condensing pressures

Hi
I've been thinking (dangerous i know) how you guys relate your servicing to optimising power consuption, particularly concerning reducing head pressures/temps , now the magic 40* has served me well, systems that are sized correctly with correct super and sub heat are genarally happy, but reading articles that state that the condensing pressure should be pressure diff required across tev + evap pressure + system losses (1.5 bar ish?) to achive lowest optimum head pressure has got me pondering.
Firstly i'm struggling to find the diff info required, secondly i suspect that the figures arrived at are lower than rules of thumb suggest.
How do you reconcile this? especially concerning the evil r407c which loves to flush.
Ill be interested in your answers

:confused:

If you have a TXV system, the minimum condensing pressure is: minimum differential pressure across the TXV to achieve its rated and desired capacity + the total pressure loss in the liquid line (this includes driers, piping, fittings, etc.) + the evaporating pressure.

Its only good in theory Mike.
If the ambient allow you to do so, fine. But what do you do when the Δt from winter to summer is over 40°C?

Sometimes you are happy just to achieve proper condensing and on the other season you stop condenser fans and pray that your evaporator will not freeze.

But..........I agree with every word you have written.;)

Hi Chemi.

Yes, there are other factors also. What I wrote is the minimum it takes to allow the TXV to operate and achieve its capacity. However, as you mentioned the system has to work at other operating conditions also. The TXV selection is based on the range of operating conditions to ensure it will work in all of them.

Obviously as the outdoor temperature increases the differential pressure across the TXV also increases. This operating condition also has to be checked just as you would for low differential pressure conditions like winter.

i agree. the fundamentals are all there, but in practice how low do you go? i know some 407 systems that just aren't happy at low head pressures

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Originally Posted by Haden4ever

i know some 407 systems that just aren't happy at low head pressures

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OK, but why? There has to be a reason. The info I replied with is the bare minimum required. You cannot go lower than the ambient temperature + the condenser design TD (condensing - ambient). If that pressure is differential is too low for the valve selected then yes it may not work very well.

If you are using hot gas for defrost, that might be a reason why the system is not happy at low condensing temperatures. When the system was designed someone may have made the assumption that the system operated at summer design conditions all year round.

Quote:

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Originally Posted by Haden4ever

Hi
I've been thinking (dangerous i know) how you guys relate your servicing to optimising power consuption, particularly concerning reducing head pressures/temps , now the magic 40* has served me well, systems that are sized correctly with correct super and sub heat are genarally happy, but reading articles that state that the condensing pressure should be pressure diff required across tev + evap pressure + system losses (1.5 bar ish?) to achive lowest optimum head pressure has got me pondering.
Firstly i'm struggling to find the diff info required, secondly i suspect that the figures arrived at are lower than rules of thumb suggest.
How do you reconcile this? especially concerning the evil r407c which loves to flush.
Ill be interested in your answers

:confused:

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It isn't easy to save energy. You need very good theoretical and practical knowledge, because you should look at the plant as a system. Several initial questions should be answered.
1. Does system save energy at lower condensing pressure? Typically yes, but sometimes no.
2. At lower condensing pressure capacity of TEV will be reduced. Do you need 100% TEV capacity all year around? We can have lower cond. pressure when ambient temp. is low, at the same time refrigeration load can be reduced for the room where major source of load is heat transmission from outside air. If the load is 70%, capacity of TEV can be reduced to 70%. Make sure that you have good oil return.
...........

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Originally Posted by Haden4ever

i agree. the fundamentals are all there, but in practice how low do you go? i know some 407 systems that just aren't happy at low head pressures

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You can lower cond. pressure step by step. Lower by 0.5 bar and monitor for a week. If any concerns, tell us. Every barrier has a solution.

low condensing & low load combination is un-desireable always . Keeping the condensing pressure up to the design is an excerise which does not offer any energy saving but in order to keep the system going ; it is an option that has to accepted .

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Originally Posted by D.D.KORANNE

low condensing & low load combination is un-desireable always . Keeping the condensing pressure up to the design is an excerise which does not offer any energy saving but in order to keep the system going ; it is an option that has to accepted .

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I don't agree. To save energy condensing pressure should be optimum all year around. During periods of cool weather, condensing pressure should be lowered. There are many barriers to reduce condensing pressure. Every barrier has a solution.

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Originally Posted by Segei

...you should look at the plant as a system.

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Absolutely. The problem people seem to have is they think of basic components, not how all of the components work together in the system under varying conditions.

Quote:

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Originally Posted by Segei

There are many barriers to reduce condensing pressure. Every barrier has a solution.

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I could not have said it better myself.;)

Every client wants to save power, these analysts come in and sell themselves as experts, " we can save you money " . Lower the condensing pressures and client saves a truck load of money.
You have a system designed for maximum performance for minimal power, you maintain the system for optimum performance. Then this " **** head " walks in and the client thinks he walks on water. And is the best thing since sliced bread.
My reaction is , if you want to save power costs turn the system off., and you will save huge amounts of money, but hell sorry above your freezer or chiller and product temps, when all else fails get the rocket scientist back and sort it out, donot ring me.
What really annoys me is these experts charge thousands of dollars for there advise and my advise is totally ignored. [ grump ].
magoo

thanks for that, plenty to consider, and as i thought no definitive answer, good job i suppose keeps us all busy .....
:D