CHAPTER 7: EVAPORATOR

Lecturer : ThS.Nguyễn Duy Tuệ Lecturer : ThS Nguyễn Duy Tuệ

1 12/2015 Chapter 7 : Evaporator ThS.Nguyễn Duy Tuệ

OBJECTIVES

structure,

function

of

some

to the

In this chapter, student can: In this chapter st dent can - understand evaporators evaporators - calculate and choose evaporator met requirements

q

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REFERENCES

[1]. Refrigeration and Air Conditioning A. R. [1]. Refrigeration and Air-Conditioning - A. R. Trott and T. Welch [2]. Evaporator catalogue

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CONTENT

GENERAL GENERAL

AIR COOLING EVAPORATOR AIR COOLING EVAPORATOR

LIQUID COOLING EVAPORATOR LIQUID COOLING EVAPORATOR

PLATE EVAPORATOR PLATE EVAPORATOR

DEFROSTING DEFROSTING

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CONTENT

CALCULATE AND CHOOSE EVAPORATOR CALCULATE AND CHOOSE EVAPORATOR

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GENERAL

The purpose of the evaporator is to receive low- pressure, low temperature fluid from the expansion 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. 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.

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AIR COOLING EVAPORATOR

- Air cooling evaporators for coldrooms, blast freezers, air-conditioning…will have finned pipe air conditioning will have finned pipe freezers coils

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AIR COOLING EVAPORATOR

- Construction materials will be the same as for air-cooled condensers. Aluminium fins on copper air cooled condensers Aluminium fins on copper tube are the most common for the halocarbons, with stainless steel or aluminium tube for ammonia. 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.

i

Th

f th t b

h th t th

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- The size of the tube will be such that the velocity it ill b of the boiling fluid within it will cause turbulence to promote heat transfer. Tube diameters will vary from promote heat transfer Tube diameters will vary from 9 mm to 32 mm, according to the size of coil.

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AIR COOLING EVAPORATOR

- Fin spacings will vary from 2 mm on a compact to 12 mm on a low-temperature to 12 mm on a low temperature

airconditioner airconditioner coldroom coil.

- Some evaporator haven’t got fin and fan; so we Some evaporator haven t got fin and fan; so we

call “natural evaporator”

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LIQUID COOLING EVAPORATOR

1. Shell and tube evaporator : Liquid cooling is mostly in shell-and-tube or Liquid cooling is mostly in shell and tube or In the shell-and-tube shell-and-coil evaporators. type, the liquid is usually in the pipes and the shell is type, the liquid is usually in the pipes and the shell is the liquid, boiling some three-quarters full of 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

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LIQUID COOLING EVAPORATOR

The speed of the liquid within the tubes should to promote internal be about 1 m/s or more, 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 will constrain the flow to a number of passes, as with the shell-and-tube condenser.

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LIQUID COOLING EVAPORATOR

this general

Evaporators of

type with dry expansion circuits will have the refrigerant within the expansion circuits will have the refrigerant within the in order to maintain a suitable continuous tubes, velocity for oil transport, and the liquid in the shell velocity for oil transport, and the liquid in the shell

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LIQUID COOLING EVAPORATOR

+ Sprayed tube liquid chiller : The spray chiller operates with a much lower refrigerant charge than operates with a much lower refrigerant charge than a conventional flooded evaporator does.

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LIQUID COOLING EVAPORATOR

- The liquid refrigerant level

in the surge drum shell is kept below the tubes and liquid is pumped to 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. 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 from choosing them. f

th

h

i

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LIQUID COOLING EVAPORATOR

- The improvement in heat-transfer coefficient in comparison to a flooded evaporator occurs because comparison to a flooded evaporator occurs because the sprayed tube permits easy escape of the vapor Bubbles Bubbles

sprayed-tube

yields

that

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f th

t

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i

- In fact, there is an optimum circulation rate in the evaporator the maximum boiling coefficient. When the flow rate is too high, the liquid film becomes thick and insulates the tubes. When the flow rate is too low some of the th t b Wh tube surfaces do not become wetted. A typical circulation rate is 5 times the rate evaporated. circulation rate is 5 times the rate evaporated

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LIQUID COOLING EVAPORATOR

For these reasons the brine can be cooled to a temperature close to its freezing point. Water can be 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. evaporating temperature close to -2°C.

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LIQUID COOLING EVAPORATOR

2. Evaporator with open tank: Liquid cooling evaporators may comprise a pipe 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 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

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LIQUID COOLING EVAPORATOR

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LIQUID COOLING EVAPORATOR

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LIQUID COOLING EVAPORATOR

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LIQUID COOLING EVAPORATOR

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LIQUID COOLING EVAPORATOR

+ Evaporator for tube ice making

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LIQUID COOLING EVAPORATOR

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LIQUID COOLING EVAPORATOR

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LIQUID COOLING EVAPORATOR

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LIQUID COOLING EVAPORATOR

3. Baudelot cooler :

- Another type comprises a bank of corrugated Another type comprises a bank of corrugated plates, forming alternative paths for refrigerant and liquid. Where water is to be cooled close to its liquid. Where water is to be cooled close to its risk of damage to the freezing point without evaporator, the latter is commonly arranged above the water-collection tank and a thin film of water runs over the tubes. H t t

hi h ith

thi

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- Heat transfer is very high with a thin moving film of liquid and, if any ice forms, it will be on the free to expand-> not damage the tube. outside, outside free to expand-> not damage the tube Such an evaporator is termed a Baudelot cooler.

