Kĩ thuật lạnh - Chapter 7: Evaporator

CHAPTER 7: EVAPORATOR Lecturer : ThS Nguyễn Duy Tuệ 12/2015 Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ 1 . OBJECTIVES In this chapter st dent can, u : - understand structure, function of some evaporators - calculate and choose evaporator met to the requirements 12/2015 2Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ REFERENCES [1]. Refrigeration and Air-Conditioning - A. R. Trott and T. Welch [2]. Evaporator catalogue 12/2015 3Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ CONTENT GENERAL AIR COOLING EVAPORATOR LIQUID COOLING EVAPORATOR PLATE EVAPORATOR DEFROSTING 12/2015 4Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ CONTENT CALCULATE AND CHOOSE EVAPORATOR 12/2015 5Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ GENERAL The purpose of the evaporator is to receive low- pressure low temperature fluid from the expansion, valve and to bring it in close thermal contact with the load. The refrigerant takes up its latent heat from the load and leaves the evaporator as a dry gas. Evaporators are classified according to their refrigerant flow pattern and their function. 12/2015 6Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ AIR COOLING EVAPORATOR - Air cooling evaporators for coldrooms, blast freezers air conditioning will have finned pipe, - coils 12/2015 7Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ AIR COOLING EVAPORATOR - Construction materials will be the same as for air cooled condensers Aluminium fins on copper- . tube are the most common for the halocarbons, with stainless steel or aluminium tube for ammonia. - Frost or condensed water will form on the fin surface and must be drained away. To permit this, fins will be vertical and the air flow horizontal, with a drain tray provided under. Th i f th t b ill b h th t th l it- e s ze o e u e w e suc a e ve oc y of the boiling fluid within it will cause turbulence to promote heat transfer Tube diameters will vary from 12/2015 8 . 9 mm to 32 mm, according to the size of coil. Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ AIR COOLING EVAPORATOR - Fin spacings will vary from 2 mm on a compact airconditioner to 12 mm on a low temperature- coldroom coil. - Some evaporator haven’t got fin and fan; so we call “natural evaporator” 12/2015 9Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ LIQUID COOLING EVAPORATOR 1. Shell and tube evaporator : Liquid cooling is mostly in shell and tube or- - shell-and-coil evaporators. In the shell-and-tube type, the liquid is usually in the pipes and the shell is some three-quarters full of the liquid, boiling refrigerant. A number of tubes is omitted at the top of the shell to give space for the suction gas to escape clear of the surface without entraining 12/2015 10Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ LIQUID COOLING EVAPORATOR The speed of the liquid within the tubes should be about 1 m/s or more to promote internal, turbulence for good heat transfer. End cover baffles will constrain the flow to a number of passes, as with the shell-and-tube condenser. 12/2015 11Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ LIQUID COOLING EVAPORATOR Evaporators of this general type with dry expansion circuits will have the refrigerant within the tubes, in order to maintain a suitable continuous velocity for oil transport, and the liquid in the shell 12/2015 12Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ LIQUID COOLING EVAPORATOR + Sprayed tube liquid chiller : The spray chiller operates with a much lower refrigerant charge than a conventional flooded evaporator does. 12/2015 13Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ LIQUID COOLING EVAPORATOR - The liquid refrigerant level in the surge drum shell is kept below the tubes and liquid is pumped to spray nozzles which ensure that the tube surfaces are covered with an evaporating liquid film. - The overall heat-transfer coefficient of the sprayed tube evaporator usually exceeds that of the flooded type, the additional complexity of the pump and spray assembly usually dissuaded designers f h i throm c oos ng em. 12/2015 14Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ LIQUID COOLING EVAPORATOR - The improvement in heat-transfer coefficient in comparison to a flooded evaporator occurs because the sprayed tube permits easy escape of the vapor Bubbles - In fact, there is an optimum circulation rate in the sprayed-tube evaporator that yields the maximum boiling coefficient. When the flow rate is too high, the liquid film becomes thick and insulates th t b Wh th fl t i t l f the u es. en e ow ra e s oo ow some o e tube surfaces do not become wetted. A typical circulation rate is 5 times the rate evaporated 12/2015 15 . Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ LIQUID COOLING EVAPORATOR For these reasons the brine can be cooled to a temperature close to its freezing point Water can be. chilled to a temperature of less than 1°C with an evaporating temperature close to -2°C. 12/2015 16Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ LIQUID COOLING EVAPORATOR 2. Evaporator with open tank: Liquid cooling evaporators may comprise a pipe coil in an open tank, and can have flooded or dry expansion circuitry. Flooded coils will be connected to a combined liquid accumulator and suction separator (usually termed the surge drum), in the form of a horizontal or vertical drum 12/2015 17Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ LIQUID COOLING EVAPORATOR 12/2015 18Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ LIQUID COOLING EVAPORATOR 12/2015 19Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ LIQUID COOLING EVAPORATOR 12/2015 20Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ LIQUID COOLING EVAPORATOR 12/2015 21Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ LIQUID COOLING EVAPORATOR + Evaporator for tube ice making 12/2015 22Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ LIQUID COOLING EVAPORATOR 12/2015 23Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ LIQUID COOLING EVAPORATOR 12/2015 24Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ LIQUID COOLING EVAPORATOR 12/2015 25Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ LIQUID COOLING EVAPORATOR 3. Baudelot cooler : Another type comprises a bank of corrugated- plates, forming alternative paths for refrigerant and liquid. Where water is to be cooled close to its freezing point without risk of damage to the evaporator, the latter is commonly arranged above the water-collection tank and a thin film of water runs over the tubes. H t t f i hi h ith thi i- ea rans er s very g w a n mov ng film of liquid and, if any ice forms, it will be on the outside free to expand-> not damage the tube 12/2015 26 , . Such an evaporator is termed a Baudelot cooler. Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ LIQUID COOLING EVAPORATOR - It may be open, enclosed in dust-tight shields to avoid contamination of the product (as in surface milk and cream coolers), or may be enclosed in a pressure vessel as in the Mojonniér cooler for soft drinks, which pressurizes with carbon dioxide at the same time 12/2015 27Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ LIQUID COOLING EVAPORATOR - It may be open, enclosed in dust-tight shields to avoid contamination of the product (as in surface milk and cream coolers), or may be enclosed in a pressure vessel as in the Mojonniér cooler for soft drinks, which pressurizes with carbon dioxide at the same time 12/2015 28Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ LIQUID COOLING EVAPORATOR 4. Plate evaporator: 12/2015 29Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ LIQUID COOLING EVAPORATOR - Plate heat exchanger evaporators are now widely used A heat exchanger of this type consists. of a number of herringbone corrugated plates assembled to form a pack - The volume of refrigerant contained in a heat exchanger of this type is approximately 2 litres for each square metre of cooling area, which is up to 10 times lower than for multi-tube designs. This helps t k f i t h l l l d ffo eep re r geran c arge eve ow an o ers a rapid response to changes in energy demand. 12/2015 30Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ LIQUID COOLING EVAPORATOR - The turbulence induced by the pattern of the channels results in very high heat transfer coeffi cients, typically three to four times greater than with conventional tubular designs. Why????? - The counter flow gives temperature differences close to the ideal. - When used as a direct expansion evaporator the refrigerant velocity should be high enough to entrain il th t i ft ti i l to a rema ns a er evapora on s comp e e. Where conditions give rise to non-miscibility, the formation of oil film on the wetted surface can impair 12/2015 31 (reduce) heat transfer. Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ LIQUID COOLING EVAPORATOR 5. Plate evaporator for freezing : Plate evaporators are formed by cladding- (cover) a tubular coil with sheet metal, welding together two embossed plates, or from aluminium extrusions. - The extended flat face may be used for air cooling, for liquid cooling if immersed in a tank, or as a Baudelot cooler, but the major use for flat plate t i t l lid d t b d tievapora ors s o coo a so pro uc y con uc on, the product being formed in rectangular packages and held close between a pair of adjacent plates 12/2015 32 . Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ LIQUID COOLING EVAPORATOR - In the horizontal plate freezer ;the plates are arranged in a stack on slides so that the, intermediate spaces can be opened and closed. - Trays, boxes or cartons of the product are loaded between the plates and the stack is closed to give good contact on both sides. When the necessary cooling is complete, the plates are opened and the product removed. 