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Re: Liquid Overfeed Systems
I'm stuck without my refrigerant tables but using steam as an example, I think this should help if I remember this right (sounds of scraping around in dusty corners of brain....)
Flash gas is the excess heat energy caused by the reduction in pressure. So for steam condensate:
7 bar = 721kj/kg
0 bar = 419kj/kg
Excess = 302kj
So the percentage of flash gas is:
(excess/enthalpy of evaporation kj @ 0 bar) x 100%
(302kj/2258kj) x 100% = approx 13%, or 13kg of gas and 87kg of liquid.
(Enthalpy of evaporation is the term that is now used for latent heat).
So if you know your mass flow in kg through the regulator, you should be able to work out the amount of flash gas your compressor needs to deal with.
Steve
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Re: Liquid Overfeed Systems
hi to all,
pls correct me if im wrong... what we did is, we calculate the percentage or quality by this equation: h= hf+xhfg
this equation is from thermodynamics.. are we correct???
also how can we measure the mass of the gas that leaves the evaporator?
do the latent heat ammonia vary in different temp???
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Re: Liquid Overfeed Systems
Quote:
Originally Posted by corineramer
what we did is, we calculate the percentage or quality by this equation: h= hf+xhfg
this equation is from thermodynamics.. are we correct???
It is straight thermodynamics so the answer should come from the formula I posted up.
Quote:
Originally Posted by corineramer
also how can we measure the mass of the gas that leaves the evaporator?
If you know the amount of liquid you are supplying to acheive a refrigerated affect, this volume of liquid will expand to vapour when it hits the evaporator.
In simple terms, you supply say 0.05 kg/second of liquid to the evap and it will expand to produce say 0.05m3/hr of vapour. That vapour density at the evaporating pressure can be found in your refrigerant properties table.
This means your compressor will need to deal with 0.05m3/hr from the evap plus flash gas generated at the hand regulator.
Quote:
Originally Posted by corineramer
do the latent heat ammonia vary in different temp???
The answer to that should be in your refrigerant tables: http://www.engineeringtoolbox.com/ammonia-d_971.html
Steve
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Re: Liquid Overfeed Systems
Quote:
Originally Posted by corineramer
also how can we measure the mass of the gas that leaves the evaporator?
That's easy. The evaporator mass flow for the gas is based on the evaporator cooling capacity divided by, hg-hf (this is also listed as hfg in the refrigerant tables)
Where,
hf = liquid enthalpy at evaporating pressure/temperature
hg = vapor enthalpy at evaporating pressure/temperature
hfg = latent heat at evaporating pressure/temperature
Quote:
Originally Posted by corineramer
do the latent heat ammonia vary in different temp???
If you look at the tables, you will find your own answer to this.;)
Now, if you calculate the refrigerant mass flow into the separator, you need to use the conditions that exist for the compressor.
hf = liquid enthalpy at condensing pressure/temperature
hg = vapor enthalpy at evaporating pressure/temperature
When you find the mass flow for one kW (or 1 Ton) using the above NRE (Net Refrigerating Effect), hg-hf, you will find this mass flow is higher than the mass flow for the evaporator (for a 1 kW or 1 Ton basis).
The difference between the two mass flows (compressor mass flow minus evaporator mass flow - gas only) is the flash gas.
