EXV and Pumpdown

[bersaga]

Need some advice here :

Have you guys ever used an Electronic Expansion Valve with a Pumpdown system i.e. one EXV combines a dual role of TXV and solenoid valve. In this case EXV will fully close when set-temp is reached , create a low pressure in the suction and activate the LP switch which then cuts off the compressor.

Or do you avoid a Pumpdown system and connect the EXV directly to compressor contactors ?

Thanks in advance !

[coolman]

I would suggest pump down.
Electronic expansiondevices to tend to fill the evaporator with liquid to obtain highest efficiency.
on startup, there always is the possibility of liquid reaching the compressor.
You will have to program the regulator so its will have a sufficient delay to reach working conditions (on danfoss AKC there will be a possibility that there will be a injection fault and the regulator will shut down)

Victor

[malik55]

Carrier is using successfully EXV with pumpdown at shut off since long time in their
30GT series called flotronic chillers.
How you will connect an EXV directly with a compressor contactor?
EXV not comes alone, it works with a processor and temperature sensors.

[chillin out]

Weve been using those for a long time now.
We use a tuscan EEC board to power it, most of our cases run off a pack from arctic circle, and each case shuts down the EEV on temp and defrost with no problems.

[bersaga]

Malik ( and the others ),

The way we do this is as follows :

We use an ALCO EC2 connected to the EXV. The EC2 has 5 temperature probes - measuring : Air In, Air Out, Coil In , Coil Out, Mid Coil.

The AirIn is used to to close the EXV on reaching set-point.

CoilOut-CoilIn is used to measure Superheat against set superheat.

Mid Coil is used for Defrost Termination on Temperature.

So when the room design temp. is reached, the EXV will close, create LP in the suction line and trigger the LP switch which cuts off the compressor.


Now , a little bit about the project ( to give you a better understanding of the situation) :

The apllication is a coldstore that keeps dried beans at 5 C. The total calculated heatload is 59kW ( max) and the equipment have been selected based on this. At commisioning, the calculated heatload is only about 25kW ( lower ambient, lower product load, thicker insulation panel , higher shading etc.)

The equipment selected are 4 units of Copeland R22 Scroll Condensing Units - each 7 1/2 hp ( 16.53kW at 0C evap and 35C ambient)

The EC2 is set to cut-in at 7C and cut-out at 4C.
Superheat setting is 6K.

We found that after the compressor cuts-out at reaching 4 C and when the room temperature starts to rise but still between 4 to 7 C, the compressor cuts -in for about 5 secs and then cuts off. This happens quite regularly.

Any experience with this type of phenomena ? Is this because that currently 4 condensing units are too big for the actual load ?

The units are configured on a one-to one basis i.e. one Evaporator to one CUD.

Will be going to site tomorrow - plan to connect a computer to the EC2 and take down all the temp readings. And anything else that you would suggest.

Thanks in advance

[chemi-cool]

Hi bersaga.

the compressor cuts -in for about 5 secs and then cuts off

It looks to me one of three possibilities,

First, the setup differencial of the LP cutout is too close.

Second, the EEV is leaking but I dont think that the problem lies here.

Third, The scroll compressors do not hold the pressure, an NRV on the discharge line might eliminate the problem.


Chemi

[malik55]

Is this problem is for one unit or all four are doing same?
You have to put your gauges and check suction side pressure, it might increasing due to EXV leakage or N.R.V as said by CHEMI.
Regarding the load I think you have to adjust each EC2 at a different temperature so if one at higher temperature satisfied it not comes on and the lower one only works.

[Peter_1]

Never use a scroll in a pump down cycle if compressor ratio goes up higher then 10:1.
http://www.ecopeland.com/Literature/..._Ref_Small.pdf page 6

[frank]

the compressor cuts -in for about 5 secs and then cuts off.

We had a similar situation with a semi hermetic on a chill room some years back and traced it to a leaky sol valve. It can also happen with re-expansion in the suction line depending how low the cut out is set.
We've also seen it many times where the compressor comes on for a few seconds after the system call and the sol valve opens, but as insufficient time is allowed for equalisation the comp starts and stops until stable conditions are achieved.
This can be overcome by installing a delay on timer into the comp circuit.

[chillin out]

I agree with Frank, if there is no timer already fitted you must fit one or some other type of control to stop the comp stoping and starting too many times an hour.

[vaidas_78]

Hi bersaga, chek thermostat mode(r4)parameters:

1 = cooling, deadband control
cut in = set-point + difference
cut out = set-point
2 = cooling, modulating thermostat
cut in = set-point
cut out = set-point – difference /2
In your case it should be set on 1.
And also chek superheat mode(u4) parameters:
1 = fixed superheat
2 = adaptive superheat
also it should be set at 1

[bersaga]

To All who contributed :

We changed the system control.

It now is controlled by Temperature i.e. the EC2 sends an On/Off signal directly to the condensing unit - not by LP switch cut-out

One question though : Is there Suction Pressure vs EXV 'opening' graph for a range of set evaporating temperature ? Does the suction vary with different opening levels of the EXV ?

[Peter_1]

This isn't possible, an EXV only controls a preset SH, whatever capacity or evaporating temperature may be.
Suppose you install this EXV on an airco, cooler or freezer...the evaporating temperature will vary widely.

It depends all on the evaporator and the compressor that th EXV is connected on.

A certain opening of the stepper motor of the EXV will give you a certain mass flow, so a certain capacity.
But this mass flow will depend on the DP over the valve.