PM valve selection

Hi there,

Danfoss has a programme called Dircalc which is for Industrial Refrigeration. This programme also selects the required components.

My question is for PM valves. There are three different ways which programme uses to select the PM valve.
1- Pressure Drop
2- Velocity
3- Size

By selecting one of the options then a valve is selected which completely differers from the other option.

In your expert opinion which one is the best and why? (if you don't mind explaining;) ).

Thanks in advance.
Cheers

Although I have not used this program, I suspect the selection method is based on what you want to accomplish with the valve.

In some cases, a control valve may have various operating conditions it may experience during different periods. In those, you might want to select the valve for a specific capacity at two different conditions. Each would have a different flow and pressure drop.

In other cases, you might not want to exceed a specific pressure loss as this may increase the total pressure loss and contribute to a performance loss of another component.

I see these as a similar to a flow coefficient or Cv calculation. For a specific orifice size you will have a specific flow capacity at a certain pressure drop across the valve.

If you change the differential pressure across the valve, the flow capacity also changes.

Now, if the valve has a variable orifice size due to a change in stroke of the control valve you could also have a range of valve positions (varying degrees of valve stroke).

Perhaps Andy or one of the other Mods or RE members who use this program can provide a better explanation.

Hi US Iceman,
Thanks for your advice.

Best regards,
LANA

If one can know this, then our member BESC5240 is best placed to answer your question.
I suggest you send him a PM Lana and post his answer afterwards here.

Quote:

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Originally Posted by Peter_1

If one can know this, then our member BESC5240 is best placed to answer your question.
I suggest you send him a PM Lana and post his answer afterwards here.

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Hi Peter,

I will try your suggestion, but I was wondering why? BESC5240 does not post on the forums?:confused:

Cheers

lana,

BESC5240 contributes to the forums every so often, but the good news is he works for Danfoss in Belgium I believe and Peter knows him.

I have already sent the PM.
Thanks a lot for your kind help.

Cheers

please see attachment

Dear BESC5240,

Many thanks for your kind reply. Very helpful.

Cheers

And what about its regulation?, how do you regulate a pm valve?, just as an expansion valve regulating the evaporating temperature and closing the txv until you have 4 degress under the tempereature regulated in the pm, or what do you do?

Church, I think you miss something.
A PM valve is only a heavy valve throtled by the device(s) (the different pilot valves) that have been screwed onto the different regulating ports.
So a PM can be a crankcase pressure regulator, a SV valve, a very big expansion valve, a suction pressure regulator,... whatever.

yep, i know. I meant as evaporating pressure control, with the correct pilot. What i dont quite enough understand, is the regulation of the superheating in an evaporator when after it u have an evaporating pressure control. What is the correct mtehod to do this in this case, step by step. I need to regulate an air conditioning system with a tex expansion valve, evaporator, and a pm just before the compressor. I did it more times but actually i have a discussion with a colleague and i want to know a second opinion.

Quote:

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Originally Posted by church2k

I need to regulate an air conditioning system with a tex expansion valve, evaporator, and a pm just before the compressor. I did it more times but actually i have a discussion with a colleague and i want to know a second opinion.

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Hi church2k,

Would you please explain what you did with PM valve on a DX system? I am very interested.

Thanks in advance.
Regards
LANA

ok. I have a dx system wit a Te5 valve in the air conditioning unit in a ship. In order to mantain the evaporating temperature positive we have installed a pm valve with the cvp pilot, so we can regulate the evaporating pressure that is going to be different that the suction pressure as its logic. the problem comes whe u try to regulate the superheating in teh evaporator. After looking the sectioning of the valve i suppose it has to be regulated until the temperature in the manometer i have already installed in the valve, will be over 5 º deg, and i have to measure the sh just before the valve respecting the 6-7 º deg upper the evaporation temperature(as u do in an usual txv).Is this correct?

Quote:

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Originally Posted by church2k

ok. I have a dx system wit a Te5 valve in the air conditioning unit in a ship. In order to mantain the evaporating temperature positive we have installed a pm valve with the cvp pilot, so we can regulate the evaporating pressure that is going to be different that the suction pressure as its logic. the problem comes whe u try to regulate the superheating in teh evaporator. After looking the sectioning of the valve i suppose it has to be regulated until the temperature in the manometer i have already installed in the valve, will be over 5 º deg, and i have to measure the sh just before the valve respecting the 6-7 º deg upper the evaporation temperature(as u do in an usual txv).Is this correct?

