Monday, December 27, 2010
【 Weak current College 】 thermal expansion valves and electronic expansion valve for controlling 】
1 Overview-saving and environmental protection are to be solved two problems. In 2002, 26 August to 4 September in Johannesburg, South Africa has the World Summit on sustainable development. At that meeting international Refrigeration Institute published the refrigeration industry for sustainable development and mitigation of climate change commitment in this file in the refrigeration industry major challenges from global warming. The impact of refrigeration industry caused global warming cause of 80% of the carbon dioxide emissions. These indirect emissions is part of the refrigeration unit running the required energy production. Refrigeration, air-conditioning and heat pump which consumes approximately around the 15% of the production of electrical energy, which indicates that the indirect effects of emissions is very serious. This file also put forward in the next 20 years the refrigeration industry must embrace the ambition to achieve one goal: every refrigeration equipment energy consumption by 30 ~ 50%. Refrigeration industry to protect the environment and energy should be put through to the life cycle of refrigerating equipment. As a refrigeration cycle, one of the four parts throttle device in the system plays a crucial role, by selecting the apply appropriate throttle bodies and refrigeration system matching the entire refrigeration equipment to reduce energy consumption. This article will throttle body works and run energy match analysis, focusing on electronic expansion valve works for analysis. 2 traditional throttle body works and matching throttling works is the refrigerant flowing through the valve when flow section suddenly shrunk, fluid velocity, pressure, pressure drop size depends on the ratio of the flow cross section shrinkage. Throttle body: 1) throttle buck. When the room temperature and pressure of the refrigerant saturation liquid flows through the throttle valve, low temperature and low pressure refrigerant liquid and gas produced a little Flash. Realize the purpose of the external receiver.
2) regulating the flow: throttle through sensor feel the evaporator outlet refrigerant superheat changes to control valve to open degree, regulation enters the evaporator of the refrigerant flow so that it flows and evaporator heat load matching. When the evaporator heat load increases the valve to open degree also increases, refrigerant volume increases, on the contrary, the refrigerant volume reduction.
3) controlling superheat: throttle bodies with control of the evaporator outlet refrigerant Superheat of function, keeping the evaporator heat transfer area of leverage, and prevent damage to the compressor suction with liquid of accidents.
Evaporation of liquid level 4) control: with level control of throttle bodies with control function of the level of evaporation, keeps the evaporator heat transfer area of leverage, and prevent suction with liquid lower suction superheat. If throttle bodies to the evaporator for volume compared with evaporation load is too large, part of the liquid refrigerant together into the compressor, wet compress or punching cylinder. On the contrary to be fluid and evaporator load too few compared to the evaporator parts of the heat transfer area failed to fully play its effectiveness, even causing evaporation pressure decrease and the refrigeration system cooling capacity reduction, reduced refrigerating factor of energy consumption of refrigerating equipment. Throttle bodies flow regulation on refrigeration appliances save energy plays a very important role. Large central air-conditioning chillers used throttling mechanism has manual throttle valves, orifice, thermostatic expansion valve, float + main throttle valve. 2.1 manual choke valve manual throttle valve is the most old-fashioned throttle valve, its appearance is similar to an ordinary stop valve. It consists of a valve body, valve, valve stem, the packing pressing and covering cap, hand wheel and bolts and otherparts. And stop valves differs in that it's a needle-shaped valve or with v-shaped notch in the cone and the valve stem adopts fine thread. When you rotate the hand wheel, you can make the valve opening slowly increase or decrease in order to ensure good performance. Manual throttle opening size, require operators frequently adjust to accommodate the load changes. Usually opening to 1/8-1/4 turn, generally not more than one circle, open will be ineffective without spending big to throttle (expansion). The throttle valve has now been replaced by automatic throttle bodies. 2.2 orifice plate orifice throttle bodies composed by two orifice, with two-level throttling. Refrigerant through the first level of orifice, refrigerant just reached saturation liquid line, and produces a small Flash gas; because the Flash gas occupies part of the space, its flow is also on the fluctuations which refrigerants into the secondary fluid flow orifice, to a certain extent (approximately 20%), thus automatically adjust the amount of refrigerant cycle function, level 2 orifice flow due to a change in the different pressure drop, and system high low dropout regulator, on balance, stability play a refrigerant expansion capabilities and complete the entire refrigeration cycle. A secondary design basis: 1 orifice) traffic formula: q = a x A x (2 x Δ p x ρ) 2 1/2) chiller standard conditions: 12 ° c/7 ° c; 30 ° c/35 ° c. Chillers in standard operating mode at full load at run time, the orifice to the evaporator for fluid and evaporation load matching. But the actual run regularly in the unit under variable conditions, varying load operation. In the large pressure conditions, the demand to reduce the load of the evaporator (range greater than 20%), orifice adjustment allowance