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    How to apply floating interstage pressure on two stage plant



    Any theories on how to apply floating suction pressure on a two stage NH3 plant.
    Mainly interested with interstage pressure with a number of chillers, a boning room, load out docks etc.
    Thanks for any responses.



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    Re: How to apply floating interstage pressure on two stage plant

    Usually, interstage pressure(absolute) determined by square root of Pd/Ps. These pressures are absolute as well. This is for two stage refrigeration. However, if you have additional refrigeration load at this suction pressure, it can be increased to improve efficiency of the plant. Actually, interstage pressure can be changed in wide rage without significant influence on plant efficiency. It will be better to discuss real pressures and temperatures.

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    Re: How to apply floating interstage pressure on two stage plant

    Hi Ranger.
    playing with inter stage pressure could upset the low stage performance and drawn power limits. but if you have plenty of high stage capacity, the added side load into inter cooler should not be a problem, are you going to use HP liquid, or pumped inter cooler liquid.

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    Re: How to apply floating interstage pressure on two stage plant

    Liquid from HP liquid receiver to intercooler/ accumulator, low pressure liquid makeup from HT to LT accumulator.
    -40 C carton tunnel & freezers on low side -45kpa
    currently 160 kpa interstage -13 C, will gradually set higher.
    Room temps on interstage vary from-1 to +10c
    VSD on high side screw.
    Condensing approx 25 to 35 deg C
    Could floating interstage help with plant efficiency if controlled properly?
    Last edited by RANGER1; 23-09-2015 at 09:07 PM.

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    Re: How to apply floating interstage pressure on two stage plant

    Lifting the inter stage control pressure will increase low stage compression ratio and power input.
    Dependent on low stage stage capacity versus high stage capacity, the low stage increase could be off set with high side savings.

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    Re: How to apply floating interstage pressure on two stage plant

    Optimum interstage pressure for condensing 25C is 143 kPa for 35C is 184 kpa. Your current pressure is in the middle. Not optimum interstage pressure can reduce efficiency up to 1%. Assume that 50% of high stage load come from low stage and 50% from high temperature rooms. Increase of suction pressure for high stage compressors will improve their efficiency 3% per 1C of higher suction temperature. Increase intersatge temperature will improve efficiency of high temperature load by 15%. At the same time efficiency of high stage from low temperature load will be reduced by 1%. 15-1=14% improvement for 50% of high temperature load or 7% for all high stage compressor. This will be around 5% for whole plant. However, low stage compressors should not be overloaded. High stage compressors: at low condensing pressure should not carry over oil(screw compressors) and at high condensing pressure should not be overloaded.

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    Re: How to apply floating interstage pressure on two stage plant

    Thanks for replies.
    Someone mentions floating interstage pressure, but I am thinking a "buzz" word at this stage, as to many variables to control it satisfactory & for real benefit.
    Maybe more for low load situations for a coolstore or similar, not a continuous process plant.
    Sometimes you have to be practical & have settings to cover al situations.

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    Re: How to apply floating interstage pressure on two stage plant

    Actually, I would't call you case as floating pressure. It should be set as high as possible but to make sure that plant perform right.
    I know several production plants where people change suction set point twice per day based on production requirements. i would't call it floating suction as well. Only floating pressure is condensing pressure. It should follow wet bulb temperature of ambient air. Typically, wet bulb temperature changes gradually during the day and condensing pressure will change gradually as well.
    The most important part of energy saving process is to evaluate system performance. Unfortunately, we have several misconceptions in our industry. For example. The higher suction pressure is the better efficiency of the plant. This is correct for the compressors. However, at higher suction pressure we need more evaporators to run to provide required cooling. Additional evaporators fans power should be used. At certain suction pressure efficiency of the plant will be maximum and further increase of suction pressure will reduce plant efficiency. Major factor of this issue is how powerful are evaporator fans.

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    Re: How to apply floating interstage pressure on two stage plant

    Segei,
    On this particular site it was mentioned & of course it gets you thinking.

    If for example all rooms on interstage are either down to temperature or close to, then interstage pressure could be tweaked up automatically.
    I don't know if it has been done, or an operator who runs the plant makes a decision.

    If one room cannot make temperature, I guess it can't happen, or you investigate room.

    It could also mean evaporators in that room are not designed for higher running conditions.
    You would then weigh up if it's worth adding new evaporator with more surface area or whatever.

    This plant is old & things are slightly experimental at this stage with plant room upgrade, compressors & controls.
    The only other plant that changes interstage set point seasonally is a cold storage plant.
    They run 0 deg C interstage in winter & -6 deg C in summer.
    Of course low side machines have to have correct size motor & oil cooler, also evaporator selection.

