Re: Evaporative Condensers
Quote:
Originally Posted by
Polisie
Hi.
Im am new to the Ammonia Refrigeration Industry. Being in the HVAC consulting side for 3 years and going into the refrigeration indutry is a big change for me.
I need to replace an evaporating condenser which has a base heat rejection of 538kW. The 538kW is the nominal heat rejection with out any location factors etc.
I am putting a proposal together as follow:
-Replacing the old coils with new coils only
-Replace the old condenser with a new conderser with the same make and model
-Replace the old condenser with a defferent make which has a larger capacity 582kW
My questions are:
What does the larger capacity of the condenser have on the exciting ammonia system?
Will this have a negative or positive impack on the system as to prolonging the life of the system?
Francois
What does the larger capacity of the condenser have on the exciting ammonia system?
An equilibrium between compressors and condenser will be reached so the condensing temperature will be lower » lower discharge pressure and small increase on the cooling capacity and decrease on the power consumption, resulting on better COP
Will this have a negative or positive impack on the system as to prolonging the life of the system?
Positive impact. The compressors will have less tear due to lower mechanical stress of the connecting rods and crank shaft and also the oil will live longer » result: a longer time between services.
Re: Evaporative Condensers
It depends. Capacities of compressors and condensers should be balanced. Over sized condensers use more energy than compressor energy that can be saved by reduced condensing pressure. However, this happen not so often. I saw over sized condensers only for plant that was deigned for certain refrigeration load,but real load is significantly lower that design one. For example, plant was designed for certain production load but production is only 50% of designed. Most likely bigger condensers will be better for your plant.
Re: Evaporative Condensers
Yes Segei, that's why the floating condensing temperature is so important on huge plants.
Re: Evaporative Condensers
Many people are over focused on floating condensing pressure. I found that this floating saves just 15-20% of condenser energy. However, condenser energy is 10% of compressor energy. So this floating will save 1-2% of plant energy. This is not a lot. Another issue that we can float condensing pressure during short period of time, usually, when outside temperature between 10C and 20C. Typically, above 20C condenser operates at full capacity(no floating). Below 10C plant operates at minimum allowable condensing pressure(no floating as well).
Re: Evaporative Condensers
Quote:
Originally Posted by
Segei
Many people are over focused on floating condensing pressure. I found that this floating saves just 15-20% of condenser energy. However, condenser energy is 10% of compressor energy. So this floating will save 1-2% of plant energy. This is not a lot. Another issue that we can float condensing pressure during short period of time, usually, when outside temperature between 10C and 20C. Typically, above 20C condenser operates at full capacity(no floating). Below 10C plant operates at minimum allowable condensing pressure(no floating as well).
That's why I said that is worth for big plants where 1-2% even at small periods of the year it could represents some Euros (cash). The expensive of this system is negligible: just know the consumers, the existing PLC, a combined temperature-humidity sensor (for the condenser) and some programming.
Do you never user floating condensing pressure?
Re: Evaporative Condensers
I read somewhere that if the condenser is sized one size bigger they use VFD motors on the fans which is controlled by temp controllers.
But you can only go so big with a condenser? If the condenser is too big it will have a negative effect on the plant. What would happen then?
Thank you all for your replies.
Re: Evaporative Condensers
Quote:
Originally Posted by
Sandro Baptista
That's why I said that is worth for big plants where 1-2% even at small periods of the year it could represents some Euros (cash). The expensive of this system is negligible: just know the consumers, the existing PLC, a combined temperature-humidity sensor (for the condenser) and some programming.
Do you never user floating condensing pressure?
May be it looks like I don't use floating condensing pressure. Actually, I've done extensive research about this issue. To float condensing pressure, you need sophisticated PLC with wet bulb temperature sensor and wet bulb approach should be set properly to get energy savings. PLC itself doesn't save energy, set point do. So it is very important to keep optimum wet bulb approach. Look at this article.
Major energy saving measure
In energy saving process many people are over focused on floating condensing pressure. Definitely, floating condensing pressure saves energy, but these savings are not significant.
Example.
