No Drop In Suction Pressure

[jwasir]

Hi All,

I designed a system to cool ambient air to -30F (500 cfm only). They need this air in a lab experiment and unit would be running only for 10-15 minutes at a time.

The unit is designed with two stages of cooling . Stage 1 is cooling from 90F to 30F and stage 2 was supposed to cool air from 30 to -30F.

Air side load calculation turned up with a load of approx 5TR@-30F and we used total 6TR@-40F on compressor side.

There are 2 independent stages (Separate compressors, TXVs etc), first is supposed to bring the temp to 30F and second is supposed to bring it to -30F.
first stage is doing the job of reducing temp. from 60 Ambient to 30F
In second stage has 1/2" coil with 2 FPI.
Presently, all we can get is 5F air temp and SST is -19F (R-404a)

The coil output, as per software is 30,000 BTU.

The comp. Output at this condition is 102,000 BTU but still the suction pressure is not dropping. (Checked Amps as per curves )

Electronic Expansion valve (Stage 2)-- set to 5.5 Deg F Super heat.

Coil-- DX- Evaporator stage 2
Tube -- 1/2"
Pattern -- 1.5" X 1.29"
Fin Height -- 12"
Fin Width -- 15"
Rows -- 10 RD
Fins/Inch -- 2
Feeds -- 5
Distributor capillary -- 5/16

What is baffling me is that even though compressor is showing a capacity of over 100,000 btuh and load on coil is only around 25,000 btuh, Suction pressure still remains in the the range of -19F and is not dropping.

When tried to change CFM to 100 from 500 surprisingly suction pressure remains almost constant. TXV valve opening (in stage 2) changes from 100% at 500CFM to 93% at 100CFM.

There is a hot gas capacity control going to the side port of distributor but that has been turned off (physically) at valve and is ensured that no hot gas is being bypassed to evaporator. System works perfectly on pump down so no possibility of having internal leakages etc. Compressor is Biter Semi-H.

I would appreciate by getting the comments on :

#1) Why Suction pressure is not dropping in stage 2, even though we have no load on coil (reduced CFM). Stage 1 suction drops (TXV opening reduces from 60% to 15% as we reduce CFMs) (Unit is airtight and there are no leakages of air infiltrations.).
#2) I know that there is a lot of air bypass in stage 2 coil as it has 2FPI. Would changing it from 2 to 6FPI make a significant difference? (Defrosting is not a major issue as the run time is fairly short for this system) Should we be looking at the circuiting.
#3) What major precautions ( Circuiting, refrigerant velocity in coil) should be taken if designing a new coil for this application

[powell]

jwasir,

I'm going to really simplify this. You either have a oversized evaporator or undersized condensing unit.

Is the unit you have really pumping 102,000 btuh's @ -19 F ? That would equate to about a 30 HP compressor.

[wambat]

I would go to 6 FPI because you seem to be bypassing too much 30*F air. It would increase heat tansfer substanchaly

[SteinarN]

What is your actual superheat. Are you absolutely 100% sure no liquid is entering the compressor? I would have set a superheat of 15-20F. You have a very high projected DT on the evaporator. I normaly set the superheat to roughly half of the DT on the evaporator. (entering air minus evaporation temperature)

The other thing that comes to mind is the charge. Are you absolutely 100% sure you have enough refrigerant in the system? How do you determine the corect charge? A seriously undercharged system with a substantly oversized EEV/TEV will give exactly that symtom you describe, high suction pressure and lack of capasity. What make is the TEV? How does it sense the evaporation temperature? By pressure or temp.sensor?

What do you mean with 2FPI?

[Brian_UK]

2FPI - 2 fins per inch

[jwasir]

Thanks to All,

I checked the system again.

Charge was good, I just went with SG.


Super heat was set for 5.4 deg and i was getting 3-4 deg at the outlet.

The EEV (Alco made) was almost 100% open.

I readjust the SP to 15 deg and I was getting 9-11 deg at the evap oulet.

Things changed drastically.

Suction went to 5 PSI from the lowest of 18PSI, evap coil temp-25F and air outlet -11F.

It seems to be that EEV dont work at 100% open.

