Havalandırma sistemlerinde enerji tasarrufu

Abstract

Günümüzde enerji maliyetlerinin artması, enerji tasarrufuna olan ilgiyi zorunlu olarak artırmıştır. Bu nun neticesinde enerji üreten tesislerde, gerek proje aşamasında titiz hesap yapılmakta gerekse işletme aşama sında enerji geri kazanma sistemleri bilinçli olarak kul lanılmaktadır. Özellikle havalandırma sistemleri enerji tasarrufu uygulamalarına çok açık sistemlerdir. Proje ne kadar mü kemmel olursa oİ3un, enerji geri kazanma tedbirleri ile büyük ölçüde yakıt tasarrufu sağlanmaktadır. Öncelikle havalandırma sistemleri incelenmiştir. Daha sonra genel olarak havalandırma sistemlerinde yapı labilecek enerji tasarrufu ve özellikle hava ısıtıcılar detaylı olarak incelenmiştir. Teknolojinin gelişmesi ile fiyatları ucuzlayan frekans konverterlerinin havalan dırma sistemlerinde kullanımı ve yapılan enerji tasarrufu belirtilmiştir. Uygulamaya ait bir örnek olarak halen inşaatı de vam eden Özel Darüşşafaka lisesi Kampus inşaatı veril miştir. Çok büyük ölçekli olan bu kampusun ısıtma ve havalandırma sisteminde yapılabilecek enerji tasarrufu etüd edilmiştir.