maximum 20%, due to the pressure increases, the actual available capacity than orifice plate evaporator load required volume, lower suction superheat, wet compress; in small pressure conditions, the demand for evaporator load increases (ranges greater than 20%), due to the pressure decreases, the actual storage capacity evaporator than evaporator load needed volume small, the suction superheat, cooling capacity, reducing the coefficient of decrease, refrigeration, cooling energy consumption increases; the low load into high load situations (range greater than 20%), increased demand for evaporator load, because the refrigerant mass flow increases, the short time the actual deposit fluid evaporator than evaporator load needed volume small, the suction superheat, cooling capacity, reducing the coefficient of decrease, refrigeration, cooling energy consumption increases; in the high load to low load (20% margin greater than), evaporator load demand decrease, reduce the refrigerant mass flow in a short time the actual deposit fluid evaporator than evaporator load required volume, lower suction superheat, wet compress, extreme conditions i.e. Unit full load suddenly downtime, reduce the demand for evaporator load 75%, since the refrigerant mass flow suddenly reduced 75%, short-time evaporator actual deposit amount minus than evaporatorLotus need fluid big 55%, suction superheat rapidly reduced, thereby reducing exhaust superheat, oil effect decreased or even cause compressor run oil. Although a secondary orifice in a range are automatically adjusted, but its cover design, variable load capacity, and refrigeration factor reduces the energy consumption of refrigeration equipment, increase generally should not be used. 2.3 thermostatic expansion valve thermostatic expansion valve is widely used in central air-conditioning chillers. It can control the evaporator for fluid and throttle saturated liquid refrigerant. According to thermostatic expansion valve structure, divided into internal balance and balance the two. Considering the refrigerant flows through the evaporator to produce certain pressure loss, reduced opening superheat, improve the evaporator heat transfer area of utilization, General since the expansion valve outlet to the evaporator outlet, refrigerant pressure drops by evaporating temperature drop of more than 2 to 3 degrees centigrade, should use exdpansion valve. Exdpansion valve works is built on the power balance. Work, by elastic metal diaphragm upper sensor pressure refrigerant in P3, the following from the evaporator outlet pressure P2 P1 and spring force. Diaphragm in three forces, both up and down, up, so that the valve hole shut down or up to regulate evaporator for fluid. When entering the evaporator capacity less than evaporator heat load of need, the evaporator outlet steam of superheat increases, the pressure above the diaphragm is greater than the lower pressure, which forced the diaphragm down drum out through the top rail compression spring and valve pin top opening, the valve hole up, for volume increase. On the contrary when for fluid-than evaporator heat load of need, the exit of the vapor of superheat reduced heat system pressure reducing, patch above the forcing is smaller than the underlying forces that make up the drum out diaphragm, spring elongation, top rail moves up and down the valve hole, on evaporator for fluid will be reduced. Thermal expansion valves for superheat by opening superheat and effective superheat, open the superheat and spring preload, effective superheat and spring of strength and valve needle itinerary. Expansion valve springs are designed according to the standard mode, the unit in standard mode, the unit full-load or load operation will maintain relatively high COP value. But in the large pressure conditions, to reduce evaporation pressure, evaporator load demand volume decrease, but on the contrary, in the suction superheat unchanged, because the pressure to reduce evaporation, the evaporator outlet pressure P1 corresponding lower, upper and lower diaphragm pressure difference larger, so the main valve to open degree increase for volume increase, but at low pressure condition, evaporation pressure increased, evaporator load demand volume increased, but the reality is in suction superheat unchanged, because of the evaporative pressure increases, the evaporator outlet pressure P1 increased accordingly, diaphragm and upper differential pressure, the main valve to open degree decrease for volume reduction, variable load. Therefore thermal expansion valves in variable conditions for fluid regulation aspects need to be further improved. Thermostatic expansion valve principle diagram shown in Figure 1:
Figure 1 thermostatic expansion valve principle sketch 2.4 float + main throttle valve float + main throttle valve is used to have a free surface of the evaporator, where horizontal full fluid evaporator for automatic adjustment of liquid. Through the floating ball valve of the evaporator can remain more or less constant level. Ball valves have a cast iron shell, with a liquid pipe and gas pipe, respectively and controlled evaporator of liquid and vapour in two parts, so the surface shell of the ball and the evaporator of the liquid surface. When the liquid level in the evaporator decreases, the shell of the liquid is reduced to float down the aperture pin valve will open, then the ball out of the volume increases, the ball out of the fluid pressure in the upper part of the formation of the valve core, primary expansion P4 reduce cartridge upside pressure Ps (including main expansion valve the upper spring force P5 and ball out fluid pressure P4) decreases, when the main expansion spool Ps greater than the lower pressure P1, promote main spool up, increasing the amount of open, main expansion valve for liquid increase; on