Mingyin Chan

Steady-state operating performance modelling and prediction for a direct expansion air conditioning system using artificial neural network:

A two-in two-out steady-state artificial neural network (ANN)-based model for an experimental variable speed direct expansion (DX) air conditioning (A/C) system has been developed for simulating its total output cooling capacity and equipment sensible heat ratio under different combinations of compressor and supply fan speeds. Experiments were carried out, and totally 169 sets of experimental data were obtained for ANN training and testing. An ANN-based model having the configuration of 2 neurons in the input layer, 2 neurons in the output layer and 6 neurons in each of the 2 hidden layers, i.e. 2-6-6-2 configuration, was thus developed. The ANN-based model developed can be used to predict the operating performance of the DX A/C system with a higher accuracy. It is expected that the model developed can help design a multivariable-input multivariable-output strategy to simultaneously control indoor air temperature and humidity. Practical applications: The work reported in this paper demonstrates that the operating performance of a DX A/C system under different compressor and supply fan speeds can well be represented using ANN model technique, as an alternative to traditional physical-based modelling techniques, with a higher predicting accuracy and less computational effort. The ANN-based model for the DX A/C system is expected to be very useful in developing an efficient control algorithm for simultaneous indoor temperature and humidity using the ANN technique.

Commuters' Exposure to Carbon Monoxide and Carbon Dioxide in Air-conditioned Buses in Hong Kong

The summer in Hong Kong is hot and humid with temperatures over 30°C and relative humidity over 70%. Air-conditioned buses were first introduced in the early 1990s and have become more and more popular. Everyday, millions of passenger-trips are taken in such buses and the air quality and thermal environment in them has become a public health issue. One major source of pollution is the passengers themselves but re-entry of engine emission is also a concern. People have always complained about the thermal environment of the buses especially during rush hours saying they are stuffy, smelly and have inadequate fresh air. We measured conditions in air-conditioned buses and non air-conditioned buses, running on 12 different bus routes in the Hong Kong Special Administrative Region (HKSAR), in both urban and rural areas. Two trips were made for each route. Portable gas analysers measured carbon monoxide (CO), carbon dioxide (CO2), temperature and relative humidity. The measurements showed there was no evidence that CO and CO2 levels in the air-conditioned buses were statistically higher than in non air-conditioned buses (p 0.05). For the rural commutes, the in-vehicle levels of CO and CO2 were not significantly higher than for urban commutes (p 0.05). The Environmental Protection Department (EPD), HKSAR had published a guidance note for airconditioned public transport facilities and most of the exposures found, including the thermal environment, were acceptable with regard to the recommendations in the note. Momentary high levels of CO2 were found, but this lasted for a short period only.

Thermal comfort in an office with intermittent air-conditioning operation

An effective way to reduce A/C energy use is to switch off air conditioning during unoccupied periods. The main concern is, however, whether occupants’ thermal comfort is sacrificed. This paper reports a study of the impacts of switching off A/C during unoccupied hours in an office on thermal comfort of occupant after returning to office and resuming the operation of A/C system. Site measurements for both indoor thermal environmental parameters and energy consumption of A/C were carried out and the thermal comfort of occupants was assessed. Measurement and assessment results indicated that up to 11% of annual energy consumption of A/C for the office could be saved, but the level of thermal comfort of occupants was not greatly degraded. Practical applications: Intermittent operation of A/C is often ignored when people talk about energy saving. It requires no additional cost and equipment, but the saving is significant. Moreover, no advanced technology or knowledge is required. The most important result is that thermal comfort level is not scarified during occupied hours.

A new capacity controller for a direct expansion air conditioning system for operational safety and efficiency

For achieving a balance between the operational safety and efficiency of a variable speed direct expansion air conditioning system, a new capacity controller that can not only simultaneously control indoor air temperature and humidity, but also select an optimized degree of superheat setting to properly balance the operational safety and efficiency has been developed, by adding a control module to a previously developed capacity controller. The core of the control module was an artificial neural network based model representing the known relationship between the inherent operational characteristics and operational stability of a variable speed direct expansion air conditioning system. Using the control module developed, the degree of superheat setting could be varied in accordance with the variation in operating conditions for ensuring a safe and an efficient operation. The performance of the new capacity controller was experimentally tested using an experimental variable speed direct expansion air conditioning system. Controllability test results showed that the system hunting was mitigated when a larger degree of superheat setting was selected by the developed control module and an improvement in COP at about 3.4% was achieved when a smaller one was selected when using the new capacity controller for simultaneously controlling indoor air temperature and humidity. Practical application : This paper reports a new capacity for a VS DX A/C system, which can not only control indoor air temperature and humidity through varying compressor and supply fan speeds, but also select an optimized DS setting for ensuring both a safe and efficient system operation. The controllability test results showed that the new capacity controller could be applied to DX A/C systems for indoor thermal control with a higher operational safety and energy efficiency.