In many countries, the energy consumed In build ings represents a significant proportion of the total e- nergy consumption of the country. Most of the energy used in buildings is consumed for space heating and vent ilation. Savings in these fields are therefore of vital importance. In thi3 thesis, installation of heating and vent ilating system at private Darüşşafaka high school campus is examined. Sophisticated HVAC system and energy reco very equipments have been proposed at this big scale pro ject. All of them are examined below : First of all, while preparing a heating and vent ilating project, design temperature is very valuable pa rameter for calculating total heat losses. This tempera ture value for Istanbul is -3 C. Heating season is six months which begins at 15 november ends 15 may. Heating and ventilating requirements are calculated according to the avarage monthly ambient temperatures. Using the only required heating and ventilating capacity is drastically reduced by means of building ambient air temperature com- pansation equipmets. In addition of using avarage mont hly ambient air temperature, building work schedule are applied to the campus. For HVAC applications, four basic types of air to air heat exchangers are used : - Rotary heat exchangers - Plate exchangers - Heat pipes - Liquid -coupled heat exchangers For private Darüşşafaka high school campus, rotary heat exchangers and liquid -coupled heat exchangers are examined. In view of its high efficiency and its ability to recover heat as well as moisture, the rotary heat excha ngers offer wide opportunities for reducing radically the installed power and energy demand. By sizing the various components at the project stage to suit the needs of the vii plant, the investment and operating costs canbe signifi cantly reduced. It is also important to satisfy the re covery demands by correct choice of the type of rotary heat exchanger. Heat recovery can be carried out by me ans of a hygroscopic rotor, i.e. a rotor which transfers moisture, as well as by means of a non-hygroscopic rotor i.e. a rotor which under normal operating conditions, transfers only sensible heat. When no humidiiication demands are made, a non hygroscopic rotor is normally installed. However, even in this type of plant, the hygroscopic rotor offers cer tain benefits, such as lower frosting point, at the same time as it eliminates the risk of extremely dry indoor air conditions during the winter. Since heat wnells are the only air to air exchan ger which recovers both sensible and latent heat, they are generally the most cost effective energy recovery de vice for HVAC systems. In addition to preventing moistu re from entering the building during the summer, heat whells also prevent moisture from being exhausted in win ter. As aresult, the need for humidification is reduced and the level of comfort increased. Liquid -coupled heat exchangers are easy to install in existing as well as new systems. A liquid -coupled system consists of the following components. - Finned coil in the exhaust air - Pipe work with a pump and control valve - Finned coil in the supply air The most important benefits of a liquid-coupled system are : - The supply air and exhaust air ducts can be run indepently of each other. - Several units on the supply and exhaust air si des can be combined into a system - Simple to incorporate heat recovery into exist ing systems - The heat recovery coil in the supply air can be utilised as a cooling coil in the summer The third proposed energy saving system is chanc ing motor speed of the fresh air unit, exhaust air units and pumps. For several years, electronic variable speed drives have been used on fan motors in large buildings variable-air-volume (VAV) ventilation systems to replace inlet vanes. The most important benefits of a variable speed machine are : viii 50% of the electrical energy consumed in private Darüşşafaka high school campus HVAC system is used to operate centrifugal or flow loads, such as pumps, fans and blowers. Most of these loads are powered by constant speed squirrel cage induction motors. Since the power consumption varies as the cube of speed, using frequency converters which adjuste the motor speed according to the required speed, the power requirements can be signi ficantly reduced. Two boilers in heating centre represent potential sources for recovering waste heat. This waste heat, bo iler flue gas, can be used for preheating the water be longs to hot water system. As a result, boiler gas flow rate and inlet, outlet gas and water temperatures are calculated. As a result, for each energy recovery system pay back period is calculated. All of the methods have been examined and especially it is decided that using a- varage mothly ambient air temperature will give better result than using constant design temperature, -3 C» by means of ambient air compansation equipment, as well as together applying with building's work schedule exactly. Since the avarage ambient air temperature for Istanbul is around 9 0, payback period for rotary heat exchanger is very long about 13.5 years. So, rotary he at exchangers are not applicable for Istanbul air condi tions. But, energy recovery from boiler exhaust gas u- sing economizer is applicable. Waste heat recovery calculations usually start with the heat balance. A heat balance is an analysis of a process which shows where all the heat comes from and where it goes. This is a vital tool in assessing the profit implications of heat losses and proposed waste he at utilization projects. The heat balance for a steam boiler, process furnace, air conditioner, etc., must be derived from measurements made during actual operating periods. The measurements that are needed to get a com plete heat balance involve : energy inputs, energy losses to the environment, and energy discharges. Energy enters most process equipments either as chemical energy in the form of fossil fuels, of sensible enthalpy of fluid streams, of latent heat in vapor stre ams, or electrical energy. For each input it ia necessary to meter the quan tity of fluid flowing or the electrical current. This means that if accurate results are to be obtained, sub ix metering for each flow is required (unless all other eq uipment served by a main meter can be shut down so that the main meter can be used to measure the inlet flow to the unit). It is not necessary to continuously submeter every flow since temporary installations can provide suf ficient information. In the case of furnaces and boilers that use pressure ratio combustion controls, the control flow meters can be utilized to yield the correct informa tion. It should also be pointed out that for furnaces and boilers only the fuel need be metered. Tests of the exhaust products provide sufficient information to deri ve the oxidant (usually air) flow if accurate fuel data are available. Industrial heat exchangers have many pseudonyms. They are sometimes called recuperators, regenerators, waste heat steam regenerators, condensers, heat whells etc. Whatever name they have, they all perform one ba sic function : the transfer of heat. Heat exchangers are characterized as single or multipass gas to gas, liquid to liquid, evaporator, con denser, paralel flow, counterflow, or crossflow. The terms single or multipass refen to the heating media pas sing over the heat transfer surface once or a number of times.' Multipass flow involves the use of internal baf fles. The nextthree terms refer to the two fluids bet ween which heat is transferred in the heat exchanger, and imply that no phase changes occur in those fluids. The principal methods of reclaiming waste heat in industrial plants make use of heat exchangers. The heat exchanger is a system which seperates the stream contai ning waste heat and the medium which is to absorb it, but allows the flow of heat across the separation boundaries. The reasons for separating the two streams may be any of the following. (1) A pressure difference may exist between the two streams of fluid. The rigid boundaries of the heat exchanger can be designed to wit hstand the pressure difference. (2) In many, if not most, cases the one stream would contaminate the other, if they were per mitted to mix. The heat exchanger prevents mixing. (3) Heat exchangers permit the use of an interme diate fluid better suited than either of the principal exchange media for transporting wa ste heat through long distances. Consideration should be given to HVAC energy reco very for both new and retrofit applications where buildi ng codes or other regulations require substantial amounts of fresh make-up air. Air-to-air heat exchangers can be used to pre-heat or pre-cool supply air from wasted heat or cooling being exhausted from the building. In addition to the obvious fuel savings, several other benefits are possible. For instance, when designed into new installations, major savings can be achieved through the reduction in the requirred capacities of he ating and cooling equipment.