cooling &freezing comp.

[afeef]

dear all
i am sure that there is diffrence between compressors used in cool rooms and compressors used for freezing rooms, any body can give me link to proof this
thanks
afeef

[NH3LVR]

Afeef;
Yes, there are differences in the Commercial end using Hermetic Compressors. Copeland's Catalog illustrates this well, the difference being the motor size mostly, as far as I know.
But I really do not have much knowledge of the smaller machines.
My comments are directed toward the larger Industrial Machines using NH3 (Vilter in particular). The only difference I am aware of is not in the Compressor itself but in the Horsepower required.
If we acquire a Compressor Skid that was used on a Low Temp Application, we might need to install a larger Motor for a Medium Temp Application.
This is not to say that there is not a difference in controls, Oil separators. and other accessories.
In the control difference area, one occasionally sees a High Suction Pressure Unloader installed to limit Horsepower requirements on startup, for instance.
Screw Compressors are a different story. V.I. will be different for different conditions. For instance a Fixed V.I. Machine optimized for Medium Temp will not be optimal for Low Temp.
Some ***** Machines may well use different valve gear, but again I am not sure of the particulars.
I will leave the difference in the Hermetics up to the guys who know them well.

[afeef]

dear NH3LVR
thanks for replay, the diffrence : the discharge pressure of the freezing comp. is higher than the cooling comp. also the suction pr. of the freezing comp. is lower than the cooling comp. , i search copland site (www.emersonclimate.com),but i could not find the document that proof my answer
any one have another site explain this diffrence please send it to me
regards
afeef

[Peter_1]

Discharge pressure has nothing to do with the fact that the compressor is used for cooling or freezing.
The discharge pressure is only determined by the condenser and the outside conditions.
We have packs running with freezing and cooling ccompressors on a common discharge line.

[monkey spanners]

Hi Afeef

On small prestcold compressors they have different displacemnts for the same hp depending on the temperature range it is designed to work in.

model designation
PL20X003607
P = Prestcold
L = motor type (K or L)
20 =nominal motor output in HP x 10
X = X for polyolester oil
0036 is swept volume in m3/second
07 = electrical supply type

So a PL20X0027 is a 2 HP compressor rated at 13.4kw at 12.5C evap./ 30C cond. and 971 Watts at -40C evap./ 30C cond.
A PL20X0036 is a 2HP compressor rated at 9.4Kw at -5C evap./ 30C cond. and 549 Watts -50c evap./ 30C cond.
A PL20X0040 is a 2 HP compressor rated at 5.6Kw at -20 evap./ 30C cond and 655 Watts -50C evap and 30C cond.
Older prestcolds are marked as H (high) M (medium) L (low)
You can get a PL30X0036 for high temperature work which is a smaller displacement with than the 2HP one above but with a 3 HP motor.
Low temperature compressors have a bigger displacement than higher temperature compressors for the same motor size.

Hope this makes sense, cheers Jon

[winfred.dela]

i am sure that there is diffrence between compressors used in cool rooms and compressors used for freezing rooms

I have tried using hermetic recips for aircon applications in
chillers (0C room temp) and
freezers (-18C room temp)

I do not want to give the models and brand to anybody coz its quite early to see the long term effect. Tried it 5 years ago to at least 10 units (chillers/freezers)

dear all
any body can give me link to proof this

I dont think so. the manufacturers always wanted to have some more money for refrigeration equipment.

[Peter_1]

Afeef, this is back to basics: if you use a compressor with a driven electrical motor designed for cooling, then it will take some power to pump the gasseous refrigerant and compress it.
Now, if you take the same compressor and let it run for a freezing application, then the swept volume will be far less then it was fo a cooling condition, so the power will also be less because there's not so much gass to compress (to the same discharge pressure!)

The link you're looking for to prove this is in every refrigerant school book in chapter 1 or 2

[Josip]

Hi, Afeef

dear NH3LVR
thanks for replay, the diffrence : the discharge pressure of the freezing comp. is higher than the cooling comp. also the suction pr. of the freezing comp. is lower than the cooling comp. , i search copland site (www.emersonclimate.com),but i could not find the document that proof my answer
any one have another site explain this diffrence please send it to me
regards
afeef

Hm,

Evaporating temperature for freezing is much lower then evaporative temperature for cooling and the same is with the pressure.

