Need Help- Calculating Net Cooling Capacity with R-22 Replacements

[terrywilson]

I am a HVAC/R Instructor at a Technology School and we are testing R-22 "drop-in" replacement refrigerants which do not require an oil change.

We are attempting to determine net cooling capacity of a unit that we have been performing trials with using the following formula -

Net Cooling Capacity =
1.08 x CFM x Temp. rise (delta T) - Motor Heat in Btu/hr

CFM = Outdoor fan cfm
Delta T = temp rise or split of condenser
Motor Heat Btu =
Volts x Amps x 3.41 x power factor % (.95) = Btu

What I need help with is this -

One of the refrigerants in the test has a much lower delta T across condenser which I believe is due to having a much lower discharge temp. and discharge superheat. This lower delta T when used with the formula gives me a lower net cooling capacity than R-22 and the other test refrigerant, yet the temp drop across the evaporator coil is more than the other two. (The refrigerants were all tested over different days at almost identical indoor and outdoor conditions)

How do you compensate for the different delta T and discharge temps when calculating the net cooling ?

An additional note - this refrigerant had a higher subcooling also which I believe translates to the better temp. drop across evap. due to less flashgas loss to net cooling effect. Also does this while drawing less compressor amperage and running cooler than the others.

I have more numbers and info available if needed.

Thanks for any info and input in advance.

Terry

[mad fridgie]

If I underatand you correctly
You have an existing system, fixed speed compressor, evap fan, cond fan.
Stable air (db, and wb) on to the cond and evap.
The net cooling is temp/Rh, leaving the evap, what happens within the system is not important. Only variables are if the coils are induced or forced air, forced air no change, induced changes in air density leaving the coils, thus so air mass flow.

[Magoo]

Hi terry,
as mad suggested you need to estabish the enthalpy change expressed as delta h, for total cooling effect. For power input the formula needs an efficiency factor as well. Suggest a kw hourmeter and data logging / trace, to incorpoate inrush currents etc., to determine actual power costs

[terrywilson]

Thanks for your replies.

So if I understand correctly -
I take the dry bulb temp of the supply air leaving the air handler and divide it by the supply air % of relative humidity ?
What will this give me exactly ?

I am looking for a method to arrive at a btu/hr rating.

Also this is a "draw through" evap coil (in the return air stream) not a "blow through" (in the supply air stream).
Does this change the calculations ?

To clarify what I am doing further -

We are testing a 3 1/2 ton Trane split heat pump in cooling and later will also be testing in heating mode.
We got base-line numbers for the system operating correctly with the correct charge for R-22.

We then ran same test with R-424A (RS-44) and R-438A (MO-99). Tests were performed at near identical conditions of 95 deg. outdoor ambient, 75 deg. indoor dry bulb, and 50% relative humidity.
Units were charged correctly per required superheat for conditions (fixed metering)

Using the original formula in my first post, R-22 and R-438A came out to the same capacity because they had the same net 11 deg. temp rise across the condenser. (They both came out to 42,651 btu)

The R-424A only had a 6 deg. temp rise across the condenser which calculates to only 22,789 btu. What the calculation doesn't account for is the R-424A has a lower discharge superheat, discharge line temp, and higher subcooling which translates to a cooler condenser so less temp rise. This is why I believe my original formula will not work for this refrigerant.

Here are some numbers.
Discharge superheats -
R-424A = avg. 27 deg.
R-22 = avg. 40 deg.
R-438A = avg. 38 deg.

Discharge line temps.
R-424A = avg. 143 deg.
R-22 = avg. 155 deg.
R-438A = avg. 150 deg.

Subcooling
R-424A = avg. 24 deg.
R-22 = avg. 11 deg.
R-438A = avg. 16 deg.

Evaporator split
R-424A = avg. 17.3 deg.
R-22 = avg. 14 deg.
R-438A = avg. 16.75 deg.

Compressor amperage
R-424A = avg. 13.54
R-22 = avg. 15.92
R-438A = avg. 15.3

I do plan to re-test using a watt-hour logging meter for the power usage comparisons.

Thanks again in advance for your time.