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    Re: How to apply floating interstage pressure on two stage plant

    Temperature differences for that plant are from 12C to 23C. Typically, plants designed for 5-8C temperature difference. Most likely they have room for improvement. If one room can't operate at higher suction pressure, additional evaporator can be installed and this will be cost effective improvement, because efficiency of whole plant will be improved.
    However, major energy savings can be achieved by lowering condensing temperature below 25C. I some plants that run at 15C condensing pressure. This is huge energy savings. Definitely, there are barriers to achieve such low condensing temperature, but every barrier has a solution

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    Re: How to apply floating interstage pressure on two stage plant

    Quote Originally Posted by Segei View Post
    Temperature differences for that plant are from 12C to 23C. Typically, plants designed for 5-8C temperature difference. Most likely they have room for improvement. If one room can't operate at higher suction pressure, additional evaporator can be installed and this will be cost effective improvement, because efficiency of whole plant will be improved.
    However, major energy savings can be achieved by lowering condensing temperature below 25C. I some plants that run at 15C condensing pressure. This is huge energy savings. Definitely, there are barriers to achieve such low condensing temperature, but every barrier has a solution
    Discharge definitely can be reduced in winter, raising for hot gas defrosts
    here is a few local ideas from a Sydney company

    http://www.environment.nsw.gov.au/re...n-tech-rpt.pdf

    http://www.foodprocessing.com.au/con...nts-1202075669

    http://www.mintrac.net.au/docs/pdf/20120314-N-E-MB.pdf
    Last edited by RANGER1; 28-09-2015 at 06:42 AM.

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    Re: How to apply floating interstage pressure on two stage plant

    Ranger.

    Is this not one of those fly by night "theoretical savings" companies that sprang into life when the Prime Minister of Australia I forget which one (5 in 5 years) was waving around the carbon pricing that would not cost anyone anything?

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    Re: How to apply floating interstage pressure on two stage plant

    Quote Originally Posted by piewie View Post
    Ranger.

    Is this not one of those fly by night "theoretical savings" companies that sprang into life when the Prime Minister of Australia I forget which one (5 in 5 years) was waving around the carbon pricing that would not cost anyone anything?
    Piewie,
    I think it is but some information still relevant.
    To me electricity is still not expensive enough to implement a lot of these heat exchangers, but maybe I'm wrong.
    The government grants were a weak excuse to upgrade your plant, when they had done nothing at all in the past (false economy), typically want everything for nothing.
    The carbon tax kicked it right along for companies like this (in links posted), it will probably happen again when things settle down.
    When we kicked carbon tax off we were one of the first, then caved in, now we will be one of the last.
    If a plant is new & purpose built it's a lot easier to justify.
    How many times do you win a job when you are the most expensive, a lot is market driven.
    At the end of the day it keeps you thinking about improved design & how you sell it.
    I wouldn't try to sell something I don't believe in personally.

    A lot of these places started small & grow over decades, so no real planning for a lot of these new ideas.
    Any heat exchanger in discharge line etc, always has the penalty of a pressure drop.
    As far as replacing old screws, yes they are worn, but not cheap either to replace.
    Client wants serious payback & package always looks like crap if not replacing like for like.
    Last edited by RANGER1; 28-09-2015 at 09:17 PM.

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    Re: How to apply floating interstage pressure on two stage plant

    There are 2 major ways to save energy in industrial refrigeration:
    1. Optimization of the refrigeration plant operation.
    2. Capital investments in energy saving equipment.
    Very often 50% energy can be saved by optimization and 50% by capital investments, However, optimization is 10-20 times cheaper than capital investments. Every company can start from optimization and continue with capital investments. Some companies are very focused on energy savings and they do two steps simultaneously.
    However, not so many people know how to do optimization. Minus 40 can do that. They are not fly by night company.

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    Re: How to apply floating interstage pressure on two stage plant

    Hi Ranger1.
    interesting read from everyone.
    Short answer shut everything down during peak load period by power supply company, generally between 6.00pm and 10.00 pm, not always easy with a meat processing plant. But most things like tunnels and holding chillers will not creep temperature too much if closed up, then run hard out at cheaper supply rates. Your client can negotiate a better off peak rate if they adopt the shut down scenario during peak load timing. Talk with power supply company, they are always talkative to keep their network peak loading spike periods and penalties down.
    Had a client that adopted that policy and started night shift later,, did the cleaning defrost what ever in that time frame, saved $50K per week.

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    Re: How to apply floating interstage pressure on two stage plant

    There is a lot of work in progress on this plant, with long delays on each improvement.
    Power bill hard to say how much they save, as add new rooms at the same time.
    Power might stay the same, so no one notices any improvement or savings, all sort of theoretical.

    We worked for a company that had it's own internal "engineer" who believed opening all hand expansion valves fully open on a plant saved money.
    Compressor load supposedly dropped, so power savings.
    We did not agree, so suddenly you are on the outer.

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    Re: How to apply floating interstage pressure on two stage plant

    I don't know any production refrigeration plant which switched of during week days. Some companies require to keep precise +/- 0.5C temperature in the coolers. During winter operation holding freezers can be switched off for period of time. Some people want to save money and kWh as well. Optimization and capital investments can save both.
    Ranger 1.
    May be it is better to ask independent adviser(minus 40 company) about opening hand expansion valve. Hopefully, this internal "engineer" will listen others people.

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