Kĩ thuật lạnh - Chapter 8: Expansion valve

CHAPTER 8 : EXPANSION VALVE Lecturer : ThS Nguyễn Duy Tuệ 12/2015 Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ 1 . OBJECTIVES In this chapter student can : , - Understand operation priciple of some kind of expansion valves - Choose the thermal expansion which is suitable for requirements 12/2015 2Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ REFERENCES [1] Refrigeration and Air Conditioning A R. - - . . Trott and T. Welch [2]. Danfoss expansion valve catalogue 12/2015 3Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ CONTENT INTRODUCTION THERMOSTATIC EXPANSION VALVE ELECTRONIC EXPANSION VALVE LOW PRESSURE FLOAT VALVE HIGH PRESSURE FLOAT VALVE 12/2015 4Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ CONTENT CAPILLARY TUBES AND RESTRICTORS OTHER LEVEL CONTROL CHOOSING THERMOSTATIC EXPANSION VALVE 12/2015 5Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ INTRODUCTION - The purpose of the expansion valve is to t l th fl f f i t f th hi hcon ro e ow o re r geran rom e g -pressure condensing side of the system into the low-pressure evaporator. - In most cases, the pressure reduction is achieved through a variable flow orifice, either modulating or two-position. Expansion valves may be classified according to the method of control. 12/2015 6Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ THERMOSTATIC EXPANSION VALVE 1. Reason why we have to use thermostatic i l ?expans on va ve Direct expansion circuits must be designed and installed so that there is no risk of liquid refrigerant returning to the compressor. To ensure this state, heat exchange surface in the evaporator is used to heat the dry saturated gas into the superheat region. The amount of superheat is usually of the order of 5 K. -> Useful superheat 12/2015 7Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ THERMOSTATIC EXPANSION VALVE 2. Operation principle : Th b i th t ti i l h- e as c ermos a c expans on va ve as a detector and power element, charged with the same refrigerant as in the circuit. - The pressure ps generated in the phial by the superheated gas passes through the capillary tube to the top of the diaphragm. An adjustable spring provides the balance of ps ;pe at the diaphragm, and the valve stem is attached at the centre. - Should the superheat fall for any reason, there will be a risk of liquid reaching the compressor 12/2015 8Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ . THERMOSTATIC EXPANSION VALVE ps>pe+pspring : Valve open ps<pe + pspring : Valve close 12/2015 9Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ THERMOSTATIC EXPANSION VALVE 12/2015 10Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ THERMOSTATIC EXPANSION VALVE If the load on the evaporator increases, f i t ill t li d th ill bre r geran w evapora e ear er an ere w e more superheat at the phial position. Then p s will increase and open the valve wider to meet the new demand. 12/2015 11Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ THERMOSTATIC EXPANSION VALVE /RAPIDFiles/01/Asset/TE55-Ver/TE55-Ver.10.03.25-.02- 2D L swf- . 12/2015 12Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ THERMOSTATIC EXPANSION VALVE 12/2015 13Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ THERMOSTATIC EXPANSION VALVE /RAPIDFiles/01/Asset/TE55-Ver/TE55-Ver.10.03.25-.02- 2D L swf- . 12/2015 14Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ THERMOSTATIC EXPANSION VALVE 12/2015 15Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ THERMOSTATIC EXPANSION VALVE Dependence of ts-ps at the bult 12/2015 16Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ THERMOSTATIC EXPANSION VALVE The power element can be limit charged so that ll th f i t ithi it h i d ba e re r geran w n as vapor ze y a predetermined temperature (commonly 0°C). Above this point the pressure within it will cease of follow, the boiling point curve but will follow the gas laws as shown in : -> the valve will remain closed. 12/2015 17Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ THERMOSTATIC EXPANSION VALVE Detector pressure for limit charged valve: 12/2015 18Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ THERMOSTATIC EXPANSION VALVE - This is done to limit the evaporator pressure h fi t t ti t hi h i htw en rs s ar ng a warm sys em, w c m g overload the drive motor. This is termed limit charging or maximum operating pressure -> MOP valve - Such valves must be installed so that the phial is the coldest part. 12/2015 19Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ THERMOSTATIC EXPANSION VALVE When soldering, we have to wrap the valve’s b d b t l tho y y a we c o 12/2015 20Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ THERMOSTATIC EXPANSION VALVE 3. External equalizer : Th i l th t ti i l li- e s mp e ermos a c expans on va ve re es on the pressure under the diaphragm being approximately the same as that at the coil outlet, and small coil pressure drops can be accommodated by adjustments to the spring setting. - If evaportor have high pressure drop due to distributor, large coil, there will be the large pressure difference between inlet and outlet coil -> Have equalizing connection 12/2015 21Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ THERMOSTATIC EXPANSION VALVE Thermostatic expansion valve with external liequa zer 12/2015 22Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ THERMOSTATIC EXPANSION VALVE 4. What is the hunting of expansion valve : E i h ti f th th t tixcess ve un ng o e ermos a c expansion valve means that: - The evaporator surface has an irregular refrigerant feed with a resulting slight loss of heat transfer effectiveness. - a time lag between the change of valve position and the effect at the evaporator outlet - Over-sized valves and incorrect phial position can also give rise to hunting. 