Terry

[Magoo]

Hi Terry,
first you have to establish the evaporator performance. To do that you need to know the actual air flow, disregard the catalogue figures and do a air on face velocity check with an anomometer [ tested and certified for accuaracy ] plot across and down as much as possible for closer average, apply the accurracy factor. Multiply the the actual face area by the lineal flow rate for air flow.
Then check air dry bulb onto evap versus the air off dry bulb. This will give you a sensible heat cooling effect.
Recheck the same with wet bulb temps that will give you the total cooling effect.
The difference will give the SHF [sensible heat factor. ] or cooling effect, moisture removal for no temp change.
Do you need the calc formula, I work in SI metric.

[terrywilson]

I may need the formula, yes.

Is sensible heat factor and sensible heat ratio (SHR) the same thing ?

Would the formula below be correct for for btu/hr capacity ?

Btu = CFM x (Evap split x 1.08) / SHR

Thanks again,

Terry

[shaaf]

Thanks for your replies.

So if I understand correctly -
I take the dry bulb temp of the supply air leaving the air handler and divide it by the supply air % of relative humidity ?
What will this give me exactly ?

I am looking for a method to arrive at a btu/hr rating.

Also this is a "draw through" evap coil (in the return air stream) not a "blow through" (in the supply air stream).
Does this change the calculations ?

To clarify what I am doing further -

We are testing a 3 1/2 ton Trane split heat pump in cooling and later will also be testing in heating mode.
We got base-line numbers for the system operating correctly with the correct charge for R-22.

We then ran same test with R-424A (RS-44) and R-438A (MO-99). Tests were performed at near identical conditions of 95 deg. outdoor ambient, 75 deg. indoor dry bulb, and 50% relative humidity.
Units were charged correctly per required superheat for conditions (fixed metering)

Using the original formula in my first post, R-22 and R-438A came out to the same capacity because they had the same net 11 deg. temp rise across the condenser. (They both came out to 42,651 btu)

The R-424A only had a 6 deg. temp rise across the condenser which calculates to only 22,789 btu. What the calculation doesn't account for is the R-424A has a lower discharge superheat, discharge line temp, and higher subcooling which translates to a cooler condenser so less temp rise. This is why I believe my original formula will not work for this refrigerant.

Here are some numbers.
Discharge superheats -
R-424A = avg. 27 deg.
R-22 = avg. 40 deg.
R-438A = avg. 38 deg.

Discharge line temps.
R-424A = avg. 143 deg.
R-22 = avg. 155 deg.
R-438A = avg. 150 deg.

Subcooling
R-424A = avg. 24 deg.
R-22 = avg. 11 deg.
R-438A = avg. 16 deg.

Evaporator split
R-424A = avg. 17.3 deg.
R-22 = avg. 14 deg.
R-438A = avg. 16.75 deg.

Compressor amperage
R-424A = avg. 13.54
R-22 = avg. 15.92
R-438A = avg. 15.3

I do plan to re-test using a watt-hour logging meter for the power usage comparisons.

Thanks again in advance for your time.

Terry

are the temp. in deg C are F

[terrywilson]

Sorry, I'm in the US.. Guess most of you here aren't.

All the temps listed are in degrees fahrenheit. F


Terry

[udarrell]

Sorry, I'm in the US.. Guess most of you here aren't.

All the temps listed are in degrees fahrenheit. F
Terry
If temps are in F, unless a very high SEER, that is a low condenser temp-split.
What SEER Rating is that condenser?

Get the low-cost wet bulb Testo Tester & good anemometer to check CFM through cooling coil.

The information on Testo - easy checking wet bulb temperatures:
http://www.amazon.com/Testo-605-H2-H.../dp/B000774B6A
-----
This is simple, except for getting an accurate CFM through the cooling coil, & should be helpful:

Cooling coil CFM X* 4.5 @sea-level, or use X* 4.35 if 1000' above sea-level, X's change in enthalpy = BTUH (Ballpark) Operating Performance.

"U Must Right Click Link & open in New Tab," look-up wet bulb enthalpy figures on chart," & figure the enthalpy change. PRINT & USE the CHART:
Wet Bulb Enthalpy Chart (http://www.udarrell.com/wet_bulb_enthalpy_chart.pdf)

Use to compare test refrigerant performances.

[shaaf]

are the temp. in deg C are F

thanx my dear
i hope you will me positive response in the future