A New Viewpoint to Studying the Condensate Retention and Drainage Process on Air Cooling Coil

Normally simultaneous air cooling and dehumidification takes place in an air cooling coil. The condensate accumulated on the cooling coil surfaces could significantly affect its thermal and hydraulic performance. The condensate retention and drainage has bee usually regarded as a static process. The total mass of condensate retained was viewed as being influenced by the balance among gravitational, air flow drag forces and surface tension retaining force only. With fixed geometric and surface characteristics and under a constant air Reynolds Number, a cooling coil was therefore considered to have a fixed amount of condensate retained. However, some researches pointed out that the total mass of condensate retained could be influenced by system operating parameters such as temperature and moisture content of inlet air. A previously related study by authors developed a mathematic model and considered this process as being a dynamic but without giving a detailed discussion. In this paper, through a further analysis of data obtained in the previously study, a new viewpoint on condensate retention and drainage process has been suggested in order to better explain the experimental results and to provide a base for proposing some future related research work.Copyright

Optimum Network Sizing for Delivering Different Media

This paper aims at fundamentally finding optimal shape of networks used in delivering different distribution networks from one supplier plant to a number of end-users in order to minimize power loss in delivery process under a constraint of total flow volume. A general equation is developed for evaluating the power loss for different shape networks. Based on the equation, a parameter, γ, is deduced which can be applied to define the volume flow into three classifications. For each flow classification, the optimal shape network will be discussed. Results indicate that the radial shape will be optimum when γ ≥ 1. When γ<1, an optimal tree shape network is deduced for evenly distributed end-users on a square.Copyright

Effect of the Indoor Environment on the Condensing Rate and the Air-side Sensible Heat Transfer Resistance of a Direct Expansion Cooling Coil

Dehumidification and cooling performance are two most important features of a direct expansion (DX) cooling coil, which works as an evaporator for a DX air-conditioning system. The condensing rate and the sensible heat transfer resistance of a DX cooling coil represent its capacity for dehumidification and is an important characteristic of the cooling process. An experimental investigation on the effects of the indoor environment, that is, indoor air temperature and relative humidity (RH), on the condensing rate and the air-side sensible heat transfer resistance of a DX cooling coil has been conducted and is reported in this paper. It was found that both the condensing rate and the air-side sensible heat transfer resistance increased with increase of indoor air RH and decreased with an increase of indoor air temperature. Correlation of the experimental results of the condensing rate and the air-side sensible heat transfer resistance under different indoor air temperatures and RH were studied.

Residential indoor humidity control in tropics and sub-tropics:

The use of direct expansion (DX) air conditioning (A/C) systems is widely seen in residential buildings in tropics and sub-tropics. However, most DX A/C systems are equipped with single-speed compressor and supply fan, relying on On—Off cycling as a low-cost approach to maintain only indoor dry-bulb temperature, whereas the indoor air humidity is not controlled directly. This also leads to potential inefficient energy use due to space over-cooling. This paper suggests a novel control strategy (termed Hi—Lo strategy) of using two-speed compressors in DX A/C systems for better indoor thermal environment control and higher energy efficiency. Experimental work has been carried out to test the Hi—Lo control strategy. Experimental results suggested that the novel control strategy can help improve the indoor humidity control without sacrificing energy efficiency. Practical application: The use of ON/OFF control at part load conditions in conventional DX A/C systems will lead to undesirable relative humidity levels. The problem is more prominent when the latent load is dominant. In order to have a better dehumidification effect, a lower indoor dry bulb temperature has been often set. On the other hand, the use of variable speed compressors in DX A/C systems can be a solution to the problem of humidity control, but its cost is comparatively high. The suggested novel control strategy is a low cost approach, which can save energy and provide a better indoor humidity control using DX A/C systems.