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Andrew5447
26-03-2013, 05:23 AM
Dear All!
Because of low condensing pressure in ammonia plant in winter time (it is difficult to do hot gas defrosting) I need condensing pressure regulator ICS + CVP and ICS + CVPP (HP) on discharge pipe.
But I have thermosyphon oil cooling with pilot receiver connected with high pressure receiver and condenser by equalizer line. And it is impossible to use check valve on drain line to high pressure receiver (it needs dP to open). What I have to do? Please help.

Andrew5447
26-03-2013, 06:21 AM
I understand by myself, to formulate a question is to answer then half. We need only pressure regulator ICS + CVP, no check valves, because we need high discharge pressure only for hot gas defrosting.

HVACRsaurus
26-03-2013, 07:37 AM
Wouldn't it be easier to just cycle off the condenser fans?

Grizzly
26-03-2013, 07:48 AM
Good point H.

Simple solutions are always the ones to try first.
I think they call it Oakums Razor or something similar?
Grizzly

Andrew5447
26-03-2013, 10:30 AM
ambient temperature maybe -30 C and windy

HVACRsaurus
26-03-2013, 12:14 PM
Can you tell us more about the condensers. How many? Water cooled? Air Cooled? VSD Fans? Are all the fans off & still low condensing?

How long during the year is this a problem?

Segei
26-03-2013, 12:53 PM
ambient temperature maybe -30 C and windy
You can isolate one condenser or install shield to prevent direct wind. What pressure do you need for the defrosting?

Andrew5447
27-03-2013, 03:14 AM
Can you tell us more about the condensers. How many? Water cooled? Air cooled? VSD Fans? Are all the fans off & still low condensing?

How long during the year is this a problem?

We have 2 air cooled condensers with 8 fans each. No VSD for Fans. All the fans off during low condensing of course. There are severe winters 3-4 month per year.

Andrew5447
27-03-2013, 03:49 AM
In winter compressor's load falls down as heat leakage drop (it is cold storage). We did not have enough hot gas for defrosting, the condensing pressure falls especially at low ambient temperature. We need of condensing pressure at least 7-9 bar, not less I think.

RANGER1
27-03-2013, 04:56 AM
Andrew, can you just consider what you posted earlier. Install PM with high pressure pilot valve or similar with hot gas connection taken off before it at your pre-set pressure 9 bar. Downstream is condenser with t/syphon return, equalizing line etc. In summer or warmer weather jut manually jack it open. Unless I'm missing something it could work. You may have to watch refrigerant level are adequate at this time, as well as enough load to generate hot gas.

HVACRsaurus
27-03-2013, 09:05 AM
Ok, I'm beginning to understand.

At times the ambient is colder than the Coldstore! I suspect that the evaporators would likely be cycled off on temperature setpoint for extended duration...

Hopefully this requires less frequent defrosting?

I am wondering if valving off one condenser would help?

Keep in mind - evaporator load is required to create hot gas. Each evaporator defrosting requires at least two similar evaporators running to create the hot gas. Are you able to set a low setpoint (say -40C) for the purposes of maintaining evaporator load? Likely a similarly modified suction pressure setpoint would also be required..

Segei
27-03-2013, 10:58 PM
In winter compressor's load falls down as heat leakage drop (it is cold storage). We did not have enough hot gas for defrosting, the condensing pressure falls especially at low ambient temperature. We need of condensing pressure at least 7-9 bar, not less I think.
Isolate one condenser.

Andrew5447
28-03-2013, 03:21 AM
Ok, I'm beginning to understand.

At times the ambient is colder than the Coldstore! I suspect that the evaporators would likely be cycled off on temperature setpoint for extended duration... Yes!

Hopefully this requires less frequent defrosting? Yes, but they need

I am wondering if valving off one condenser would help? It is possible

Keep in mind - evaporator load is required to create hot gas. Each evaporator defrosting requires at least two similar evaporators running to create the hot gas. Yes, because of this we do not have enough hot gas for defrosting - compressor's load falls down.
Are you able to set a low setpoint (say -40C) for the purposes of maintaining evaporator load? Likely a similarly modified suction pressure setpoint would also be required.. yes it is possible, but it is not the best solution

Thank you HVACRsaurus you are understanding!

Andrew5447
28-03-2013, 07:45 AM
Thanks, Ranger 1, I think you are right. If we stop equalizing line, the high pressure receiver will be on pressure that is higher then in condensers lines and the pilot receiver, they will be overfilled by liquid. The working area of flooded condenser go down and at some I see liquid can drain to high pressure receiver. I hope it will be without hammering.
The question is how to combine condensing pressure regulator and thermosyphon oil cooling.

Iceman717
28-03-2013, 08:14 PM
I have an A4AB valve in the discharge line up to the older condensers. Back before we expanded the plant it served to keep 130 psi of hot gas in the plant for defrosting purposes and would let any excess go to the roof. The first step in the condenser control sequence was to power that valve, opening it wide.

