How about that rule of thumb that states your condensing temperature should be about 30 degree's above ambient? Is the high side pressure(at service port)actually the condensing temp of the refrigerant?
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How about that rule of thumb that states your condensing temperature should be about 30 degree's above ambient? Is the high side pressure(at service port)actually the condensing temp of the refrigerant?
That's an old rule of thumb for unitary air conditioning equipment and self-contained refrigerators. These days you could see a 20 degree temperature difference because of efforts to save electricity by making the condensers larger.
In built up a/c or larger refrigeration systems the rules of thumb are: 10 deg TD for low temperature, 15 deg TD for medium temperature, 20 deg TD for high temperature or air conditioning.
You are right about the high side pressure representing the condensing temperature.
thanks for the help Dan. Are these numbers for all refrigerants? I do understand that they are rules of thumb and many other factors come into play in charging a system.
I think you could use these numbers for all refrigerants. Glide has to be taken into consideration when using the blends... meaning that the pressure you read at the discharge would correspond to the bubble point and not the dew point of the refrigerant.
Gary Lloyd has a technique called the TECH method which looks at 8 significant temperatures:
Condenser air in, air out, saturated discharge temperature and leaving liquid temperature.
Evaporator air in, air out, suction temperature leaving the evaporator, and saturated suction temperature.
By looking at what is happening to the air as well as what is happening to the refrigerant, you have a much more reliable method of establishing a proper charge.
angryk was refering to ambient + 30F for the condenser.
Dan was ,I think, refering to the TD of the saturated refrigerant temperature and the temperature of the load applied on the cooling coil,(i.e. box temperature )
The only variation I have seen ,is that of heat pumps. The outddor coil is usually slightly oversized ( to provide better heat rejection ) and in the reverse cycle better (more) heat absorbtion from the outdoor air. Also the indoor coil is usually a little undersized to give a higher heat output, and in the reverse a little colder coil to remove moisture. These are the the rules of thumb, but then there is also a rule of thumb in air conditioning of 17-21F TD on air across the cooling coil(supply & return)..but I tried one time to see what it would be like in a refrigeration walk-in and you could barely see any TD in the air temperatures in the box, but the temperature adventually drops...I thought it would have been about 5-10F??anyone know?
Mike
It is a rule of thumb. Many things can make it be off. Type of expansion valve and evap load are the biggest. As far as the discharge pressure, it will be differant than the condensing pressure. There is a lot of turbulance in the condenser. So there can be a fair sized pressure drop in the condenser. I usually assume less than 25 psi difference between discharge pressure and condensing pressure.
I have no problem with using rules of thumb to ballpark the system, and then when it is closer to design temp, gather up all of the relevant data and fine tune the system. But techs should not confuse rules of thumb with doing it right. And newbies shouldn't even be taught rules of thumb, until they are thoroughly familiar with fine tuning.
Actually I was referring to ambient versus saturated refrigerant temperatures in regard to condensers. (I was thining deg F, however.) Thus, the rule of thumb proposed by Angryk would be way off with Centigrade degrees.Quote:
angryk was refering to ambient + 30F for the condenser.
Dan was ,I think, refering to the TD of the saturated refrigerant temperature and
the temperature of the load applied on the cooling coil,(i.e. box temperature )
But you bring up a good point in general, Mike, that holds to the question. The same pattern appears with the evaporators and their sizing quite often, as well. The colder the temperature one is attempting to achieve, the more justifiable it becomes to increase the size of both heat exchangers simply because of the relative effect upon compressor ratios and energy consumption.
Been there, done that and scratched my head the same way. It boils down to bypass factor, or how much of the air is treated as it flows through the coil, along with the TD which for aforesaid reasons is less than an a/c TD. Fin spacing is greater, for example to permit frost accumulation.Quote:
but I tried one time to see what it would be like in a
refrigeration walk-in and you could barely see any TD in the air temperatures in
the box, but the temperature adventually drops...I thought it would have been
about 5-10F??anyone know?
Mike
But I have measured the same small rise that you describe.
I think, superheat, that if you assume 20 deg F pressure drop through a condenser that you are incorrect. The truest place to read condensing pressure is where the liquid condenses, which is not realistic, since it is midway in the condesner. Maybe reading liquid pressure is the best if you have the ports available... something to think about. But as much as 25 psig sounds like a whole lot of pressure drop that I am not used to seeing through a properly sized condenser and certainly way too large an allowance for half-way through the condenser, were one to assume that is where condensing takes place.Quote:
So there can be a fair sized pressure drop in the condenser. I usually
assume less than 25 psi difference between discharge pressure and condensing
pressure.
Interesting and good questions all, though. Thought provoking stuff. I must say, if you have more than 5 psig pressure drop through a condenser, that you would be safer to use the liquid pressure to establish condensing pressure. And am willing to concede that maybe liquid pressure should be the only thing read as condensing pressure.. because that is where we establish saturated liquid properties.
Dan
PS: Let's always refer to the difference in air temperature versus a saturated refrigerant as "Temperature Difference" or TD. And refer to the difference in air inlet temperatures versus air outlet temperatures as "Delta Temperature" or dT.
I fined that 30 deg. over ambeant temp.is close when unit is at temp. I CHARGE TO FULL SIGHT GLASS NOTE HI PSI.THEN CHECK SUPER HEAT .THEN TD AT EVAP COIL YOU CAN,T CONTROLL HI SIDE PSI.(old coil dirt ect)unit needs proper amount of liquid when at temp.out side units need fan control or head master to have proper psi. drop at txv :D :D