İzolasyon malzemesi seçiminin elektrikli defrost 
ısıtıcıların ömrüne etkisinin incelenmesi
The effect of dielectric isolation material selection to the usage life of electrical defrost heaters
Yazar: ALİ İLKER TUĞRU
Danışman: PROF. DR. ADNAN DİKİCİOĞLU
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Malzeme ve İmalat Bilim Dalı
Derece: Yüksek Lisans, 2016
Konu: Makine Mühendisliği = Mechanical Engineering

Özet:
Endüstriyel soğutma ve endüstriyel proses sektörlerinde sıklıkla kullanılan soğuk oda evaporatörleri ve doğal akışkanlı oda soğutucuların çalışma koşullarında karşılaştığı en önemli ve en sık periyoda sahip problem buzlanmadır. Soğuk oda evaporatörü serpantinlerinde, kanatçıklar arasında buz tabakaları oluşur. Bu buz tabakalarının oluşma sebebi, havada bulunan nemdir.
Donmuş muhafaza odalarında veya gıda proses alanlarında, mahalin kapısı açılıp kapandıkça içeriye giren taze hava, mahale bir miktar su buharı taşır. Mahalin kapısı sık açılıp kapanmasa bile mahalde bulunan gıda maddeleri de ortama proses sırasında nem aktarır. Bu nem, operasyon sıcaklığı 0 °C'nin altında olan evaporatör serpantin kanatçıkları üzerinde yoğuşarak suya dönüşür. Daha sonra bu su damlacıkları donarak buz kristallerine dönüşür ve katmanlar halinde kalınlaşır. Bu durumu sakıncalı kılan birçok sonuç olmakla birlikte, içerinden en önemlisi, buzlanmanın evaporatör kapasitesine olan etkisidir.
Serpantin kanatçıkları arasında oluşan buz katmanları, hava akışını bloke ederek ve kanatçıklar ile boru yüzeyleri üzerinde ısıl yalıtım işlevi görerek, evaporatörün kapasitesini düşürür. Bu sebepten dolayı bu buz tabakalarının eritilerek ortamdan uzaklaştırılması gerekir.
Evaporatör serpantinleri üzerindeki buz tabakalarını eritmenin yöntemlerinden biri elektrikli defrost yöntemidir. Elektrikli defrost yönteminde, serpantindeki kanatçıkların içinden serpantin boruları doğrultusunda geçecek şekilde elektrikli çubuk defrost ısıtıcılar yerleştirilir. Bu ısıtıcı çubukların yaptığı arızaların başlıca sebebi, güç kablolarının ısıtıcı gövdelerine girdiği noktalardaki yalıtım malzemelerinin zamanla yapısal bozulmaya uğrayarak görevlerini yerine getirememeleridir. Bu yüzden, yalıtım malzemesi seçimi yapılırken, sıcaklık, nem ve zaman gibi faktörlerin etkisiyle yapısal bozulmaya en az uğrayan, dielektrik özellikleri iyi malzemeler seçilmelidir.
Bu tez çalışmasında izolasyon malzemesi seçiminin, elektrikli defrost ısıtıcıların ömrüne etkisi incelenmiştir. Elektrikli çubuk defrost ısıtıcıların karşılıklı olarak kullanım ömrü testine tabi tutulabilecekleri bir test yöntemi tasarlanmış ve test düzeneği kurulmuştur. Tasarlanan test yöntemi ile hangi yalıtım malzemesine sahip defrost ısıtıcının daha uzun süre test koşullarına dayanarak çalışacağı gözlemlenmek istenmiştir. Buradan yola çıkılarak, deneye tabi tutulan malzemelerden hangisinin elektrikli defrost ısıtıcılar için daha uygun olduğu hükmüne varılması hedeflenmiştir. İki farklı yalıtım malzemesine sahip, diğer tüm özellikleri aynı olan elektrikli defrost ısıtıcılar numune olarak alınmış ve test edilmiştir. Test sonuçları literatür taramaları ile karşılaştırılmıştır.

Abstract:
Unit air coolers are used in cold rooms and proses cooling systems for industrial cooling. The most frequent and important problem for unit coolers is frosting. Frost layers occur between heat exchanger fins and tubes. This frost is caused by the moisture in the air.
Every time the door is opened, fresh and humid air enters the cold room. Even if the door of cold room opens rarely, the food and goods inside the room diffuses moisture during industial operation time. If the operation temperature is below 0 °C, the moisture condenses on the fins of heat exchangers. Condensed mointure turns into water and then water turns into frost layers. This frost layers thicken and grow in time.
