How can I reduce evaporator pressure?

[mansari]

Hi, we are commissioning our Hydrochloric Acid (HCl)gas condensation system. The HCl gas is condensed in a Kettle type heat exchanger, functions as "evaporator" for the refrigeration unit.

As per design, vapor HCl enters the heat exchanger at -25C and condenses. ***** 22 liquid enters the heat exchanger at a temperature of -36 C and 0.26 bar g pressure and leaves as gas at the same temperature and then goes to suction of the compressor, where its pressure increases to 14.5 bar g and temperature to 82 C. In condenser, temperature of condensed liquid ***** brought to 40 C. After the condenser there is an economizer where temperature of liquid ***** is dropped to -9.7 C through partial flashing of a split stream, before the refrigerant reaches to the throttle valve installed upstream of the evaporator.

My problem is, we are unable to bring the evaporator pressure below 0,9 bar and therefore temperature of vapor ***** does not fall below -25 C (saturation point of F-22) which is not suitable for us because then HCl will condense at -20 C resulting in higher system pressures.

My questions are,

1. Evaporator pressure is 0.9 bar g and suction pressure of the compressor is 0.2 bar g. Where is the pressure drop being created? is it the inlet valve or impact of suction? At compressor inlet, we have Danfoss SVA 300 valve installed.
2. We have not yet commissioned the economizer and temperature of liquid ***** is +30 C at the inlet of throttle valve, instead of -9.7 C. Does this deviation from design makes pressure and temperature of the evaporator higher?
3. How we can bring the temperatures in the evaporator lower? The heat exchanger is new and does not have fouling.

Look forward to see experts advices.

Mansari

[nike123]

1. Evaporator pressure is 0.9 bar g and suction pressure of the compressor is 0.2 bar g. Where is the pressure drop being created? is it the inlet valve or impact of suction? At compressor inlet, we have Danfoss SVA 300 valve installed.

Maybe this explain pressure drop:

SVA-DL (Delta pressure Low)
is designed with a restriction in the opening
function. High differential pressure can be
applied from the side port and reduced
pressure can be applied from bottom port. SVADL
is a two-step valve for pressure relief.

SVA-DH (Delta pressure High)
is designed without restriction in the opening
function. As a result of its balanced design this
valve is able to open at all differential pressures
with limited torque.

.......................

[nike123]

1. Evaporator pressure is 0.9 bar g and suction pressure of the compressor is 0.2 bar g. Where is the pressure drop being created? is it the inlet valve or impact of suction? At compressor inlet, we have Danfoss SVA 300 valve installed.

SVA-DL (Delta pressure Low)
is designed with a restriction in the opening
function. High differential pressure can be
applied from the side port and reduced
pressure can be applied from bottom port. SVADL
is a two-step valve for pressure relief.

SVA-DH (Delta pressure High)
is designed without restriction in the opening
function. As a result of its balanced design this
valve is able to open at all differential pressures
with limited torque.

.......................

[mansari]

Thanks Nike, The valve we have at compressor suction is SVA-DL-300 from Danfoss.

[nike123]

Thanks Nike, The valve we have at compressor suction is SVA-DL-300 from Danfoss.

I am not sure that that is what makes that pressure drop, but it deserve considering.
What is equivalent length of suction line?
What is compressor displacement? What is suction pipe diameter?
Is there anything other between evaporator and compressor?

[mansari]

Total length of vapor return line from Evaporator to Compressor suction is 75 m. Compressor capacity at inlet is 326 kg/minute and line diameter is 12 inch.

Between the evaporator and compressor suction, we have two knock out vessels, one outside package, other inside the package.

Pressure drop between evaporator and second knock out vessel is only 5 kPa (0.7 psi). Major difference is seen between second knock out and compressor where suction gage indicates some 70 kPa (10 psi) less pressure. Line length between second evaporator and compressor is only 10 m, with 12 inch size

[Gary]

2. We have not yet commissioned the economizer and temperature of liquid ***** is +30 C at the inlet of throttle valve, instead of -9.7 C. Does this deviation from design makes pressure and temperature of the evaporator higher?

Yes... without the economizer, the evaporator must cool the +30C liquid refrigerant as well as cool the HCI.

[nike123]

Pressure drop between evaporator and second knock out vessel is only 5 kPa (0.7 psi). Major difference is seen between second knock out and compressor where suction gage indicates some 70 kPa (10 psi) less pressure. Line length between second evaporator and compressor is only 10 m, with 12 inch size

Did you measured that 70kPa between compressor suction (after Danfoss) valve and inlet or outlet of 2nd knock out vessel?
That pressure drop is almost 10K temperature difference and measuring pipe temperature could easily tell you where is restriction.

What "second evaporator" are you talking here?