12/2015 23Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ THERMOSTATIC EXPANSION VALVE + Solutions: Th hi l h ld l h l t d th- e p a s ou a ways e oca e on e horizontal outlet, as close to the evaporator as possible and not on the underside of the pipe. - Electronic expansion valves are now becoming increasingly popular for many applications. 12/2015 24Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ THERMOSTATIC EXPANSION VALVE 12/2015 25Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ THERMOSTATIC EXPANSION VALVE 12/2015 26Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ THERMOSTATIC EXPANSION VALVE 12/2015 27Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ THERMOSTATIC EXPANSION VALVE 12/2015 28Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ THERMOSTATIC EXPANSION VALVE + Installation diagram: need solenoide valve before thermostatic expansion valve wledge-center/multimedia/refrigeration-system- overview/ 12/2015 29Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ THERMOSTATIC EXPANSION VALVE 12/2015 30Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ ELECTRONIC EXPANSION VALVE 1. Benefits of electronic expansion valve: Th l t i i l ff fie e ec ron c expans on va ve o ers a ner degree of control and system protection. The benefits can be summarized as: - Precise flow control over a wide range of capacities . - Rapid response to load changes . - Better control at low superheats so that less evaporator surface is required for superheat. More surface for evaporation results in higher evaporating temperature and better efficiency 12/2015 31Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ . ELECTRONIC EXPANSION VALVE - Electrical connection between components ff t fl ibilit i t l t hi h io ers grea er ex y n sys em ayou , w c s important for compact systems. - The valve can close when the system shuts down, which eliminates the need for an additional shut off solenoid valve. 12/2015 32Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ ELECTRONIC EXPANSION VALVE 2. Kinds of electronic expansion valve : With l t i i l th ifi- e ec ron c expans on va ve, e or ce size is varied by : a stepper motor, and a pulse width modulated (PWM) type. - In each case a controller is used in conjunction with the valve. The controller is pre-confi gured for the refrigerant and valve type and it receives the information from sensors, for example, pressure and temperature at the evaporator outlet. This enables the superheat to be determined. The output signal to the valve initiates the orifice 12/2015 33Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ adjustment. ELECTRONIC EXPANSION VALVE 2. Kinds of electronic expansion valve : With electronic expansion valve the orifice- , size is varied by : a stepper motor, and a pulse width modulated (PWM) type. - In each case a controller is used in conjunction with the valve. The controller is pre-confi gured for the refrigerant and valve type and it receives the information from sensors, for example, d t t t th t tl tpressure an empera ure a e evapora or ou e . This enables the superheat to be determined. The output signal to the valve initiates the orifice 12/2015 34Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ adjustment. ELECTRONIC EXPANSION VALVE + Installation diagram for electronic expansion valve : 12/2015 35Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ ELECTRONIC EXPANSION VALVE Asset/ETS6-Ver/ETS6-Ver.20.08.12-.02-2D-L.swf 12/2015 36Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ ELECTRONIC EXPANSION VALVE 12/2015 37Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ ELECTRONIC EXPANSION VALVE 12/2015 38Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ ELECTRONIC EXPANSION VALVE 12/2015 39Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ ELECTRONIC EXPANSION VALVE In the case of the PWM valve it is the l ti hi b t th i d l i hi hre a ons p e ween e open ng an c os ng w c determines the capacity of the valve. The valve is either open or closed and each time interval of a few seconds will include an opening period depending on the signal. 12/2015 40Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ ELECTRONIC EXPANSION VALVE Files/RAPIDFiles/01/Asset/AKV10-Ver/AKV10- Ver.10.01.27-.02-2D-L.swf 12/2015 41Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ ELECTRONIC EXPANSION VALVE 12/2015 42Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ ELECTRONIC EXPANSION VALVE 12/2015 43Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ ELECTRONIC EXPANSION VALVE 12/2015 44Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ ELECTRONIC EXPANSION VALVE Under partial load condition or floating d i hi h h t lcon ens ng pressure, w c appens a ow ambient temperature, the condensing pressure decreases Thermostatic expansion valves tend to. hunt, but systems with electronic components operate at partial load in exactly the same and stable manner as at full load. 12/2015 45Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ LOW PRESSURE FLOAT VALVE 1. Description : Flooded