Effect of the ventilation on the performance of a multi-split air conditioning system

Havalandırmanın çoklu-split bir iklimlendirme sisteminin performansına etkisi

Abstract

Air conditioning and ventilation for residential and commercial buildings are highly demanded due to concerns on thermal comfort and healthy environment of the living space in modern society. Multi-split air conditioning system, featuring variable refrigerant flow technology, so-called multi-split variable refrigerant flow system is finding its way in residential and commercial buildings. The multi-split variable refrigerant flow systems have one outdoor which is connected to several indoor units with different configurations and capacities. The outdoor unit consists of two or three compressors, one of which is variable speed compressor driven by an inverter frequency drive. The precise capacity control is provided by this inverter driven compressor and individual electronic expansion valves located in each indoor unit. Depending on the indoor cooling and heating loads, the required refrigerant mass flow rate is provided by varying the inverter frequency of the variable speed compressor, and electronic expansion valves are operated in such a way to keep the indoor temperature at the indoor thermostat set temperature. The first multi-split variable refrigerant flow system was installed more than 20 years ago in Asia. Due to the long history of this technology, the multi-split variable refrigerant flow systems have been widely studied experimentally and numerically. Previous studies mostly focused on control strategies of the variable speed compressor, and electronic expansion valves of multi-split systems. It was observed that the frequency of the compressor and the electronic expansion valve openings of the indoor units should be controlled simultaneous. On the contrary, it was also claimed that the major control parameter was the electronic expansion valve opening. In addition to studies related to the control strategy, the performance of the multi-split type systems has been investigated. One of the main drawbacks of these multi-split variable refrigerant flow systems is that they can not provide any ventilation to the indoors which is required by the American Society of Heating, Refrigeration and Air conditioning Engineers (ASHRAE) regulations. The multi-split variable refrigerant systems condition the indoor air by circulating it through their indoor units without adding any fresh air. That’s why; additional ventilation systems should be installed to the buildings. Thus; integration of the multi-split variable refrigerant flow system and the ventilation system gains importance in actual applications. Despite large number of previous studies, there is not an actual operational study under varying outdoor conditions which considered the effect of the ventilation on the performance of the multi-split variable refrigerant flow system. In this study, a field performance evaluation of a multi-split variable refrigerant flow system integrated with a heat recovery ventilation system is investigated experimentally under varying outdoor conditions in the cooling season. The effect of the ventilation on the performance (indoor temperature control, outdoor unit energy consumption, efficiency) of the multi-split variable refrigerant flow system is investigated. Two multi-split variable refrigerant flow systems and four heat recovery ventilation units were installed in an actual office suite for the field performance tests. The multi-split variable refrigerant flow systems were operated either stand-alone or in conjunction with heat recovery ventilation units during the experiments. Both systems, as well as indoor and outdoor environments were fully instrumented for measurement and comparison purposes. Parametric tests were performed in the cooling seasons of 2006 and 2007. It was found from the experimental evaluation that for both cases; ventilation assisted and nonventilated multi-split variable refrigerant flow systems, they could maintain the set temperature of 25°C. It was also observed that heat recovery ventilation units increased the indoor humidity ratio by around 0.002 kg/kg during the ventilation, and according to the ASHRAE summer comfort zone, the ventilation assisted multi-split variable refrigerant flow system provided relatively uncomfortable indoor environment compared to the non-ventilated case. It was also obtained that, as expected due to the additional ventilation cooling load, the ventilation assisted multi-split variable refrigerant flow system consumed around 18% higher energy compared to the non-ventilated multi-split variable refrigerant flow system. It was found that the ventilation system did not affect the efficiency of the multisplit variable refrigerant flow. Overall, it was concluded that even though the ventilation system did not affect the indoor temperature control and the efficiency of the multi-split variable refrigerant flow system, it did affect the indoor thermal comfort and energy consumption of the outdoor units.

Özet

Hem evsel, hem de ticari binalarda konforlu ve sağlıklı bir yaşam alanı oluşturmak için, iklimlendirme ve havalandırma sistemleri sıklıkla istenmekte ve uygulanmaktadır. Çoklu-split Değişken Soğutkan Akışlı (DSA) sistem olarak da bilinen bir dış ve birden fazla iç üniteden oluşan çoklu-split iklimlendirme sistemleri hem evsel, hem de ticari binalarda kendine yer bulmaya başlamıştır. Bu tip sistemlerde dış ünite içinde, içlerinden biri değişken hızlı kompresör olmak üzere iki ya da üç kompresör bulunmaktadır. Genel olarak, iç ortam soğutma ve ısıtma yüklerine bağlı olarak azalan ve artan sistem soğutkan kütlesel debi ihtiyacı, değişken hızlı kompresörün frekansı değiştirilerek sağlanmaktadır. Böylece hassas sistem kapasite ayarı yapılabilmektedir. Değişken hızlı kompresöre ek olarak, her iç ünitede bir Elektronik Kısılma Vanası (EKV) bulunmaktadır. Bu EKV’ler sayesinde, her iç üniteden geçen soğutkan kütlesel debisi, termostat sıcaklığını sağlayacak şekilde ayarlanabilmektedir. Çoklu-split DSA sistemleri iklimlendirme yapabilmesine rağmen, iç ortama, yönetmeliklerde belirlenen hususlar çerçevesinde taze hava sağlayamamaktadırlar. Bu nedenden dolayı da, binalara çoklu-split DSA sistemi ile beraber, havalandırma sistemleri de takılması gerekmektedir. Böylece, çoklu-split DSA sistemleri ile havalandırma sistemlerinin birleştirilmesi, gerçek uygulamalarda önem kazanmaktadır. Bu çalışmada, ısı geri kazanım havalandırma sistemi ile birleştirilmiş bir çoklu-split DSA sisteminin soğutma sezonundaki saha, değişken dış hava koşulları altında deneysel olarak incelenmiş ve havalandırmanın çoklu-split DSA sisteminin performansına (iç ortam sıcaklık kontrolü, dış ünite enerji tüketimi, sistem verimi) etkisi araştırılmıştır.