300 cfm per ton?
Comments
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if the pressure drop through the evaporator is less than 5 pounds they use an internally
Over 5 pounds externally equalized
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(also didnt' uinderstadn until about an hour ago that "equalize" here doesn't mean what equalize usually means in refrigeration systems. Here it means it is equalizing the thermostat bellows in the txv rather than equalizing the suction and discharge sides of the compressor.)
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So I know what a TXV does, but I have a question about the details. The TXV controls the flow of refrigerant through the evaporator in response to changes in the cooling load. It does this by sensing the difference between the temperature of the refrigerant when it enters the evaporator and when it exits. If the temperature difference rises that means that more heat is being removed, so the valve opens and more refrigerant is allowed to flow. If the temperature difference falls that means less heat is being removed, so the valve closes and less refrigerant flows.
In normal operation, with a constant fan speed you'll get variation in heat removal depending on the temperature and moisture content of the incoming air.
So here's my question: going back to the original post, if you reduce the fan speed, what happens to the coil temperature? My guess is that it drops, the cooling load is lowered, the TXV senses that and reduces the flow of refrigerant. But because of the way the TXV works, in order for the flow to stay reduced the temperature difference has to stay somewhat smaller than it does in normal operation, so the coil stays colder.
But that's just my guess. It occurs to me that it could also be the case that with a lower flow and the compressor operating at the same speed the refrigerant enters the evaporator hotter, so even though the temperature difference is smaller the exit temperature is higher.
I don't know. Anyone know the answer?
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if less refrigerant is flowing in to the evaporator it will stack up in the condenser as a liquid at a lower temp because less heat is being added o the condenser from the evaporator.
a cooler coil temp and less airflow doesn't necessarily mean less energy is being absorbed by the evaporator. it depends on the delta t of the air, the volume of air, and the amount of moisture condensed. or to put it more simply it depends on the wet bulb delta t and volume of air of both operating points.
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If the temperature and humidity of the incoming air are kept constant, along with the airflow, a cooler coil always results in more cooling, both latent and sensible. In the temperature ranges that HVAC operates a cooler coil will always increase the latent cooling more than the sensible cooling, which lowers the sensible heat ratio and means more dehumidification relative to the amount of cooling.
But the question I still have is the opposite: if you drop the airflow and keep everything else constant, what happens to the coil temperature? Intuitively, it seems to me that it would drop. So let's say you drop the airflow by 25%. The total cooling doesn't drop by 25%, the coil temperature drops so you're getting more cooling per unit of air and maybe total cooling drops by 20%. This squares with my understanding of how the TXV works, it closes and restricts the flow of refrigerant. The compressor keeps spinning at the same rate but delivers less refrigerant.
But I'm just guessing, I don't know.
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A TXV doesn't sense the temperature difference between the input and output of the evaporator. This doesn't work like hot water baseboard.
In some evaporators, there may be no difference in temperature between the input and output. That's dependent on pressure. Flow is controlled by superheat of the gas leaving the evaporator. The objective is to keep the evaporator as full as you can without risking liquid going back to the compressor.
But I believe you're correct, you'll get less and less flow, unless latent load keeps the refrigerant boiling. You're battling that more and more as you attempt to drop the dew point.
Single pipe 392sqft system with an EG-40 rated for 325sqft and it's silent and balanced at all times.
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@ChrisJ : "A TXV doesn't sense the temperature difference between the input and output of the evaporator."
The TXV balances two inputs. One input is a sealed tube of refrigerant that is physically connected to the outlet of the evaporator. The pressure in that tube is entirely determined by the outlet temperature of the evaporator. The other input is connected to the refrigerant at the inlet to the evaporator, the pressure there is entirely determined by the inlet pressure in the evaporator.
If either the outlet temperature drops or the inlet pressure rises, the valve moves in a direction that reduces the flow. If either the outlet temperature rises on the inlet pressure drops, the valve moves in the other direction and increases the flow.
Clearly the inlet pressure varies, if it didn't you could control the valve with a much simpler temperature-only sensor on the outlet.
Intuitively it seems (in other words, I'm guessing) that a reduced flow of refrigerant leads to a reduced inlet pressure. If that's the case, then the system balances when the outlet temperature is lower.
