CPR During HG Defrosting

[jwasir]

Hi All,

Does the system actually require CPR during or right after the HG defrost, for a single evaporator system.

Is it possible to cut off the solenoid valve, if HP goes really high during defrost?

Problemwith CPR is that they come in low capacities from Danfoss/Sporlan and create Big PD in the system, during normal cooling cycle?

Has anyone use the HG defrosting systems w/o CPR?

[philfridge]

Hi when using hot gas defrost on a single evaporator system cpr is not needed as it reduces the defrost heating capacity ,just set defrost period for ten to fifteen minutes 4 times per day and have a defrost termination temperature set at 30c
regards phil

[powell]

Hi All,
Does the system actually require CPR during or right after the HG defrost, for a single evaporator system?

First let's look at the definition of a CPR valve:

"An outlet pressure regulator which maintains a predetermined maximum outlet pressure. Designed to prevent compressor motor overload."

Motor overload occurs after defrost and during high load conditions. You must look at the performance chart of the compressor you have and determine the max SSP and RLA and then look at the data of this system during defrost and high load conditions to make a decision whether to remove the CPR valve.


Is it possible to cut off the solenoid valve, if HP goes really high during defrost?
You need to adjust the defrost term switch to prevent this.


Problem with CPR is that they come in low capacities from Danfoss/Sporlan and create Big PD in the system, during normal cooling cycle?

Is the CPR valve undersized per the mfg charts? What compressor do you have? There are bigger valves made. Does the PD affect the pull down time? The CPR valve might not be adjusted properly.


Has anyone used the HG defrosting systems w/o CPR?Not on a single evap system.

A suction accumulator is also required for this system.

Finally, most of the major mfgs here in the states, ie Heatcraft, Hussmann and Krack will not sell a single evap HGD system due to compressor failures resulting from liquid floodback. Russell and Witt will but they have a warranty disclaimer.

[Peter_1]

Most compressors can handle the very short higher evaporating pressure after a defrost.
If not, you always can use a MOP TEV which gives the same result without DP over it.
Is your HG a 3way valve or a 4 way valve?

[jwasir]

I use HG with 2 solenoids......

They flip flop during HG.

I tried to adjust the defrost controller and set the termination temp to 40F but after some time suction goes to 60-70PSI and discharge jumps to 350 PSI and cuts the system from HP cutout.

We tried CPR from Danfoss and sporlan but the max capcity we can get is 2 TR.

There are some valves a/v from PARKER but very expensive.

Any ideas how to resolve this??

[powell]

I tried to adjust the defrost controller and set the termination temp to 40F but after some time suction goes to 60-70PSI and discharge jumps to 350 PSI and cuts the system from HP cutout.
I suspect you have a HG solenoid valve sticking open.

[jwasir]

Powell,

It's not for 1 particular system.......

Our design includes CPR, which I want to get rid off, as the PD is too high.

But when 1 system was shipped w/o CPR, it was always cutting off on HP during defrost.

We tried adjusting defrost controller but didn't work and finally have to install CPR.

What should be the termination temp?

[powell]

jwsair,

What system do you have? Compressor and evap model, refrig etc....

[abbasi]

well to supplement the question can any1 suggest how to use HGBP in reverse cycle systems/heat pumps?

[jwasir]

Powell,

As I mentioned, not for any particular system but we make medium temp and low temp refrigeration systems.

Most of the systems are complete with HG defrosting and we have to use CPR to prevent system to cutoff on HP during defrost.

Can suggestions, how we can eliminate CPR.

What should be the cut off coil temp.?

[SteinarN]

Do you have a LL solenoid closing the line at defrost?

If you have, one possible solution might be to open the condenser inlet valve, thereby dumping some refrigerant into the condenser, when the compressor suction or discharge gets to high . A simple pressostat should do it.

[jwasir]

LL solenoid stays open during defrost.

If we leave discharge solenoid open, which usally is bigger than HG solenoid, do we get enough refrigerant flow in the coil during defrost?

[SteinarN]

Defrost is like this:
LL close, hot gas/condenser solenoids flip-flop.

Then condenser solenoid only is controlled by the discharge or suction pressure, stays closed until pressure becomes excessive, open for a short moment to dump refrigerant into the condenser, pressure goes down, valve closes.

For this to operate it is required to have sufficient pressure drop in the defrost line to get a reasonably high discharge pressure with a reasonable low suction pressure. The amount of condensation in the evaporator will be small or non existent at the end of the defrost. A bit longer defrost time follows, but it should work ok.

Have you seen the PM valves from Danfoss? They are in different sizes from medium to large. I have used one as suction pressure regulator at a 15kW system once. Pressure drop of something like 0,5 psi.

[Peter_1]

As earlier said, just change your TEV to a MOP one, +/- same price and all the problems solved.
Less prone to failure also.

[jwasir]

Steinar N,

If we leave the LL soln off during deforst and if it it extremely cold day and condenser is 80-90% flooded, won't the evap gets starved of HG?

I normally size HG line 1 size smaller than discharge for more Pr. drop.

PM valves from danfoss is again a expensive option.


Then condenser solenoid only is controlled by the discharge or suction pressure, stays closed until pressure becomes excessive, open for a short moment to dump refrigerant into the condenser, pressure goes down, valve closes.

But if you think, we can add a HP control to open discharge valve during defrost, I'll try the one.

Have you ever tried this option?

[SteinarN]

I suppose you use speed regulation on the condenser fan in cold ambients. What might happen is the refrigerant might migrate to the condenser through the TEV, LL and receiver during defrost. This is easily overcome by installing a back valve in the liquid line between LL solenoid and evaporator.
Do not install this back valve between receiver and LL solenoid as liquid trapped in this section of line between the back valve and a closed LL solenoid might burst the line if the line temperature is raised.