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Re: Liquid Overfeed Systems
Hi to all,
Example
evaporator cooling capacity = 178.2637 KW
Temp of ammonia in separator = -10C
Condensing temp of ammonia = 40C
For evaporator mass flow for gas
From ammonia table at -10C
hf = 135.4 KJ/kg
hg = 1433 KJ/kg
hfg = hg - hf = 1433 - 135.4 = 1297.6 KJ/kg
Mass evap gas = (178.2637 KJ/sec)/1297.6 KJ/kg
= 0.13738 kg/sec
For compressor mass flow for gas
From ammonia table at 40C (condensing temp)
hf = 371.9 KJ/kg
From ammonia table at -10C (evaporating temp)
hg = 1433 KJ/kg
hfg = hg -hf = 1433 - 371.9 = 1061.1 KJ/kg
Mass comp gas = (178.2637 KJ/sec)/1061.1 KJ/kg
= 0.16800 kg/sec
Mass comp gas = Mass evap gas + Mass flash gas
Mass flash gas = Mass comp gas - Mass evap gas
= 0.16800 - 0.13738
= 0.03062 kg/sec
Pls correct us if we're wrong
In getting the mass flowrate with overfeed of 400%
= mass evap gas x 4
= 0.13738 kg/sec x 4 = 0.54952 kg/sec
In getting the volume flowrate with overfeed
from ammonia table at -10C
vf = 1.5338 L/kg
Vol. flowrate = 0.54952 kg/sec x 1.5338 L/kg
= 0.84285 L/sec
In getting the work of compressor, Wc
Suction pressure, Ps = Psat at -10C
From ammonia table at -10C
Ps = 2.908 Bar or 290.8 KPa
hg = 1433 KJ/kg
Discharde pressure, Pd = Psat at 40C
From ammonia table at 40C
Pd = 15.54 Bar or 1554 KPa
hg = 1473.3 KJ/kg
Wc = Mass comp gas x (hg at 40C - hg at -10C)
= 0.16800 kg/sec (1473.3 KJ/kg - 1433 KJ/kg)
= 6.7704 KW
pls correct us if we're wrong.. thanks masters :)
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Re: Liquid Overfeed Systems
hi to all,
i have a question in our ageing process... we have to maintain the temp of our product to 4C... we're confused in getting the load because there's no temp difference... but we're to maintain this temp... how can we get the load in order for us to select the equipment.. we will use a water chiller in maintaining this temp... pls help, tnx
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Re: Liquid Overfeed Systems
Quote:
Originally Posted by
corineramer
hi to all,
i have a question in our ageing process... we have to maintain the temp of our product to 4C... we're confused in getting the load because there's no temp difference... but we're to maintain this temp... how can we get the load in order for us to select the equipment.. we will use a water chiller in maintaining this temp... pls help, tnx
Load comes from heat. Your product will heat up by respiration (biological activity) and heat will leak into the cooled space (heat transmission). Your refrigeration should carry away this heat. Heat will also come from any lights, fans, or people in the cooled space. Even if you load your product in at 4c, it will warm up eventually.
You have to work out this heat load and this information is readily available in text books and on the internet.
In simple terms, you have to work out the equipment needed to carry away this heat and from this you get your equipment requirements. Look for water chiller suppliers and download their information. A water chiller will be rated in kW and you need to supply it with enough refrigerant and enough water to move the heat.
Steve
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Re: Liquid Overfeed Systems
Quote:
Originally Posted by corineramer
we're confused in getting the load because there's no temp difference...
Steve mentioned where the heat comes from for your load. The remaining issue to determine is the desired temperature range for the water chiller flow and the evaporating temperature of the chiller.
If you use a high temperature range (larger TD for the water), the flow rate will be smaller (smaller water pump).
A lower temperature range (smaller TD for the water), the flow rate will be much higher (larger water pump).
Depending on what you select for the supply chilled water temperature to the coil(s) and the return water temperature (from the coils), you are finding the TD required. This is a balance between the requirements of the cooling process and the costs of the equipment (operating and initial equipment costs)
When you have the temperature range and flow rate established, then you can select the evaporating temperature for the chiller.
Once this is completed, you have the load and the evaporating temperature, so now you can add this to the refrigeration capacity requirements and select the chiller.
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Re: Liquid Overfeed Systems
hi us iceman,
im selecting the the compressor using the grasso site that you gave... what does refrigerating capacity at 2940 min^-1 mean??? min raised to -1, what does it stand for?
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Re: Liquid Overfeed Systems
Quote:
Originally Posted by
corineramer
hi us iceman,
im selecting the the compressor using the grasso site that you gave... what does refrigerating capacity at 2940 min^-1 mean??? min raised to -1, what does it stand for?