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Hi Church2k,

Thanks for the explanation.
This is the first time I heard this type of evaporator pressure regulating method. The usual one I am familiar with is to use simple EPRV (evaporator pressure regulating Valve). This EPRV prevents the evaporating pressure to drop from the set point.

Lets see what others have to say on this one.

Interesting ....:confused:

Cheers

Hi church2k,

As I understand it this is a very standard situation.

- you have an evaporator with a TE5 as expansion valve
- you have a PM+CVP in the suction line as an evaporator pressure regulator (EPRV).

Setting:
- let the system run in nominal conditions (with the load it has been designed for) if possible

- set the CVP (on the PM), looking at your gauge (manometer mounted between evaporator outlet en PM inlet, or on the pressure connection on the PM) in a way pressure does not go below the minimum pressure (in your case 5°C?).
The minimum pressure is defined by
- frost protection (0°C) or,
- by a minimum air outlet temperature.

- if the selection of the TXV has been made correctly, there is no need for superheat adjustment. If however the valve is to big or someone has tampered with the setting, a readjustment could be necessary. Be carefull with reducing static superheat (spring force adjustment): in low load conditions and/or high pressure differential over the valve this can lead to instable behaviour (hunting) of the TXV.

Best regards
BESC5240

And if the selection of the txv has been messed, can i regulate sh in the same way as the expansion system, i mean, if i have regulated the pm at 5º, may i have 10 degrees just before the valve?

Based on what I have read so far (quickly ;)) the question is about set points and requirements.

Let's forget about the type of valve and concentrate on the required function.

The normal requirements for using an EPRV (evaporator pressure regulating Valve) is exactly what BESC5240 said. You want to maintain a minimum evaporator temperature for; a) frost prevention or b) minimum discharge air temperature.

I am in complete agreement with BESC5240. Start-up the system and let it reach a stable operation at full load. Leave the EPRV wide open during this time. Check the evaporator superheat. You should NOT normally have to adjust this.

As the evaporating pressure begins to decrease when the cooling load also decrease, this is when you want to adjust your EPRV. Set it to control the desired evaporating pressure. By controlling the pressure, you control the temperature in a basic sense.

All the TXV cares about is a stable operation so that it can control the superheat properly.

This is only a basic description of one possible alternative. Capacity variations from high to low capacity, single large capacity coils to multiple smaller capacity coils can affect how all of this works to some degree.

I think that i´m not explaining as well as i want to. All you are saying that when load drops is when i have to regulate the superheating, i do know this, the question that i have since the first post is where should i measure this superheating not when. Before the pm valve or after it?????

Before. I'm assuming this is where the bulb for the TXV is mounted. Is this correct?

The TXV is controlling superheat, so you want to measure it where it is controlled from. This is the power element bulb.

The EPRV is controlling the upstream pressure between the evaporator outlet and the inlet to the EPRV.

Are you trying to adjust the TXV superheat at reduced capacity?

Yes i´m doing in this way, i had a discussion with a colleague that said that when you grow rise the pressure before the valve without regulate the txv (when its already in low charge) the risk of propelled liquid is rising so, and that has not sense for me that thought that it was not correct because when you rise the pressure in the pm, the evaporating temperature rises as well and the superheating has to be forcedly higher.I think this happens when you are going out of the normal capacity of regulation of the bulb, for example when in low charge you change the evaporating temperature from 0º to 5º or 6º.

I use the pressure drop selection method, depending on what equivalent saturated temperature drop you can allow you system to have.

ie
a PM valve in the wet gas return line of an evapporator. Design satuarted evaporating temp -6*C (241 kPa) and the compressors are operating at -10*C (190 kPa) SST and the combined wet gas return and suction pressure drop is 2*K (typical for a large complex industrial facility) then you have 2*C of equivalent pressure drop available = approx 25 kPa
so if you select a valve with 15 to 20 kPa pressure drop you will have a bit up you sleeve as far as capacity goes

Hope this helps

Brads