evaporators require a constant liquid- level, so that the tubes remain wetted. A simple float valve suffices (applied), but must be located with the float outside the evaporator shell - Since the surface of the boiling liquid is agitated (stir) and the constant movement would cause excessive wear in the mechanism. The float is th f t i d ithi t h bere ore con a ne w n a separa e c am er, coupled with balance lines to the shell 12/2015 46Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ LOW PRESSURE FLOAT VALVE 2. Figure of low pressure float valve: 12/2015 47Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ LOW PRESSURE FLOAT VALVE - Such a valve is a metering device and may not provide positive shut-off when the compressor is stopped. Under these circumstances, refrigerant will continue to leak into the evaporator until pressures have equalized, and the liquid level might rise too close to the suction outlet. To provide this shut-off, a l id l i d d i h li id liso eno va ve s nee e n t e qu ne. - Since the low-pressure float needs a solenoid valve for tight closure this valve can be used as an, on–off control in conjunction with a pre-set orifice and controlled by a float switch 12/2015 48Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ LOW PRESSURE FLOAT VALVE + Figure of float switch : 12/2015 49Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ HIGH PRESSURE FLOAT VALVE 1. Description : A float valve can be fitted which will pass any drained liquid from the condenser direct to the evaporator. The action is the same as that of a steam trap. The float chamber is at condenser pressure and the control is termed a high-pressure float 12/2015 50Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ CẤP DỊCH BẰNG VAN PHAO KIỂU CAO ÁP 2. Installation diagram: 12/2015 51Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ HIGH PRESSURE FLOAT VALVE In this diagram, the first upper coil superheat the vapour the second one boil off the remaining, liquid 12/2015 52Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ CAPILLARY TUBES AND RESTRICTORS The variable orifice of the expansion valve can be replaced in small systems by a long thin tube, , . This is a non-modulating device and has certain limitations, but will give reasonably effective control over a wide range of conditions if correctly selected 12/2015 53Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ CAPILLARY TUBES AND RESTRICTORS + Capillary adjustment : 12/2015 54Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ CAPILLARY TUBES AND RESTRICTORS - Tube bores of 0.8–2 mm with lengths of 1–4 m are common The capillary tube is only fitted on. factory-built and tested equipment, with exact refrigerant charges. - Mass flow is a function of pressure difference and the degree of liquid subcooling on entry. Th ill b i d l l i l- e cap ary tu e s use a most exc us ve y in small air conditioning systems and is self regulating within certain parameters Increasing. ambient temperature -> increasing load and the condensing pressure rise->more refrigerant flow. 12/2015 55Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ CAPILLARY TUBES AND RESTRICTORS - Tube bores of 0.8–2 mm with lengths of 1–4 m are common The capillary tube is only fitted on. factory-built and tested equipment, with exact refrigerant charges. - Mass flow is a function of pressure difference and the degree of liquid subcooling on entry. Th ill b i d l l i l- e cap ary tu e s use a most exc us ve y in small air conditioning systems and is self regulating within certain parameters Increasing. ambient temperature -> increasing load and the condensing pressure rise->more refrigerant flow. 12/2015 56Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ CAPILLARY TUBES AND RESTRICTORS Figure shows how the device is applied in a reversible air conditioner. 12/2015 57Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ OTHER LEVEL CONTROLLS - If a small heater element is placed at the required liquid level of a flooded evaporator, together with a heat-sensing element, then the latter will detect a greater temperature if liquid refrigerant is not present. This signal can be used to operate a solenoid valve. - The thermostatic or electronic expansion valve can also be used to maintain a liquid level. The phial d h t l t b th l d t b lb tan a ea er e emen are o c ampe o a u a the required liquid level. If liquid is not present, the heater warms the phial to a superheat condition and 12/2015 58Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ the valve opens to admit more liquid. OTHER LEVEL CONTROLLS 12/2015 59Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ CHOOSING THERMOSTATIC EXPANSION VALE We have to follow manufacturer catalogue for choosing expansion valve Following this is. Danfoss’s catalogue: 12/2015 60Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ CHOOSING THERMOSTATIC EXPANSION VALE 12/2015 61Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ CHOOSING THERMOSTATIC EXPANSION VALE 12/2015 62Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ CHOOSING THERMOSTATIC EXPANSION VALE + Lưu ý : Trong ví dụ trên ta chưa kể đến hệ số hiệu chỉnh do độ quá lạnh của môi chất trước khi vào VTL. Độ quá lạnh lỏng càng lớn thì NSL tăng do sự hóa hơi đột ngột sau van giảm 12/2015 63Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ CHOOSING THERMOSTATIC EXPANSION VALE 12/2015 64Chapter 8: Expansioln valve ThS.Nguyễn Duy Tuệ