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LIQUID COOLING EVAPORATOR

- It may be open, enclosed in dust-tight shields to avoid contamination of the product (as in surface 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 pressure vessel as in the Mojonniér cooler for soft drinks, which pressurizes with carbon dioxide at the same time

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LIQUID COOLING EVAPORATOR

- It may be open, enclosed in dust-tight shields to avoid contamination of the product (as in surface 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 pressure vessel as in the Mojonniér cooler for soft drinks, which pressurizes with carbon dioxide at the same time

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LIQUID COOLING EVAPORATOR

4. Plate evaporator:

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LIQUID COOLING EVAPORATOR

- Plate heat exchanger evaporators are now widely used. A heat exchanger of this type consists widely used A heat exchanger of this type consists of a number of herringbone corrugated plates assembled to form a pack assembled to form a pack

t h

f i

k

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- 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 low and offers a l to keep refrigerant charge level t d ff l rapid response to changes in energy demand.

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LIQUID COOLING EVAPORATOR

- The turbulence induced by the pattern of the transfer coeffi channels results in very high heat transfer coeffi channels results in very high heat cients, typically three to four times greater than with conventional tubular designs. Why????? conventional tubular designs. Why?????

- The counter flow gives temperature differences

close to the ideal.

that th t

ft

ti

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- When used as a direct expansion evaporator the refrigerant velocity should be high enough to entrain remains after evaporation is complete. oil il t Where conditions give rise to non-miscibility, the formation of oil film on the wetted surface can impair formation of oil film on the wetted surface can impair (reduce) heat transfer.

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LIQUID COOLING EVAPORATOR

5. Plate evaporator for freezing : - Plate evaporators are formed by cladding Plate evaporators are formed by cladding (cover) a tubular coil with sheet metal, welding together two embossed plates, or from aluminium together two embossed plates, or from aluminium extrusions.

- The extended flat

d t b

d ti

lid

t

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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 evaporators is to cool a solid product by conduction, the product being formed in rectangular packages and held close between a pair of adjacent plates. and held close between a pair of adjacent plates

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LIQUID COOLING EVAPORATOR

- In the horizontal plate freezer ;the plates are the the

arranged in a stack on slides, so that so that arranged in a stack on slides intermediate spaces can be opened and closed.

- Trays, boxes or cartons of Trays, boxes or cartons of

the product are 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.

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LIQUID COOLING EVAPORATOR

Figure of contact freezer :

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DEFROSTING

- Air cooling evaporators working below 0°C will accumulate frost which must be removed will accumulate frost which must be removed transfer. periodically, since it will obstruct heat Where the surrounding air is always at 4 C or 4°C or Where the surrounding air is always at higher, it will be sufficient to stop the refrigerant for a period and allow the frost to melt off.

th t it d i

M th d

d

- For lower temperatures, heat must be applied the frost within a reasonable time and to melt ensure that it drains away. Methods used are as follows:

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DEFROSTING

- Electric resistance heaters: Elements are

within the coil or directly under it. within the coil or directly under it

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DEFROSTING

- Hot gas: A branch pipe from the compressor discharge feeds superheated gas to the coil. The discharge feeds superheated gas to the coil The compressor must still be working on another evaporator to make hot gas available. evaporator to make hot gas available.

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DEFROSTING

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DEFROSTING

- Reverse cycle: The direction of

flow of

the refrigerant is reversed to make the evaporator act refrigerant is reversed to make the evaporator act as a condenser.

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DEFROSTING

- Water defrost

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DEFROSTING

In each of these cases, arrangements must be made to remove cold refrigerant from the coil while made to remove cold refrigerant from the coil while defrosting is in progress. Drip trays and drain pipes may require supplementary heating. may require supplementary heating.

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CALCULATE AND CHOOSE O EVAPORATOR

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Heat transfer equation:

F = Qo / k.∆t F = Q / k ∆t

,

Qo : Evaporator capacity, W k : Overal heat coefficience, W/m2.K ∆t : Logarit temperature difference, K F : heat transfer area; (m2)

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Experence of some evaporator heat

transfer

coefficiences : coefficiences :

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- With air cooling evaporator,

temperature difference between room and refrigerant is usually difference between room and refrigerant is usually 10K

- Heat transfer coefficience:

+ Room temperature:

k 12 8 W/

-40oC -> k=11,6 W/m2.K -20oC -> k=12,8 W/m2.K 2 K 20 C -10oC -> k=14 W/m2.K

0oC -> k= 17,5 W/m2.K 0oC > k= 17 5 W/m2 K

Note : If we want to have more exactly result,

please refer to manufacturer catalogue please refer to manufacturer catalogue

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CALCULATE AND CHOOSE O EVAPORATOR

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Data of air cooling evaporator :

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Catalog

dàn

lạnh

hãng Gao

Xiang

http://www.alibaba.com/showroom/gaoxiang- http://www alibaba com/showroom/gaoxiang- evaporator.html

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CALCULATE AND CHOOSE O EVAPORATOR Ví dụ 1 : Tính chọn dàn lạnh cho kho lạnh có nhiệt độ không khí vào dàn lạnh là 15oC, nhiệt độ 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 ốống là -28oC.

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:

10,29oC -> Dàn lạnh 2B020 -> Kiểm tra lại Qo ?

ạ

ạ

Giải: Cách 1 : Cách 1 : - Tìm được độ chênh lệch nhiệt độ logarit , Cách 2 : Ước lượng lại hệ số truyền nhiệt k của

hãng chế tạo -> F=?

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Ví dụ 2 : Tính chọn thiết bị bay hơi

ệ

ộ

;

làm lạnh nước cấp cho các FCU với nhiệt độ nước vào 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 ạ chất là 0 oC.

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Giải:

Độ chênh lệch nhiệt độ logarit : 9,26oC 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 ??????????

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