Still delta T seems to be high, may be because of high bypass with 2 FPI coil.

Planning to change the coil to 6FPI but fear it might freeze.

Any inputs will be highly appreciated!

[wambat]

6 FPI is normal for low teperature work , and for the air on the second stage at 30 *F you only have 21 grains of moisture/lb and are only moving 40#/min for a very short time so 6 FPI sounds reasonable to me. The evaporator coil should be able to have a delta t of 10*F so you should need to be about -40*F to get air off at -30*F

[SteinarN]

5 psi suction is reasonable, that is -40F evaporating. You now need a higher capasity coil to get -30F air out.

You need roughly 10kW (little less than 3TR) cooling capasity to cool 0,23m3/s (500CFM) from -1C to -34C (30F to -30F)

At -40C (-40F) evaporating/30C (86F) condensing, and a few degres subcool, you have a suction gas volume flow of 55m3/h. You need 16-18 1/2" feeds or loops in your evaporator to get a reasonable gas speed of 10m/s in the evaporator tubes. I would also go for 6FPI.

If you have only 5 loops in your existing evaporator you must have a horendous pressure drop on the refrigerant side. This causes your actual evaporation temperature to be significantly higher than what your evaporator outlet pressure indicates.

What is your displacement or volume flow of your low side compressor? What is the size of the suction connection?

Edit:
You said -11F air out in your last post. Assuming 30F air in and 500CFM, that translates to a cooling load of roughly 6,5kW. You need 10kW to reach -30F. You now have a reasonable suction pressure on your low stage compressor (5psi, -40F). In other words, you are 3,5kW short in cooling capasity on your low stage compressor to acheive -30F air out. You cant increase the suction pressure to get the extra 3,5kW. Something has to be done with the compressor also. Are you absolutely 100% sure no liquid is still entering the compressor? What happens to the suction pressure and air out if you increase the superheat to, lets say, 25F?
What model is the compressor? What is the condensing temperature?

[daddyfixit]

Hello jwasir...

This is interesting.. I've seen a couple of cascade systems for circuit board testing. They would bring a small chamber (36 cu ft.) from 100*C+ down to about 0*C in about 10 minutes. Is your system cooling a larger space?

[jwasir]

Steinar N,

Once again thanks for your valuable comments.

The compressor is Bitzer 6G-30.2Y (6 cyln- 30 HP)

Compressor will give 5TR @ -40/110F.

I am with you guys and will change the coil to 6FPI, 5/8", 4 feeds-- the pr. drop as per software is 1.2 PSI.

The suction line size is 1 5/8". It's only 5-6'in length.

I tried increasing the SH till 30F, the SST stays same but the coil temp. increases and surprisingly the air outlet was reduced -- went to -15F.

Does EEV will be like a open ball valve when 100% open?


There is no room to cool, the air is used for some experiments and is 1 pass only.

[SteinarN]

Your compressor has a displacement of 127m3/h @50Hz. You should have a compressor with a displacement of only roughly 75-80m3/h as your required gas flow at -40F is roughly 55m3/h.

You also need to double check your calculations on the number of loops in your evaporator. I get a number of roughly 8 5/8" loops with a compressor with a dicplacement of 75m3/h. With your compressor you should have something like, lets say, 13 5/8" loops.

Interesting about the superheat. What type of sensors has the EEV? Where is they placed? What is the dimension on the "manifold" outlet tube on the evaporator?

Edit:
With 4 5/8"loops and a compressor with a displacement of 80m3/h you will have a gas speed at the end of the loops of a staggering 27m/s. You will have a pressure drop of 1.2psi in the 3 last meters on every loop. That pressure drop translates to a loss of evaporating temperature of 1,4C or 2,5F. You get additional pressure drop in the rest of the loops as well as in the suction manifold of the evaporator.

[SteinarN]

Does EEV will be like a open ball valve when 100% open?


Not really. An EEV is sort of a orifice valve. The opening in the valve is a hole of maybe only 3-4mm diameter in your valve at max opening. As the valve closes, a conical nedle is lowered into the hole, gradually decreasing the passage.