Amount of frosting in the unit air coolers depends on the cold room thermal conditions. If the cold room has higher humidity and lower temperature, frost  appearance and layer growth gets fast.
Frosting problem is inconvenient for unit air coolers. First of all, it decreases the capacity of the unit coolers. Secondly, if frosting grows between fins and tubes, it damages the heat exchanger.
Frost is a good thermal insulator. When frost layers cover heat exchanger fins, it blocks heat transfer between fins and air. The thicker frost layer is, the lower heat transfer occurs. Furthermore, frost layers block air flow between fins. If air flow rate decreases, heat transfer decreases too. 
The consequences of frost layer ocurrance means money and time loss for cold room operators. Frost layers can not be avoided, so it must be eliminated regularly. Elimination of frost layers is called defrosting. Frost is melted by defrost methods, the water is collected and removed out of the unit coolers.
There are four basic defrost types for unit air coolers; electric heat defrost, hot gas defrost, air defrost and water defrost. Electric heat defrost and hot gas defrost are used generally for cold room operations. Electric heat defrost method uses electrically insulated resistance bars to generate heat and melts down the frost over fins.
Compressor of the cooling cycle compresses the gas to the condenser in high temperatures. This hot gas is taken before condenser and given into evaporator tubes for defrosting of the fins. This method is called hot gas defrost.
Water defrost and air defrost methods can be used in rooms that have temperatures over 0 °C. Warm water is sprayed on the fins in water defrost method. This warm water melts down the frost. Air defrost method uses only the air flow which is created by the fan.
Electric defrost heaters are used in electric heat defrost method. This heaters are located in the heat exchangers in the direction of tubes and controlled by a system. This system works with a time-controlled switch or a thermostat.
When frost layers occur, the system sets the heaters on, and heat is generated for defrosting. After all frost layers are melted down, the system closes the electric heaters and cooling operation starts again.
This heaters are thin bars that resistance heaters mounted inside. Electric heater bars are made of stainless steel. Phase and neutral cables enters the bar and feeds the resistance. This cable entrances are dielectrically isolated. The isolation is done to avoid water and moisture enterance to the electric heater. If water enters to the bar, it brakes down the electric defrost heater, and leakage current occurs. Leakage current is dangerous and must be avoided for safety of the operators.
When an electric defrost heater breaks down, defrost operation does not work properly, the frost can not be melted down and heat exchanger fins remain blocked by frost layers. Therefore, the isolation material must be appropriate for dielectric isolation. This thesis studies the effect of dielectric isolation material to the usage life of the electric defrost heaters.
Generally elastomers which have good dielectric properties are choosen for electric defrost heater isolations. When defrost operation starts, the bar is heated and the isolation material is affected by the heat. If the material is sensitive to temperature changes or elevated temperatures, it can loose mechanical properties and it may be broken down.
First of all, a method is designed for examining the isolation material in elevated temperatures of electric defrost heaters. This method depends on accelating the usage of the defrost heater. Normally, a defrost heater works and stops in every 4-6 hours. Melting down of frost layer takes 10-15 minutes.
This periods are too long for examining the defrost heater. Electric defrost heaters works for 3 minutes and stops for 12 minutes in designed method. 3 minutes is enough for heating the defrost bar to elevated temperatures. Also 12 minutes is enough for cooling the bar to the room temperature.
This period is repeated until the electric heater is broken down. At least two electric defrost heaters which have the same properties but different isolation material are taken into test in this examination method. The material of isolation of electric defrost heater that broken lastly is more suitable according to the other.
Secondly, an experimental setup is constructed. This setup includes a testing pool, an electric panel, a water pump and tank, pipes, fittings and vanes. Electric defrost heaters are located in the pool and their wires are connected to the electric panel.
The test period is repeated automaticly in water and air. When pool is empty, the test is done in air and when pool is full, the test is done in water. All of the operation is controlled by the electric panel.
Thirdly, two electric defrost heater samles are ordered. One of the semples has natural rubber isolation material. The second one has chlorosulfonated polyethylene (CSPE) synthetic rubber isolation material. These samples are tested according to the designed examination method. Test results are reported. Mechanical and thermal properties of these isolation materials are searched in literature.
Lastly, test results are compared with the literature search. The results are convenient with the information which is taken from the literature. However, this study can not give the usage life of an electrical defrost heater. A coefficient may be found by another study, and this coefficient may be used to calculate life of an electrical defrost heater.
3 minutes of heating and 12 minutes of cooling the heater bar is called one circle. How many circles are done for a electrical defrost heater till the end of the experiment is called circle number. If this circle number is multiplicated with the coefficient, real life of an electrical defrost heater may be calculated approximately.