[mansari]

Thanks Gary and Nike,

Gary ! Am I correct in understanding that even without taking economizer in service, desired tremperatures of -36 C is possible at the evaporator via throttle valve? In the design, vendor specifies throttle inlet valve inlet temperature of -9.7 C whereas we are supplying at +30C since economizer is not operational.

Nike ! Pressure sensing point is located downstream of the danfoss valve SVA-DL, just before the compressor suction.

It was a typo, I meant second knock out pot here. Evapoator at process locations with a heat duty of 3 MM Btu/h (250 Ref tons). Capacity of Refrigertion Unit is 3.7 MM Btu/h (312 Ref Tons).

One knock out pot installed closed to the process side condenser (evaporator for the refrigeration unit). Elevation of this knock out pot is higher than the heat echanger and it's drain is connected with liquid inlet so that any liquid separated here, can be recycled back. Vapors from the top of this knock out pot are transferred to another knockout (York call it Accumulator) via 12 inch line.
The two pressure measuring points are, second knock out vessel pressure and then inlet to the compressor after the danfoss.

Checking the temperature profile and will share with you. mansari

[Peter_1]

A little schematic of the setup would help a lot and mention on this plan the pressures and temperatures you've measured.

[HallsEngineer]

Has the suction strainer on the compressor been cleaned recently?. The economiser needs to be in circuit if that is what the designer is intended. The duty of the expansion device will increase largely which means less flash gas so better compressor efficency.
What is your Suction superheat?

[mansari]

Dear Peter and Hall Engineers,

I have the schematic available and complete set of operating conditions, downloaded from the PLC, but I am new to this forum, please guide me, how to attach that.

No we never cleaned the suction strainer. The system is new and thoroughly cleaned, dried and tested on almost absolute vacuum (-99.5 kpa) before charging the refrigerant.

Econnomizer is included in the scheme but the control logic brings it in service when load exceeds 85%, and right now we do not have that much load.

Is it advisable to commission the economizer on manual mode?

Average suction super heat is between 50 and 55 C. I am unable to understand that why temperature at suction id poritive (+15 to +20 C) when return of refrigerant from vaporizer is at -24 to -25 C and system is all cold insulated. I suspect malfunctioning of temperature controlled hot bypass line between compressor discharge line and compressor suction knock out vessel, to protect compressor getting liquid its suction.

[mansari]

One more question I have, At a given heat load, what determines the pressure in the evaporator, throttle valve differential or the compressor suction? I know this is a very stupid question but I believe, "whenever in confusion, better to ask the knowledgable". If the suction valve SVA-DL is offering 70-80 kPa (10-12 psi), I am afriad, we will never achieve the required low temperature at evaporator.

[benji]

Hey guys i would think according to what you guys are discussing is to fit an in-line accumulator after the evaporator, so first the evap. then the accumulator then the compressor, this would drop your suction pressure so as to the liquid vapour coming back due to sub-cooling you have a lower static head so the heavier or bigger droplets fall into the accumulator boiling off into vapour and the compressor sucking off the vapour. this would drop your suction pressure saving on energy there by having a lower condensing temperature.

[HallsEngineer]

I think your SV-dl valve is in the wrong place as you want the least amount of pressure drop possible on your suction line. I would also ask why the compressor is not at full load as the valve also works on mass flow of the refrigerant and if the unit is not charged properly this will not work either. I would remove this valve and change it for a normal shutoff valve and then get the economiser commisssioned. The hot gas bypass is probably not for this purpose either it is probably to stop the plant tripping on lp while it unloads or delivery to suction relief valve. I would set this as required and then check my suction superheat is well below 10'c!!!!

[mansari]

Benji

I am sorry, could not elaborate the situation properly. We do not want to fit an accumulator between the evaporator and compressor. Two of them are already there in between.

What I am trying to get the solution, how to bring the pressure (and corresponding temperature) down from 90-100 kPa to 25-20 kPa to achieve my process side temperatures below -25 C when refrigerant temperature is around -35-36 C. Compressor maintains its suction pressure at the setpoint of 20 kPa but the upstream pressure before the danfoss valve SVA-DL is 90 kPa. Why there is so much pressure differential across the valve ?

[mansari]

Dear Hall

Thank you very much for your kind response. Normally in a Chemical plant environment, vendor packages like this one, are not engineered in house and our scope is limited to supply of specification and requirements to the vendor, who then provides the appropriate unit, meeting our needs.

I have checked the specification we provided and then the material balance sheet supplied by the vendor. Both are identical and the vendor ensures a total differential of only 8 kpa (1.2 psi) across the whole system upstream of the compressor, i e from evaporator to the suction, including this DL valve. Is it possible?

Now we are calling the vendor's representative at site because your response supports our assumption that this DL valve has some problem, either a wrong selection, wrong installation or some blockage.