[jwasir]

As per Bitzer software, the compressor will give 0.41 m3/hrof mass flow rate with R-404a and -40F/110 @ 60Hz.

And only 4 feeds @ 500 CFM as per coil sotware.-- Coil --5/8"-6 FPI-Row--10--flowrate-791-lb/h--Pr. drop- 1.5 PSI.

Can U please show mw how you calculate the mass flow rate and the coil feeds?

[SteinarN]

At -40/+110F, zero subcool and -13F suction the capasity is 13kW, mass flow is 518kg/hr acording to Bitzer software. The specific volume of the gas at a pressure corresponding to -40F and a temperature of -25F is 0,15m3/kg acording to Coolpack. 518Kg multiplay with 0,15Kg/m3 gives a suction gas flow of 77,7m3/hr. With that number you can calculate your gas speed in your evaporator outlet with different number of loops and different dimension on the tubes.

I have used DIRcalc from Danfoss for simplicity. I enter the evaporating temperature, suction temperature, condensing temperature and liquid temperature. I can guess on 13 loops in the evaporator. I then divide the capasity, 13kW, on 13 loops and get 1kW on each loop. I enter 1kW in capasity. I then get the gas speed and pressure drop in different tube size. The gas speed in a 5/8" tube at 1kW duty is 12,3m/s which is a reasonable speed. That speed gives 0,1psi pressure loss each meter tube. Assuming 2 meter tube at the end of the loop with only gas in it, that 2 meter gives a pressure drop of 0,2psi. In addition the pressure drop in the rest of the loop where it is a mix of gas and liquid.

If I was to select only 4 loops, then the capasity on each loop would be 13kW divided by 4 gives 3,25kW each loop. That would give a gas speed of totally ridiculous 40m/s. The pressure drop would be 0,8psi for one meter tube which is totally unacceptable.

[SteinarN]

Coil --5/8"-6 FPI-Row--10--flowrate-791-lb/h--Pr. drop- 1.5 PSI.

How du you enter the evaporating conditions in the coil software, as pressure or temperature?

[jwasir]

Hi Steinar N,

Thanks for the detailed reply.

I have gone through your calculation in Imperial units and they are very accptable.

But I don't know, if we can incorporate the suction line calculations for the coil sizing.

If I input 13 feeds in the coil software, the pressure drop in complete coil is only 0.1PSI and it gives warning that feeds have to be reduced to get atleast 0.51PSI for refrigerant flow.

The optimum feeds I am getting is 4 at 500 CFM and 30F inlet conditions and -40F SST.

Can U please give it a thought and let me know.

Also, in danfoss software, the fin density is negligible.

Regards,

[jwasir]

In software the input is entered as Ent. DB temp and SST, SH etc.

If EEV has an orifice of 3-4mm then what was really happening, when it was 100% open?

Why was it not maintaining the SH and also when I set the SH to 16, it was only maintianing it to 8-9 deg.

Only after the change the value to 30deg, then I got 30 deg SH.

Can U solve the mystry?

[SteinarN]

Hi Steinar N,

Thanks for the detailed reply.

I have gone through your calculation in Imperial units and they are very accptable.

But I don't know, if we can incorporate the suction line calculations for the coil sizing.

If I input 13 feeds in the coil software, the pressure drop in complete coil is only 0.1PSI and it gives warning that feeds have to be reduced to get atleast 0.51PSI for refrigerant flow.

The optimum feeds I am getting is 4 at 500 CFM and 30F inlet conditions and -40F SST.

Can U please give it a thought and let me know.

Also, in danfoss software, the fin density is negligible.

Regards,

Hmmm, strange.

What is the cooling load from the coil software? It should be roughly 10kW. Does the software tell, or do you enter, the absolute or gauge pressure? Does the software calculate the mass flow and/or the gas volum flow?

Something is completely wrong here. 4 loops give a gas speed of 40m/s in a normal 5/8" tube for your compressor at -40/+110F. It will however be slightly less in your evaporator tube due to that tube have thinner walls and slighly larger internal diameter. Even so, 35m/s is still a totally unacceptable high speed. The pressure loss would be more than 0,5psi for one meter tube!