12/2015 33Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ LIQUID COOLING EVAPORATOR Figure of contact freezer : 12/2015 34Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ DEFROSTING - Air cooling evaporators working below 0°C will accumulate frost which must be removed periodically, since it will obstruct heat transfer. Where the surrounding air is always at 4°C or higher, it will be sufficient to stop the refrigerant for a period and allow the frost to melt off. - For lower temperatures, heat must be applied to melt the frost within a reasonable time and th t it d i M th d densure a ra ns away. e o s use are as follows: 12/2015 35Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ DEFROSTING - Electric resistance heaters: Elements are within the coil or directly under it. 12/2015 36Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ DEFROSTING - Hot gas: A branch pipe from the compressor discharge feeds superheated gas to the coil The. compressor must still be working on another evaporator to make hot gas available. 12/2015 37Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ DEFROSTING 12/2015 38Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ DEFROSTING - Reverse cycle: The direction of flow of the refrigerant is reversed to make the evaporator act as a condenser. 12/2015 39Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ DEFROSTING - Water defrost 12/2015 40Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ DEFROSTING In each of these cases, arrangements must be made to remove cold refrigerant from the coil while defrosting is in progress. Drip trays and drain pipes may require supplementary heating. 12/2015 41Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ CALCULATE AND CHOOSE EVAPORATOR Heat transfer equation: F = Q / k ∆to . Qo : Evaporator capacity, W k : Overal heat coefficience, W/m2.K ∆t : Logarit temperature difference, K F : heat transfer area; (m2) 12/2015 42Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ CALCULATE AND CHOOSE EVAPORATOR Experence of some evaporator heat transfer coefficiences : 12/2015 43Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ CALCULATE AND CHOOSE EVAPORATOR - With air cooling evaporator, temperature difference between room and refrigerant is usually 10K - Heat transfer coefficience: + Room temperature: -40oC -> k=11,6 W/m2.K 20 C k 12 8 W/ 2 K- o -> = , m . -10oC -> k=14 W/m2.K 0oC > k= 17 5 W/m2 K- , . Note : If we want to have more exactly result, please refer to manufacturer catalogue 12/2015 44Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ CALCULATE AND CHOOSE EVAPORATOR Data of air cooling evaporator : 12/2015 45Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ CALCULATE AND CHOOSE EVAPORATOR 12/2015 46Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ CALCULATE AND CHOOSE EVAPORATOR Catalog dàn lạnh hãng Gao Xiang alibaba com/showroom/gaoxiang-. . evaporator.html 12/2015 47Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ CALCULATE AND CHOOSE EVAPORATOR 12/2015 48Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ CALCULATE AND CHOOSE EVAPORATOR 12/2015 49Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ CALCULATE AND CHOOSE EVAPORATOR 12/2015 50Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ CALCULATE AND CHOOSE EVAPORATOR 12/2015 51Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ CALCULATE AND CHOOSE EVAPORATOR 12/2015 52Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ CALCULATE AND CHOOSE EVAPORATOR 12/2015 53Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ CALCULATE AND CHOOSE EVAPORATOR 12/2015 54Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ CALCULATE AND CHOOSE EVAPORATOR 12/2015 55Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ CALCULATE AND CHOOSE EVAPORATOR Ví dụ 1 : Tính chọn dàn lạnh cho kho lạnh có nhiệt độ không khí vào dàn lạnh là -15oC, nhiệt độ không khí ra khỏi dàn lạnh -20oC; năng suất lạnh Qo=2,5kW; nhiệt độ bay hơi của môi chất R22 trong ống là -28oC. 12/2015 56Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ CALCULATE AND CHOOSE EVAPORATOR Giải: Cách 1 : - Tìm được độ chênh lệch nhiệt độ logarit : 10,29oC -> Dàn lạnh 2B020 -> Kiểm tra lại Qo ? Cách 2 : Ước lượng lại hệ số truyền nhiệt k của hãng chế tạo -> F=? 12/2015 57Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ CALCULATE AND CHOOSE EVAPORATOR Ví dụ 2 : Tính chọn thiết bị bay hơi làm lạnh nước cấp cho các FCU với nhiệt độ nước vào TBBH là 12oC; nhiệt độ nước ra khỏi là 7oC; năng suất lạnh TBBH là 100 kW; nhiệt độ sôi của môi chất là 0 oC. 12/2015 58Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ CALCULATE AND CHOOSE EVAPORATOR Giải: Độ chênh lệch nhiệt độ logarit : 9,26oC Hệ số truyền nhiệt k chọn 350W/m2.K Diện tích truyền nhiệt : 30,85 m2 Chọn TBBH mã hiệu ?????????? 12/2015 59Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ CALCULATE AND CHOOSE EVAPORATOR 12/2015 60Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ CALCULATE AND CHOOSE EVAPORATOR 12/2015 61Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