the other hand the main volume decrease of expansion valve. Ball out of the liquid and the main expansion valve core and upper differential pressure (Δ P = P1-Ps) form the scale, the size of the adjustment for the amount of fluid, when shell of level within rose to the ceiling, the float valve PIN will be opening in close, P1, main Ps ＞ the expansion valve closed and stop feeding vaporizer level at this time no longer rising, this can prevent the evaporation of liquid level too high compression caused by wet and guarantee the evaporator for fluid and evaporation load matching. Because the upper part of the main expansion valve core is based on standard conditions of spring, so the unit in standard mode, the unit full-load or load operation will maintain relatively high COP value. But in small pressure differential mode, condensation pressure decrease, P1, P1 relative to the valve the upper spring force is small, so that the main valve to open degree is small, for low volume, leading to a needed evaporation level to a certain time lag, the system reduces the refrigerating factor, the energy consumption of refrigeration equipment, the same is true under variable load. Float + main throttle in variable conditions for fluid regulation needs to be further improved. Float + main throttle valve principle diagram shown in Figure 2:
Figure 2 float + main throttle valve principle sketch 3 electronic expansion valve works and control 3.1 electronic expansion valve-suction superheat control suction superheat control systems consist of electronic expansion valves, pressure sensor, temperature sensor, controller, work pressure sensor to the evaporator outlet pressure P1, temperature sensor, the compressor suction superheat passed on to the controller, the controller will signal processing, then the output directive to the electronic expansion valve with stepper motor home, open the valve to the desired location. To keep the evaporator need for fluid. Electronic expansion valve with stepper motor is based on the evaporator outlet pressure P1 change, compressor suction superheat dynamic real-time output changes, this dynamic real-time output change in time to overcome a variety of operating conditions and a variety of load cases main expansion valve change spring force, so that the valve to open degree meetevaporator for fluid needs to be volume of evaporator can be matched with the evaporation load, i.e., electronic expansion valve can be set through the controller to the effective control of the superheat. In addition, electronic expansion valve from full-closed to full open state when it only takes a few seconds, the response and the action fast, open and close feature and speed can beMan-made; electronic expansion valve can be 10%-100% range for precise adjustment and adjustment range according to different product characteristics are set. Selection of electronic expansion valves – suction superheat control units both in standard mode, design, full load, variable load operation to maintain a high level of COP value. Electronic expansion valve-suction superheat control refrigeration system schematic diagram shown in Figure 3:
Figure 3 electronic expansion valve-suction superheat control refrigeration system schematic diagram 3.2 electronic expansion valve-liquid level control level control systems consist of electronic expansion valves, liquid level sensors, liquid level controller. When the liquid level in the evaporator bottom change, evaporator of liquid level sensor will change in the proportion of liquid level with 4-20mA signals to liquid level controller, liquid level controller will signal processing, then the output directive to the electronic expansion valve with stepper motor home to open degree increase, decrease, to maintain refrigerant liquid level in the limit of the range. Electronic expansion valve with stepper motor is in accordance with changes of refrigerant liquid momentum real-time output changes, this dynamic real-time output change in time to overcome a variety of operating conditions and a variety of load cases main expansion valve change spring force, so that the valve to open degree meet evaporator for fluid needs to be volume of evaporator can be matched with the evaporation load, i.e., electronic expansion valve can be set through the controller to the effective control of the evaporation of liquid level. Use electronic expansion valve-liquid level control, units both in standard mode, design, full load, variable load run are maintaining a high level of COP value. Electronic expansion valve-level control of general application in the suction superheat is below 2 ° c of refrigeration equipment, and electronic expansion valve-suction superheat general application in the suction superheat 5 ℃ or so of refrigeration equipment, the former than the latter more effective use of evaporation and increase evaporation load, gets higher COP value. Electronic expansion valve-liquid level control refrigeration system schematic diagram shown in Figure 4:
Figure 4 electronic expansion valve-liquid level control refrigeration system schematic diagram 4 closing the throttle bodies in order to save energy, in different conditions and different load assure evaporator for fluid and evaporation load matching. Energy-saving approach is timely control superheat (control level), the effective regulation of flow in real time. Electronic expansion valve in the superheat control (level control), flow regulation are better than traditional throttle bodies, and reaction speed, adjust the broader, more significant energy saving effects, has broad application prospects. References 1, International Institute of refrigeration (HR) Director dr.f.billiard. refrigeration and sustainable development
2, Han Bo Qi, Lee Woods. principles and applications of refrigeration and air conditioning in 1998.