[mansari]

Referring to the post from Nike, I have downloaded the specification brochure from Danfoss website. It exactly says what Nike has written. DL valves have restriction in flow direction, and therefore offer higher differential. On the other hand, DH valves do not have such restriction and hence can be operated at any pressure differential. Does this mean, our vendor selected a wrong type of valve for this application, requiring very low pressure drop?

Should we replace the installed DL with a DH valve?

[mansari]

Would like to share with all of you, a very interesting finding, we saw only a little while ago.

We decided to check the accuracy of the installed pressure transmitter first. Shut down the unit, installed a calibrated local pressure gage, next to the transmitter on the same tapping. Due to the compressor shut down, the system pressure was at 200 kPa g(29 psi g). Calibrated gage indicated 200 kPa whereas the pressure transmitter was showing 130 kpa g, 70 kPa less than the gage.

I therefore conclude from this exercise, the faulty pressure gauge was reading the actual of 90 kPa as 20 kpa and the compressor was ''happy " at its setpoint, therefore was operating at low capacity (actual current drawn by the machine was almost half of the full capacity current)

If the pressure transmitter would have read the correct pressure, compressor would draw more flow from the suction line to meet its suction set point of 20 kpa, thereby lowering the evaporator pressure and temperature.

Is my understanding of the situation correct?

[US Iceman]

So, the conclusion is the higher pressure indication was causing the compressor to run partially loaded resulting in higher than desired suction pressure?

[RefrigNoob]

Mansari, at what altitude are you located? Could I guess you're around 5000ft above sea level?

[mansari]

Iceman ! Yes this is my viewpoint. It will be confirmed when we will run the machine again with new pressure transmitter.

Rafiq ! My positioning is at sea level (8 meters above the sea level to be exact), our plant is facing a creek, linked with Arabian Sea

[nike123]

Dear Peter and Hall Engineers,

I have the schematic available and complete set of operating conditions, downloaded from the PLC, but I am new to this forum, please guide me, how to attach that.

Since you don't have jet 15 posts you cannot post links.
Make few posts with 10 or more letters until you got 15 posts.
Then upload your file/s to some file server like rapidshare or mediafire and post link to that file/s in your 16th post.

[mansari]

Thanks Guys for your interest and response. We got our problem fixed. It was the faulty pressure transmitter, sensing a pressure some 70 kPa less than actual, coincidentally equal to the machine set point. After replacement of the instrument, we got the evaporator temperature and pressure at -36 C and 30 kPa g respectively. The refrigeration unit is perfect now.

[nike123]

Thanks Guys for your interest and response. We got our problem fixed. It was the faulty pressure transmitter, sensing a pressure some 70 kPa less than actual, coincidentally equal to the machine set point. After replacement of the instrument, we got the evaporator temperature and pressure at -36 C and 30 kPa g respectively. The refrigeration unit is perfect now.

Good for you!
That proves that your measuring should always need to be confirmed with service gauges.

[Peter_1]

Mansari, thanks for posting the solution. This is something that rarely happens here on RE. As soon the problem disappears, the poster also disappears.
This is how a post should always end. Thank YOU.
We all still can learn from your and any problem in fact.

I'm anyhow still interested in the schematic. Reply once to a post of 4 Mansari and you can post your attachment afterwards.

[mansari]

Peter ! Thanks a lot for your reply. I will not disappear Sir. We have started low temperature process of HCl separation from VCM for the first time so may face more problems around the refrigeration unit, therefore I would like to remain in touch with you guys. This site I found very useful with tons of rich experience around.

Certainly I will attach the schematic for you. The scanned image is with me, and asking by IT boys to help me how to attach. Please bear with my poor computer literacy, me an old man from the days of slide rule calculations.

[Sridhar1312]

Normally for such systems(Chlorine Liquefaction, LPG liquefaction) Flooded evaporator is ideal and we have used Phillips Float valve with liquid level maintained .For oil return we used to have different tap offs in the evaporator so that the even at low loads the oil return is taken care.
Pressure difference could be due to error in transmitter.

[Peter_1]

Peter ! Thanks a lot for your reply. I will not disappear Sir. We have started low temperature process of HCl separation from VCM for the first time so may face more problems around the refrigeration unit, therefore I would like to remain in touch with you guys. This site I found very useful with tons of rich experience around.

Certainly I will attach the schematic for you. The scanned image is with me, and asking by IT boys to help me how to attach. Please bear with my poor computer literacy, me an old man from the days of slide rule calculations.

Mansari, we always appreciate the valuable contribution of the older - sorry for the word- members. They have a lot of experience which sometimes can't be learned in books.

[mansari]

Dear Sridhar

The level transmitter at the evaporator (HCl Condenser) is fine. It was the faulty transmitter installed on ***** side, at suction of the compressor. Thanks for your advice, I have noted and will certainly consider when specifying a new level transmitter for evaporators.