Presentation on theme: "Metering Devices North Seattle Community College HVAC Program"— Presentation transcript:
1 Metering Devices North Seattle Community College HVAC Program Instructor – Mark T. Weber, M.Ed.
2 RULE 1 Life is not fair - get used to it RULE 1 Life is not fair - get used to it. RULE 2 The world won't care about your self-esteem. The world will expect you to accomplish something BEFORE you feel good about yourself. RULE 3 You will NOT make 40 thousand dollars a year right out of high school. You won't be a vice president with car phone, until you earn both. RULE 4 If you think your teacher is tough, wait till you get a boss. He doesn't have tenure. RULE 5 Flipping burgers is not beneath your dignity. Your grandparents had a different word for burger flipping; they called it Opportunity. RULE 6 If you mess up, it's not your parents' fault, so don‘t whine about your mistakes, learn from them. RULE 7 Before you were born, your parents weren't as boring as they are now. They got that way from paying your bills, cleaning your clothes and listening to you talk about how cool you are. So before you save the rain forest from the parasites of your parent's generation, try delousing the closet in your own room. RULE 8 Your school may have done away with winners and losers, but life has not. In some schools they have abolished failing grades and they'll give you as many times as you want to get the right answer. This doesn't bear the slightest resemblance to ANYTHING in real life. RULE 9 Life is not divided into semesters. You don't get summers off and very few employers are interested in helping you find yourself. Do that on your own time. RULE 10 Television is NOT real life. In real life people actually have to leave the coffee shop and go to jobs. RULE 11 Be nice to nerds. Chances are you'll end up working for one.
3 Key Points on Expansion Valves: Thermal Expansion Valve Systems are charged primarily by sub-coolingSince the TXV is designed to regulate a constant superheat, the actual superheat just needs to be “checked” to make sure the valve is performing correctlyDesign Superheat: degreesField Superheat: 8-28 degrees
4 CompressorSensing bulb & capillary tubeDiaphragmEvaporatorCondenserEqualizer tubeLiquid refrigerantfrom condenserLiquid refrigerant to evaporatorNeedle valveTXV Valve bodySuperheat spring
5 CompressorSensing bulb & capillary tubeAs the vapor refrigerant enters the condenser, it starts to give off heat. It will remain at its saturation temp until it has completely turned into a liquid.DiaphragmEvaporatorCondenserEqualizer tubeLiquid refrigerantfrom condenserLiquid refrigerant to evaporatorNeedle valveTXV Valve bodySuperheat spring
6 CompressorSensing bulb & capillary tubeWe want about 10 – 15 % liquid refrigerant at the bottom of the condenser. Because it is still warmer than the air passing by it, it will continue to give off heat. This is when its sensible temp falls below its saturation temp. This is subcooling.DiaphragmEvaporatorCondenserSubcooled refrigerantEqualizer tubeLiquid refrigerantfrom condenserLiquid refrigerant to evaporatorNeedle valveTXV Valve bodySuperheat spring
7 CompressorSensing bulb & capillary tubeDiaphragmEvaporatorOn the other side, the liquid refrigerant enters the bottom of the evaporator. Because it is cooler than the air passing over it, it absorbs heat. We want about 10 – 15% liquid at the bottom and for it to turn completely into vapor by the time reaches the top 25%. Because it is still cooler than the air it will continue to absorb heat and its sensible temp will rise above its saturation temp. This is superheat.CondenserEqualizer tubeLiquid refrigerantfrom condenserLiquid refrigerant to evaporatorNeedle valveTXV Valve bodySuperheat spring
8 CompressorSensing bulb & capillary tubeDiaphragmEvaporatorCondenserEqualizer tubeLet’s see this in operation.With the compressor off, we have approx. equal pressure on both sides and the sensing bulb is about 1/3 liquid refrigerant and 2/3 vapor.Needle valveNow, let’s add some gauges and thermometers.
9 CompressorSensing bulb & capillary tubeDiaphragmEvaporatorCondenser95Equalizer tubeNeedle valveThere is a call for cooling and the compressor and condenser fan start. The condenser fan is pulling 95 degree air over the condenser coils.Because the needle valve is closed, the compressor quickly builds up pressure on the high side of the system.
10 364CompressorSensing bulb & capillary tubeDiaphragmEvaporatorCondenser95Equalizer tubeNeedle valveLets make this a SEER 13 system. With a SEER 13 system, we want the condenser coil to have a saturation temperature 15 degrees warmer than the air passing over the coil. With R410a and a saturation temp of 110 (95+15), the pressure needed is 364 PSIG.Because the needle valve is closed, the compressor quickly builds up pressure on the high side of the system.
11 364CompressorSensing bulb & capillary tubeDiaphragmEvaporatorCondenser12695Equalizer tubeThe indoor blower has also started and the air passing over the evaporator coil is 75 degrees and 60% R.H.. With a 60% R.H., we want a temperature drop of 12 – 17 degrees. In addition, we want our superheat in the range of 10 – 15 degrees = a temperature difference of 32 degrees. 75 – 32 = 43 degrees for our saturation temperature. That equals a pressure of 126 PSIGNeedle valveBecause the needle valve is closed, the compressor suction quickly lowers the pressure on the low side of the system.
12 364CompressorSensing bulb & capillary tubeDiaphragmEvaporatorCondenser12695Equalizer tubeTo keep the two sides in balance, the needle valve has to open and close to allow the correct amount of refrigerant pass throughNeedle valveIf the needle valve were to remained closed, the pressure on the high side would continue to rise and the pressure on the low side continue to drop.