But I'm willing to accept that my intuition is wrong. There are other pieces to the system. What happens in the compressor and the condenser when the refrigerant flow is reduced? Does it come in hotter, or under higher pressure? I don't know.
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Remember, as mentioned earlier, many txv's sense the outlet pressure, not inlet. Pecmsg said this is based on evaporators having more than 5 psi drop, I guess more than that and they add the external equalizer which now measures the outlet pressure.
The outlet pressure is what's important, they only measure the inlet because it's cheaper, easier and works good enough on smaller equipment.
I believe the condenser's pressure is going to primarily be effected by the outdoor temperature. As the TXV closes, the evaporator should have the same amount of liquid in it regardless. Your subcooling (extra liquid in the condenser) and superheat should remain fairly consistent with a proper working TXV regardless of varying conditions.
How much the TXV is open I think depends on the pressure drop from the highside to the lowside. More condenser pressure (hot sunny day) means the valve will be closed more than on say a cool night. I think when it's 50s-60s out is when a TXV on an air conditioner is most likely to be close to wide open if not wide open.
But honestly, I think you're asking stuff that's beyond my knowledge. Someone like @pecmsg or @ratio or @EBEBRATT-Ed should be answering.
Single pipe 392sqft system with an EG-40 rated for 325sqft and it's silent and balanced at all times.
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the equalizer is to allow movement inside the valve.
as the suction line gets colder the valve closes to maintain the SH setting.0 -
Allow movement inside the valve? I don't understand.
Single pipe 392sqft system with an EG-40 rated for 325sqft and it's silent and balanced at all times.
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The diaphragm from the power head and spring are enclosed, The equalizer line allows them to move up and down
Below its internally equalized.
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It does,
But I thought it was to give the txv the ability to compare the pressure in the refrigerant charged bulb to the actual pressure in the evaporator?
Compared to just venting the other side of the diaphragm to atmospheric pressure, for example. That would still allow it to move.
Single pipe 392sqft system with an EG-40 rated for 325sqft and it's silent and balanced at all times.
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it modulates the setpoint of the the thermostat to the evaporator pressure.
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it doesn’t read pressure it just allows movement.
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That's how I interpret it…
It's a differential pressure control. In a perfect world both sides should always be identical. The spring / adjustment give a slight offset to give superheat.
Single pipe 392sqft system with an EG-40 rated for 325sqft and it's silent and balanced at all times.
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depends on what refrigerant is in the sensing bulb too. In the old days it was r22 or r12 but it might not match the system refrigerant now.
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I believe you need to use one that's designed for the refrigerant you have. The ones I have are all clearly labeled, for example R410A.
I don't think it would work otherwise.
Single pipe 392sqft system with an EG-40 rated for 325sqft and it's silent and balanced at all times.
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The TXV senses pressure at the evap inlet or at the evap outlet if externally equalized. It senses temperature at the evap outlet.
The valve modulates to maintain a constant superheat. It senses the evap pressure and the evap outlet temp and tries to maintain a constant superheat between these two settings.
If you increase fan speed and put more load on the coil the outlet temp (bulb) rises increasing the superheat the valve opens and lets more refrigerant into the coil to reduce the superheat back to its setting
If you slow the fan you decrease the coil load the coil outlet temp drops and the superheat drop and the valve moves to adjust.
Its complicated because anything that changes like fan speed the whole system will try to rebalance it is not absolute there are to many moving parts. Changing fan speed will change suction pressure, discharge pressure and the system rebalances but the constant with a TXV is that it will try to maintain the same superheat
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Now let's discuss how capillary tubes work and how they're partially variable…………………….
😁
Single pipe 392sqft system with an EG-40 rated for 325sqft and it's silent and balanced at all times.
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Fascinating! I read something like that years ago, but I'm glad you posted it again, things make even more sense to me now.
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I wonder if supco still employs anyone like the person that wrote that.
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Just to throw a twist into this - on the little Trane air handlers with the electronic expansion valve there is a sensor on the distributor tube after the expansion valve that senses the temperature of the saturated liquid. Then there is a temperature sensor on the suction line that senses the temperature of the superheated vapor. The controller modulates the valve to maintain a set superheat.
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that sensor should be on the outlet of the evap.
measuring inlet temperature does nothing.0 -
Well you need something to measure the temp or pressure going in the coil and out of the coil because both TXvs and EXvs control superheat
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