Means revs per minute (rpm) as in say 160m3/hr of refrigerant @ 2940 rpm
Steve
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Re: Liquid Overfeed Systems
Quote:
Originally Posted by
corineramer
hi us iceman,
im selecting the the compressor using the grasso site that you gave... what does refrigerating capacity at 2940 min^-1 mean??? min raised to -1, what does it stand for?
min^-1 means 1/min
so 2940 min^-1 is 2940 / min or 2940rpm
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Re: Liquid Overfeed Systems
Quote:
Originally Posted by corineramer
what does refrigerating capacity at 2940 min^-1 mean???
Others have addressed the engineering notation for the 2940 min^-1, so I believe we can move past this point.
However, I did want to clarify the statement itself.
When the Grasso literature says capacity at 2940 RPM, are they stating the kW (refrigeration) at 2940 RPM? Or, does the literature list cubic meters per hour?
Capacity is kW (or Tons), not m^3/hr. You may have already seen this, but I wanted to point out the fact that you must read the small notes sometimes hidden in the technical information.
The other issue that sometimes gets overlooked is the use of subcooling. Some manufacturers will use subcooling on ammonia compressors to inflate the performance values quoted.
Very seldom will you find subcooling from the condenser in an ammonia system, IF THE CONDENSER IS PIPED CORRECTLY.
Capacity is capacity, but you have to be careful on how the manufacturer states you will get that capacity at a specific operating condition.
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Re: Liquid Overfeed Systems
Quote:
Originally Posted by
US Iceman
Capacity is kW (or Tons), not m^3/hr. You may have already seen this
Generally, capacity is directly proportional to swept displacement and (that is the same) to rotational speed.
2940rpm is the synchro speed for 50Hz asynchronous three-phase 2-poles motors (50/s x 60 s/min = 3000/min), but at 60Hz it is more likely to be 3520 rpm, with a net increase in capacity of 20%.
Which is why capacity is specified together with the swept volume or rotational speed.
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Re: Liquid Overfeed Systems
Quote:
Originally Posted by NoNickName
Generally, capacity is directly proportional to swept displacement and (that is the same) to rotational speed.
Not always... If the screw compressor speed is slowed down below about 50% of the full speed, the capacity drops off very quickly.
In general terms though, I agree with what you said. But in no way did I intend to make a point that the capacity does not change with speed.
corineramer, here is a little equation to help you with this...
(Hz X 120)/ number of motor poles = nominal RPM
Here is an example:
(50 Hz X 120)/4 pole motor = 1500 RPM
(60 Hz X 120)/4 pole motor = 1800 RPM
(50 Hz X 120)/2 pole motor = 3000 RPM
(60 Hz X 120)/2 pole motor = 3600 RPM
The actual full load RPM will be slightly less than this due to slippage of the rotor in the stator. I thought this might help you understand the relationship with motor speed, Hertz, and motor construciton.
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Re: Liquid Overfeed Systems
hi,
thanks for that info... us iceman where can i get a clearer drawing of the evapco horizontal recirculator system?? we need it for our lay out, we're gonna use cad in drawing it... right now im selecting the equipments and following the procedure in the brochures but i dont know if im doin it right.. hehehe.. tnx again for the big help
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1 Attachment(s)
Re: Liquid Overfeed Systems
Hi corineramer,
Try this as an example to start with. This example is for an open drive refrigerant pump, not a hermetic pump.
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Re: Liquid Overfeed Systems
hi us iceman,
thank you so much sir... u have contributed a lot to our work... just a little push and where nearly finished..
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Re: Liquid Overfeed Systems
I'm glad to help you guys.
Push, push, push.... Did that help you get the project done?:D
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Re: Liquid Overfeed Systems
hi us iceman,
yup!! nearly, we dont know bout the piping because there are a lot of things that we dont know yet, like the inclination of hte pipes the oil traps that must be "u" shape.. i saw it in the brochure.. we're not famaliar with that but will solve it, hehehe:D also do u have the price of surge drum, cooling towet and evaporative condenser of the EVAPCO??