For clarification: When I talk of 4 loops, and you talk of 4 feeds, do we both talk about the refrigerant dividing into 4 separate flows after the EEV and flowing through 4 different parallell lops or circuits?

[SteinarN]

In software the input is entered as Ent. DB temp and SST, SH etc.

If EEV has an orifice of 3-4mm then what was really happening, when it was 100% open?

Why was it not maintaining the SH and also when I set the SH to 16, it was only maintianing it to 8-9 deg.

Only after the change the value to 30deg, then I got 30 deg SH.

Can U solve the mystry?

I can't tell.

Is the correct refrigerant entered in the regulator? How is the evaporator temperature measured? By pressure sensor or by temperature sensor? Where is the two different sensors placed? What is the dimension of the suction tube between those two sensors?

[750 Valve]

guys is it possible there is a miscommunication between you both?

jwasir is talking "coil feeds" - I understand this to be the number of pipes coming off the distributor to feed a multicircuit coil (eg 4 circuits of X amount of passes)

SteinarN is talking "loops" of the coil - I understand this to be the number of passes each individual circuit in a multi circuit coil does starting from distributor to suction line header, in other words how many rows of tube are in each circuit (eg X circuits of 13 passes each)


just a thought

[SteinarN]

guys is it possible there is a miscommunication between you both?

jwasir is talking "coil feeds" - I understand this to be the number of pipes coming off the distributor to feed a multicircuit coil (eg 4 circuits of X amount of passes)

SteinarN is talking "loops" of the coil - I understand this to be the number of passes each individual circuit in a multi circuit coil does starting from distributor to suction line header, in other words how many rows of tube are in each circuit (eg X circuits of 13 passes each)


just a thought

I'm talking about this:
jwasir is talking "coil feeds" - I understand this to be the number of pipes coming off the distributor to feed a multicircuit coil (eg 4 circuits of X amount of passes)

and not talking about this:
SteinarN is talking "loops" of the coil - I understand this to be the number of passes each individual circuit in a multi circuit coil does starting from distributor to suction line header, in other words how many rows of tube are in each circuit (eg X circuits of 13 passes each)

It would have been interesting to get some news from jwasir......

[jwasir]

Sorry for the Delay.....

I think we both are on the same track.

I'm ALSO talking about this:4 LOOPS and 20 PASSES
jwasir is talking "coil feeds" - I understand this to be the number of pipes coming off the distributor to feed a multicircuit coil (eg 4 circuits of X amount of passes)

From the Sofware --

Cooling Load -- 11.5KW

Pressure Entred -- Gauge pressure.

Refr. Flow Rate -- 642 Lb/Hr

Pr. Drop -- 1.5 PSI

Evap temp is measured by temp sensor, came with EEV.

Both the temp and Pr. sensors are mounted on the evap outlet header, may be 2" from the main suction header.

The suction header is 1 5/8".

[SteinarN]

Now we are getting somewhere. You said 642lb/hr, that is 291kg/hr. To get 11,5kW with that mass flow at -40°C (-40°F) you need a superheat of 50°C (90°F) and a liquid temperature of only 28°C (82°F). With a superheat of 10°C (18°F) you need a liquid temperature of as low as 6°C (42°F). This is clearly impossible with your setup without any SG/LL heat exchanger and no low temp water cooled condenser.

Your compressor has a circulation rate of roughly 520kg/hr (1140lb/hr) at -40°F/+110°F and a normal superheat. This gives a capasity of roughly 13kW.

Your evaporator software seems to be calculating wrong results and/or some inputs are wrong.

Are you entering any value for superheat, entering liquid temperature or entering refrigerant quality?

[jwasir]

Steinar N,

Thanks for the prompt reply.

I'll quickly check the software and will get to you.

Meanwhile, can you please explain what is the impact of mass flow rate for the final air outlet.

What formula do you use for calculations using superheat, entering liquid temperature and entering refrigerant quality?