You then have the refrigerant in the evaporator available to defrost without any of that refrigerant leaving the "defrost" system unless opening the condenser inlet solenoid. As you sugests, your aditional (HP) pressostat controls the condenser inlet solenoid.

This should be a simple and efficient system and at the same time control the discharge pressure efficiently.

Install suction acumulator also.

As Peter_1 sugest you could use a MOP TEV to control the evaporating pressure after defrost if the compressor doesnt tolerate the high suction.



I supose you connect the defrost line to the evaporator between the TEV and the distributor. The distributor will act as the main pressure drop device.

[jwasir]

I understand than I can leave my LL soln ON during defrost?

I've never noticed that refrigerant migrates back to cond from TXV.

I suppose that TXV is bigger restriction than open evaporator and more pressure difference, thus refrigerant will always flows to the path of least resistance.

[SteinarN]

Where are your hot gas line conected to the evaporator?

[jwasir]

HG is injected through a side port after the distributor nozzle!

[SteinarN]

After the distributor nozzle??
Is it an evaporator with more than one loop?
If so you have a short tube between the TEV and the distributor?

[jwasir]

YES SIR, we have 12 loops........

And we used ASC from sporlan, where nozzle can be moved away from the distributor and has a side port for HG.

[wambat]

Some good information on hot gas defrost:
http://www.keepriterefrigeration.com/Products/Defrost_Cycle.htm

[SteinarN]

YES SIR, we have 12 loops........

And we used ASC from sporlan, where nozzle can be moved away from the distributor and has a side port for HG.

Ahh, I learned something new here....

My reason to advice on closed LL during defrost is to be sure that no liqiud is entering the evaporator from the liqiud line during defrost. That may be a valid assumption if controlling condenser inlet solenoid by low HP setting and ambient is high.
Dependent on your pressure in the evaporator, that liquid will possibly suply a cooling loead and lenghten the defrost time.

The reason to advice on a back valve is to make sure no liquid is unintentionally leaving the defrost system through the TEV and liquid line. You have high pressure at the TEV outlet (hot gas inlet), and dependent on your resistance in the dustributor lines, that pressure may be higher than your receiver/condenser pressure a long time of the year.

I'm not saying those measures is necessarry but they will add a certainty to where the refrigerant is (flowing) in the systemm at defrost.

[jwasir]

Do you still think we should have HP control to open discharge solenoid during defrost?

We are going to change our design for ALL the units.

The plan will be to remove the CPR from the exisiting systems and add HP switch.

I'll appreacite by getting any more new ideas!

[SteinarN]

Do you still think we should have HP control to open discharge solenoid during defrost?

You most likely will find your self in some sort of excessive pressure trouble if you remove the cpr. LP will most likely be above 0*C in defrost, and for a frezer, that may be a to hard task. It also depends on how high the HP goes. Only way to say for sure is to make some tests.

One other thing to be avare of, is it a pump down system? If so, if defrost is started from pump down stop, then there isn't much refrigerant available in the evaporator for the defrost.

[jwasir]

If we dont remove CPR's then there will be more pr. drop.....

Yes, its a pump down system but during Pump down stop if defrost starts, we force the LL soln to open, so that evap is not starved during defrost.....

I'll remove CPR from one system and do some tests before making this a standard design.

[jwasir]

Steinar N,

I did a test as suggested.

I've Set the HP cut for defrost soln as Cut In -- 350 PSI and Cut out -- 200 PSI.

After 3-4 minutes, discharge soln opens and cuts back out in 5-10 seconds. The suction never goes more than 50 PSI.

The termination sensor was mounted on the last U-bend of the evaporator coil.

I noticed that the coil stayed warm till the middle and after that the bends were getting cold even during HG defrosting.

I changed the sensor to the middle centre of the coil, it worked much better there.

What should be the ideal location -- inside the fins at inlet or inside the fins at outlet OR it should be mounted on the U-bends?

What should be the termination temp.?

[SteinarN]

Steinar N,

I did a test as suggested.

I've Set the HP cut for defrost soln as Cut In -- 350 PSI and Cut out -- 200 PSI.

After 3-4 minutes, discharge soln opens and cuts back out in 5-10 seconds. The suction never goes more than 50 PSI.

The termination sensor was mounted on the last U-bend of the evaporator coil.

I noticed that the coil stayed warm till the middle and after that the bends were getting cold even during HG defrosting.

I changed the sensor to the middle centre of the coil, it worked much better there.

What should be the ideal location -- inside the fins at inlet or inside the fins at outlet OR it should be mounted on the U-bends?

What should be the termination temp.?

Since your suction is as low as it is, I would have set a higher cut out, say 250-300 psi. You will then probably get slightly higher suction as well and higher defrost capasity.

The reason the evaporator is getting cold towards the outlet is the low suction pressure and low refrigerant amount in the defrost system allowing for sensible heat defrost only (no condensing of the hot gas in the evaporator).

The correct location of the defrost sensor may be at the outlet tube (same as TEV thermo bulb).

Termination temp maybe 10-20*F above freezing.

How long time does it stay in defrost to reach above freezing at evaporator outlet?

[jwasir]

SteinarN,

Thanks for the reply.

I'll check the time and get back to you.

I have another issue, different from defrosting and I appreciate your views on it.

While performing a test in a low temp air chiller, the outlet temp from the coil was -25F.

At the fan inlet (Plug Fan), which is 10" from the coil, the temp is 6F.

At the outlet of the 10" dia duct, which is only 6" long, temp is again 6F.

Air qty -- 500 CFM.

Where and what we are doing wrong.

Checked the temp display and the sensor and all found OK.

Tried testing with different tester and same results, when sensor is at the coil outlet, temp is -25F and at the fan inlet temp is 6F and same at the fan outlet.