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Re: Liquid Overfeed Systems
Quote:
Originally Posted by corineramer
we dont know bout the piping because there are a lot of things that we dont know yet, like the inclination of hte pipes the oil traps that must be "u" shape...
What oil traps are you talking about?
Quote:
Originally Posted by corineramer
also do u have the price of surge drum, cooling tower and evaporative condenser of the EVAPCO??
Sorry, that is one area I can't help you with.
But, it is nice to hear you are making progress. Keep up the good work.:D
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Re: Liquid Overfeed Systems
hi us iceman,
yup, we also managed to get a hold of an ASHRAE copy for the sizing of the pipes, hehehe... we'll have to read it...
do u know where can we find a tunnel freezer for ice cream and filling machine?? i didnt like the one that we got because im not sure about the sanitary and its second hand.. its for the quality.. as for the filling machine we need to fill them in our container... our ice cream does not come on sticks and wrapper.. its in bulk cans..
thank you so much! rock on!!!\m/
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Re: Liquid Overfeed Systems
Quote:
Originally Posted by corineramer
do u know where can we find a tunnel freezer for ice cream and filling machine??
Sorry, I won't be much help there either. Josip or Andy may have some ideas, but since I have a very limited exposure to ice cream making I just don't know who the manufacturers are.
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Re: Liquid Overfeed Systems
hi us iceman,
ok tnx.. i'll just pm them...
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Re: Liquid Overfeed Systems
hi us iceman,
do u know something that can help us in piping our ammonia system??
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Re: Liquid Overfeed Systems
What is your question? I will try to answer them...
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Re: Liquid Overfeed Systems
hi us iceman,
for the suction diameter of the compressor and outlet diameter of the low pressure receiver are different... we dont know how to size them? our equipments have different diameters.. how can we pipe them? by using the larger diameter or the smaller or you can calculate for it??
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Re: Liquid Overfeed Systems
Always calculate the pipe size you need. The connections put on the equipment is the manufacturers attempt at balancing cost versus requirements.
When you are designing the system you have to know the operating conditions for each piece of equipment.
Let's look at an evaporator...
If the evaporator is selected for a specific evaporating temperature, then you will have some pressure loss (due to flow) to the next piece of equipment. The difference in pressure between these two components is determined by the mass flow and pipe size.
If the pipe is too small, you will have a higher pressure loss.
If the pipe is larger (for the same mass flow), the pressure loss will be reduced.
The pressure at the end of this pipe will affect the performance of that device.
I never recommend using the connection size as the required pipe size.
You do have to connect the pipe to the two components, so what you want to find is the required pipe size based on allowable pressure loss (this is what you use for your system design and equipment selections).
After you know the required pipe size, you will probably find that you will need some pipe reducer fittings to make the terminating connections.
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Re: Liquid Overfeed Systems
hi us iceman,
how can we compute for the amount of ammonia in the system??? what im trying to do is get the volume of the low pressure receiver and then multiply it with the density of ammonia and try add them up.. is this right?
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Re: Liquid Overfeed Systems
Volume of system X density = mass
You have to be careful if the volume you use is either gas or liquid (or both in a two phase line).
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Re: Liquid Overfeed Systems
corineramer,
How is your project developing? Are you guys done yet?;)
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Re: Liquid Overfeed Systems
hi guys,
its been a long time.. i've been checking this site but everytime i vist this site its not available...thank you for the big help on our school project.. ive been busy fixing my transcript and all other sort of stuff.. we've graduated already thank you so much we owe you one, big time.. i think we hav the highest project all thanks to you all... this site rules!!! thank you thank you..
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Re: Liquid Overfeed Systems
Everyone talks about the "how to" of liquid overfeed but what about the benefit?
What is the effect of liquid overfeed ratio on the refrigerant heat transfer coefficient?