Refrigeration plant has 2 screw compressors. Each compressor is 250 HP. Suction pressure is 3.5 psig. Evaporative condenser has water pump of 20 HP and fans of 30 HP.
Initially this plant was operated at fixed condensing pressure of 150 psig. Recently, operation of this plant has been changed to floating condensing pressure. Currently, condensing pressure floats from 120 psig to 130 psig. These changes have saved 58 HP or 43.25 KW of energy for mentioned refrigeration plant. However, action of switching operation from fixed to floating condensing pressure can be divided on 2 steps.
Step 1. Condensing pressure was lowered from 150 psig to 125 psig. This step saved 52 HP of compressor power. This is 90% of total energy savings of 58 HP.
Step 2. Floating of condensing pressure from 120 psig to 130 psig saved 20% of condenser fan power. This savings are not significant, because condenser fan power is small compare to total refrigeration plant power. This energy savings are 30 x 0.2 = 6 HP or 10% of total energy savings.
When can we use floating condensing pressure? Typically, condensing pressure can be floated when outside temperature is between 50 degF and 70 degF. When outside temperature is below 50 degF, majority of the refrigeration plants operate at minimum condensing pressure and there is no floating. When outside temperature is above 70 degF condensers operate at full capacity and there is no floating as well.
Unfortunately, real floating of condensing pressure can be applied for a limited period of time. It means that this energy saving measure will save less than 10% of total energy savings. However, lowering condensing pressure can save more than 90% of total energy savings and this is major energy saving measure. There are several barriers (hot gas defrosting, liquid injection oil cooling, oil carry over and etc.) to run refrigeration plant at low condensing pressure. However, every barrier has a solution.
Re: Evaporative Condensers
Quote:
Originally Posted by
Polisie
I read somewhere that if the condenser is sized one size bigger they use VFD motors on the fans which is controlled by temp controllers.
But you can only go so big with a condenser? If the condenser is too big it will have a negative effect on the plant. What would happen then?
Thank you all for your replies.
Every condenser can have fan VFDs. They will slow the speed of the fans and save energy. This can happen when outside conditions cooler than design condition or/and refrigeration load is smaller than design refrigeration load.
I don't think that over sized condensers are bad for plant. However, to save energy you should use them wisely.
Re: Evaporative Condensers
I think that minimum condensing pressure saves much more energy than floating condensing pressure. Sandro. What is the typical minimum condensing pressure for the plants you design?
Re: Evaporative Condensers
Re: Evaporative Condensers
I saw plants operating at 4 bars head pressure. However, this is Canada. For the Portugal, 6 bars will be a reasonable number. Reducing head pressure by 0.2 bar will save more energy than floating condensing pressure. Reducing minimum head pressure from 8 bars to 6 bars can save 10 times more than floating condensing pressure. This is my point.
Re: Evaporative Condensers
4-6 bar I think is too low to the plant works well...but I believe it works.
Re: Evaporative Condensers
4- 6 bars it is not low, it is optimum for winter operation. So this should be goal for every refrigeration plant. Definitely, we have several barriers to achieve this goal, but every barrier has a solution.
Re: Evaporative Condensers
Capacities of compressors and condensers should be balanced.
Re: Evaporative Condensers
Quote:
Originally Posted by
Bonnie2012
Capacities of compressors and condensers should be balanced.
The balance will be self-balance. the thermodynamic equilibrium will be reached by its own.
Re: Evaporative Condensers
[QUOTE=Bonnie2012;259884]Capacities of compressors and condensers should be balanced.[
/QUOTE]
I agree that capacities of compressors and condensers should be balanced. What is the criteria of balancing? When we talk about energy savings, criteria should be total(compressors+condensers) minimum energy use.
Re: Evaporative Condensers
Quote:
Originally Posted by
Sandro Baptista
The balance will be self-balance. the thermodynamic equilibrium will be reached by its own.
I agree Sandro. Sooner or later plant will achieve balance. Only one question. Will it achieve minimum energy use at this point?
Re: Evaporative Condensers
Quote:
Originally Posted by
Segei
I agree Sandro. Sooner or later plant will achieve balance. Only one question. Will it achieve minimum energy use at this point?
Yes, the concern of energy spend is a point to pay attention.