Regards and Thanks,

[jwasir]

COIL - 12" X 15" X 10 RD -- 4 FPI

5/8"-8 Tubes High

Entering Air -- 500CFM @ 30F

LOOPS -- 4, PASSES -- 20

From SOFTWARE # 1

Cooling Load -- 11.5KW

Refr. Flow Rate -- 642 Lb/Hr

Pr. Drop -- 1.5 PSI

Air Leaving COIL-- -33.1F

Entering Vapour -- 0.25

Superheat -- 6F

From SOFTWARE # 2

Cooling Load -- 10.5KW

Refr. Flow Rate -- 950 Lb/Hr

Pr. Drop -- 4.5 PSI

Air Leaving COIL-- -20.5F

Entering Vapour -- N/A

Superheat -- 6F

Liquid Temp -- 100F

Software # 1 is with R-404A (Actually used), Software # 2 is with R-507

[SteinarN]

Now we are nearly there. You need to set the entering refrigerant quality to 0,65. Increase the superheat to 25°F. Increase capasity to 13kW, alternatively increase entering air temperature so the sofware put out 13kW. Set leaving air to -30°F. The software should calculate a mass flow of roughly 1150lb/hr. Pressure drop should not be more than 1 PSI. Enter 6 FPI.

[SteinarN]

Steinar N,

Thanks for the prompt reply.

I'll quickly check the software and will get to you.

Meanwhile, can you please explain what is the impact of mass flow rate for the final air outlet.

What formula do you use for calculations using superheat, entering liquid temperature and entering refrigerant quality?

Regards and Thanks,

The mass flow rate alone doesn't tell anything other than what the compressor pumping capasity is at that particular operating conditions.

I normaly set a superheat roughly half the difference between evaporating temperature and entering air. In this case that is half of 70°F which is 35°F. That is a bit to much, so in this case enter roughly 20-25°F.

When determining capasity, entering liqiud temperature and refrigerant quality, you have to match those numbers closely to your compressor. You simply take those numbers from your compressor calculation software. At -40/+110°F your compressor duty is 13kW. It is dependent on your evaporator software what numbers you next enter. If you can enter leaving air and capasity, and the software calculates the entering air, that is fine. If you enter the liquid temperature then you must use the condensing temperature from your compressor software, in this case 110°F. If you can not enter the liquid temperature, then you must enter various liquid quality values to get the mass flow the same as from the compressor software. In the end you shall have same values in both the evaporator and compressor software.

[jwasir]

518Kg multiplay with 0,15Kg/m3 gives a suction gas flow of 77,7m3/hr. With that number you can calculate your gas speed in your evaporator outlet with different number of loops and different dimension on the tubes.

How will we calculate the refrigerant speed with mass flow rate?

[SteinarN]

How will we calculate the refrigerant speed with mass flow rate?

It is actually volum flow. When we know the volum flow, we can calculate the speed in different sized tubes.

But this is really not necessarry as the pressure drop is more important. Select the number of loops so you get a reasonably pressure drop in your evaporator. As the absolute pressure is low at -40°F, so should the pressure drop also be.

[jwasir]

Now we are nearly there. You need to set the entering refrigerant quality to 0,65. Increase the superheat to 25°F. Increase capasity to 13kW, alternatively increase entering air temperature so the sofware put out 13kW. Set leaving air to -30°F. The software should calculate a mass flow of roughly 1150lb/hr. Pressure drop should not be more than 1 PSI. Enter 6 FPI.

Things changed DRASTICALLY by changing the above:-

Cooling Load -- 12.2KW

Refr. Flow Rate -- 1220 Lb/Hr

Pr. Drop -- 0.7 PSI

Air Leaving COIL-- -30.5F

Entering Vapour -- 0.65

Superheat -- 20F

Liquid Temp -- N/A

Practicall, what is the vapour/liquid condition in the coil?

Does it stays same for medium temp and low temp coils?

Thanks for the kind help!!

[jwasir]

It is actually volum flow. When we know the volum flow, we can calculate the speed in different sized tubes.

How can we convert ft3/hr to ft/s?

[Toosh]

How can we convert ft3/hr to ft/s?

Hi Jwasir try this site

http://www.ryanlinks.com/convert1.htm

Norm

[SteinarN]

Practicall, what is the vapour/liquid condition in the coil?

Does it stays same for medium temp and low temp coils?

Thanks for the kind help!!