Once we expanded I had liquid injection to deal with and couldn't let the pressure on the roof drop off so I started isolating condensers in the Winter. 2 of my 3 condensers are isolated now with outside temperature running about 7C. I don't have to worry about an equalizing line since my main receiver is controlled-pressure. I run it about 80 psi which low enough to get good liquid flow down from the condensers to the receiver and high enough to deliver liquid to the pump package and the intermediate loads. My pilot receiver, being upstream of the float-controlled condenser outlet valve, stays at full head pressure and I keep a decent flow of liquid to the LIC on the screws.

Segei
30-03-2013, 02:48 AM
Do you really need defrosting if -30C outside? What is the suction temperature? If your suction temperature is -30-35C, you don't need defrosting.
If you really need defrosting, it should be done once per week or every second week, because rate of frost formation will be very slow. Lower set point for the freezers, run additional evaporators, run additional compressor and defrost.

cricri
30-03-2013, 10:23 AM
10189

better than an explanation, a drawing

try this, you need a check valve, 2 ICS/CVP. The ICS valve fitted to the hot gas line is designed to prevent too low discharge pressure at the compressors.
the liquid to oil coolers must be connected at the bottom of liquid receiver and the liquid line to LP vessels higher.
fans controlers have to be downstream of the check valve.

regards

Segei
30-03-2013, 11:20 PM
10189

better than an explanation, a drawing

try this, you need a check valve, 2 ICS/CVP. The ICS valve fitted to the hot gas line is designed to prevent too low discharge pressure at the compressors.
the liquid to oil coolers must be connected at the bottom of liquid receiver and the liquid line to LP vessels higher.
fans controlers have to be downstream of the check valve.

regards
I have couple concerns about this drawing. Probably, check valve should have opposite direction. Pressure in TSR will be very low and it will be not enough to move liquid to LPR or IPR.

cricri
31-03-2013, 08:55 AM
Sorry Segei
old guy, old school, wrong symbol but gas flow still from compressor to condenser. The topic was hot gas defrost and thermosyphon oil cooling, so this is to prevent the draining of the liquid receiver to the hot gas line during defrost and oil or discharge temperature trip-out of the compressor. In any case, the condenser capacity should be adapted to the air temperature, at least, valving of one condenser is necessary.

Andrew5447
01-04-2013, 05:23 AM
Thank you cricri,
but apart from the check valve the ICS 1 for Hot Gas have to control pressure not before but after itself to manage about 6 - 7 bars inside evaporators defrosting pressure (CVC pilot). ICS2 is CVP (HP) (set 8 - 9 bars).

cricri
01-04-2013, 08:05 AM
Hi Andrew,
ICS1 is not absolutely necessary, the control is upstream. It's to prevent a too low discharge pressure causing oil carryover and a noisy compressor.

Andrew5447
01-04-2013, 01:11 PM
Hi, Cricri,
I think ICS1 + CVC we need to protect LP evaporators, plate freezers of excessive high pressure (can be as example in summer). But check valve may be not absolutely necessary.
Regards

RANGER1
10-04-2013, 01:00 PM
Thanks, Ranger 1, I think you are right. If we stop equalizing line, the high pressure receiver will be on pressure that is higher then in condensers lines and the pilot receiver, they will be overfilled by liquid. The working area of flooded condenser go down and at some I see liquid can drain to high pressure receiver. I hope it will be without hammering.
The question is how to combine condensing pressure regulator and thermosyphon oil cooling. Amot valve or similar for t/syphon oil cooler, as I think you should have this regardless in your climate. Amot type basically controls oil temp by bypassing or mixing oil around oil cooler to maintain set temp (50 deg C).

Kherson
13-06-2013, 03:42 AM
May I suggest that your problems and solutions are with your Hot Gas Defrost system. I have seen this exact problem before and all we did was install a pressure regulator for the Hot Gas System and increasing the length of the defrosts and/or their frequency as required. We ended up running the Hot Gas System around 80 to 90 psi and the funny thing was that because our pressure was always constant throughout the year, no more small ammonia leaks at the evaporators...
In my opinion, the overall hot gas system is one of the most overlooked pieces of a well functioning system. You will have to spend sometime experimenting with the individual evaporators to see what does work best. I have found that more frequent defrosts with shorter durations works best...
Kherson

Segei
14-06-2013, 02:38 PM
It is not energy efficient to increase frequency of defrost. 80- 90 psig is it pressure in hot gas line or pressure in the evaporators?

Andrew5447
17-08-2013, 05:38 AM
Thank you All!
Now I install only one ICS with CVP(HP) and EVM (wide open for summer) before condenser on discharge line and I use classical Thermosyphone oil cooling.
And I use ICS for Hot Gas Defrost Line to control pressure not before but after itself to manage about 6 bars inside evaporators defrosting pressure (CVC pilot).
Cold winter put everything in its place.