Modelling an air handling unit, building and occupant, regarding energy, moisture and frost protection

Abstract

Frost formation in heat exchangers is an often accruing issue in cold climates. The aim of the project is to calculate the risk of frost formation in different Swedish cities by modeling vapor and frost behavior in heat exchangers.

Popular Abstract

Objectives of this paper are firstly, to analyze selected indoor and outdoor climate conditions, secondly, to propose a reasonable simulation model, thirdly, to analyze risk of frost formation and discuss different frost protection strategies.
Using psychometrics. ASHRAE heat exchanger calculation. condensation and frost calculation methods, heat plate exchanger and enthalpy wheel were simulated using Python programming language. Calculations were done using two methods.
The first method was to take the heat exchanger as a whole. using efficiency as a static number for simplification purposes. The heat exchanger was assumed to be a dimensionless unit. Using this type of approach. the performances of the crossflow plate heat exchanger and enthalpy wheel were simulated for every hour of the year.
The second method was to simulate the crossflow plate heat exchanger in more detail. A two dimensional model was subdivided into small squares. Subdivision of the heat exchanger module allowed to simulate the exchanger in greater detail to see where the condensation and frost happened and if it blocked exhaust airstreams.
The calculation of the heat exchanger module was done in the following steps: The first step was to divide the U value according to the subdivision of the heat exchanger. Then, estimated flow rates were subdivided accordingly. Then, each subdivision was considered as a ’’small’’ independent exchanger, while the whole module was considered as a chain of many small exchangers. Putting the values into matrixes made it possible to analyze data for every hour of the year.
It was concluded that the risk of frost in heat exchangers was highest in Kiruna and Ostersund due low outdoor air temperatures during the winter, however heat recovery systems in Copenhagen and Gothenburg still had a risk of frost formation even though these cities had relatively warm weather conditions. The study showed that moisture supply was unevenly occurring within building’s apartments, suggesting that the moisture load in a heat exchanger could be reduced by having less, but bigger heat exchangers that gather exhaust air streams from several apartments at once.
Out of several simulation models that were created during the study, the detailed model that takes condensation effect into consideration, was proven to be worthy to be used in further investigations, due to results that more accurately represent real word conditions.
Risk of frost formation became significantly lower in every city when heat recovery ventilation was changed into energy recovery ventilation. Because heat was not the only type of energy that could be recovered, changing heat recovery ventilation system into energy recovery ventilation system enabled latent as well as heat energy recovery, making this system more efficient. Even though heat recovery ventilation was simulated only with the simplified model, it is safe to assume that it will still be effective in reality. Although bypass strategies were effective at eliminating the risk of frost in heat recovery units, they were not as efficient from energy perspective. The more air is by‐passed the cross‐flow plate heat exchanger, the fewer opportunities there are to recover energy from exhaust air, which leads to lower supply air temperatures and higher energy need for heating a living space.

• author: Daugela, Marius
• supervisor: Dennis Johansson
• organization: Division of Energy and Building Design; Department of Architecture and Built Environment
• alternative title: Simulating an energy recovery ventilation system to find out risk of frost formation and energy saving potential.
• course: AEBM01 20222
• year: 2022
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• language: English
• id: 9103591
• date added to LUP: 2023-05-09 14:25:38
• date last changed: 2023-05-09 14:25:38