The vapour quality is determined by the difference in the entering liquid temperature and the evaporating temperature. As this difference increases, more of the liquid must evaporate just to bring the liquid/vapour mixture down to evaporating temperature. This is happening in the TEV and to some extent in the distributor. To bring the temperature down from 110°F to -40°F it is necessarry that 65% of the liquid evaporates in the TEV. Only 35% of the entering liquid is consecuently evaporating in the evaporator and performing cooling duty.

[SteinarN]

How can we convert ft3/hr to ft/s?

You get the speed in ft/s when you divide the flow in ft³/s by the cross section of the pipe in ft².

[SteinarN]

Things changed DRASTICALLY by changing the above:-

By the way, what number of loops did you get?

[jwasir]

Total loops after changing vapour quality to 0.65 is 8.

What should be the vapour quality in medium temp coils?

In low temp, does the coil tube diameter (3/8, 1/2 or 5/8), plays any significant role?


To bring the temperature down from 110°F to -40°F it is necessarry that 65% of the liquid evaporates in the TEV. Only 35% of the entering liquid is consecuently evaporating in the evaporator and performing cooling duty.

How did you get this MAGIC number??

[SteinarN]

Total loops after changing vapour quality to 0.65 is 8.

What should be the vapour quality in medium temp coils?

In low temp, does the coil tube diameter (3/8, 1/2 or 5/8), plays any significant role?



How did you get this MAGIC number??

The vapour quality depends on the refrigerant, condensing temperature, evaporator temperature and subcool.

I got the "magic" number from eyeballing a log p-h chart.

The smaller the coil tube diameter, the more efficient is the coil, but at the same time more expensive to manufacture as it is required more total tube lenght.

When calculating an evaporator, use the numbers from the compressor calculation at desired operating condition:
Enter the capasity in kW, enter the superheat, enter the evaporating temperature and enter different vapour qualities until you get the same mass flow in the evaporator calculation as you have in the compressor calculation.

It may be possible to do it differently depending on your evaporator software.

[jwasir]

Steinar N,

THANKS A TON FOR YOUR GREAT HELP!!!

[jwasir]

Steinar N,

Why we need to have low pressure drop in the evaporator?

Also, How can we determine the vapour quality from the PH chart?

[SteinarN]

Steinar N,

Why we need to have low pressure drop in the evaporator?

Also, How can we determine the vapour quality from the PH chart?

The refrigerant inlet is at the air outlet side of the evaporator. If you have say -40°F evaporating which is measured at evaporator outlet, -30°F air outlet and a large refrigerant pressure drop, say 3 psi, then this give a higher evaporating temperature/pressure at refrigerant inlet side/air outlet side of -33°F. It is then probably not possible to reach -30°F air outlet with only -33°F evaporating at the air outlet side of the evaporator.

Draw a vertical line starting at the point for saturated liquid at condenser pressure and down to the evaporator pressure line. At this point your line intersects a vapour quality line. This is your vapour quality.

[jwasir]

Steinar N,

When I draw a vertical line, I get 0.65 vapour quality.

Does this means there will be 35% liquid and 65% vapour in the coil?


Now we are getting somewhere. You said 642lb/hr, that is 291kg/hr. To get 11,5kW with that mass flow at -40°C (-40°F) you need a superheat of 50°C (90°F) and a liquid temperature of only 28°C (82°F). With a superheat of 10°C (18°F) you need a liquid temperature of as low as 6°C (42°F). This is clearly impossible with your setup without any SG/LL heat exchanger and no low temp water cooled condenser.

How can calculate the subcooling with the given mass flow rate?

[SteinarN]

Yes, 35% liquid and 65% gas straight after the TEV and the distributor. But the remaining 35% liquid evaporates gradually through the evaporator.

Its not possible to calculate subcooling by mass flow. It was only an example of what the necessarry subcool had to be to get it to sum up with that mass flow and that capasity. But that mass flow and capasity numbers was total unrealistic.

However I did calculations to get the necessarry subcool at those conditions. Got those numbers from CoolPack.

[jwasir]

SteinarN,

Once again BIG thanks.

Whenever U have time, can U show me which software touse in coolpack and how?