Condensing temperature (pressure) can be the same but capacity of freezing compressor is very small (speaking and using the same type and size of compressor). In this case electrical motors for both compressors are of the same size.

It is much better to use cascade (double stage) system for freezing.

Freezing system, first stage -40C(-40F)/-10C(14F) second stage -10(14F)/35C(95F)

Cooling system -10(14F)/35C(95F)

Speaking about compressors there is no difference in construction (maybe in assembly of valves) within the piston compressors.

Assembly (not constructional) differences also exist within screw compressors due to oil pressures and oil piping.

Size of electrical motor for any type of compressor depends on working pressure but not suction, only on discharge/condensing pressure and size of compressor.

Best regards, Josip

[NH3LVR]

Josip;
I have never replied to of your posts before and dislike having to disagree.
However,I must (gently) disagree with two statements.
Screw Compressor internal arrangements change the VI. Optimum VI is different for medium and low temp if the Head is the same. On application may require 5.0 VI and another 3.2. In the case of a fixed VI compressor this will require modifications to the compressor body.
Also the size of the motor can vary according to the Suction pressure. I recently replaced a 250HP with a 350HP motor because we raised the Suction pressure on the intermediate of two stage system.
NH3lvr

[Samarjit Sen]

The basic difference between a compressor being used for cooling and the one being used for freezing is that the evaporative temperature or the pressure for the cooling compressor shall be higher than that of the freezing. The condensing temperature or the pressure will not change as the same is dependent on the condensing unit. The lower the evaporative temperature the lower would be the power consumed.

Basically there is no difference in the two except what has been stated above.

Peter 1 and Josip have said it very correctly as I am personally involved in such applications.

With best wishes

[jamcool]

Have seen where a walk-in-freezer becomes a walk-in-cooler and all that was done was the defrost timers was taken out,the fan delay was jumpered and the stat adjusted to a cooler settting and it works fine so far.

[TXiceman]

dear all
i am sure that there is diffrence between compressors used in cool rooms and compressors used for freezing rooms, any body can give me link to proof this
thanks
afeef

It wowuld hel pif you would tell us if you are looking at commerical machines or industrial applications.

On a commerical machine like a Copeland, they use smaller a motor size on a freezer machine of a given displacment than the same displacment compressor in a cooler.

When you are into screws, to optimize the machine, a medium Vi (like a 3.2) is used in a cooler and a higher Vi (like a 5.0) in a freezer application.
The Vi or volume index is defined as the trapped gas volume at the suction divided by the trapped gas volume at the discharge. It is also the pressure ratio raised to the k power.

Newer screw compressors us a variable Vi (it has a slide vlave and a slide stop to adjust the Vi. The control panel is sconstantly looking at the compressor operating conditions and adjust the slide stop to match the Vi to provide optimum performance.

I don't know of any artlices to specifically "prove" your position. Best thing to do is to make machine slections and see the performance under varying conditions.

Ken

[Samarjit Sen]

Dear afeef,

What iare the temperature and the capacities that you are looking for. You may attain the conditions with the same type of compressors . Of course I am talking about semi hermetic or open type reciprocating.

[taz24]

Most good Semi hermetics are marked as Hi - Low dependent on the aplication.
Scrolls and hermetics are slightly different because the way they cool themselves.

Cheers taz.

[Josip]

Hi, NH3LVR

Josip;
I have never replied to of your posts before and dislike having to disagree.
However,I must (gently) disagree with two statements.

No problem we are here to discuss and help each other.

Screw Compressor internal arrangements change the VI. Optimum VI is different for medium and low temp if the Head is the same. On application may require 5.0 VI and another 3.2. In the case of a fixed VI compressor this will require modifications to the compressor body.

Agree 100%.

In the case of a reciprocating compressor, the volume of the refrigerant sucked into the cylinder decreases and the refrigerant pressure increases as the piston ascends. When the pressure exceeds the discharge side pressure and the force of the spring on the discharge plate valve, the refrigerant in the cylinder pushed open the valve and passes to the discharge side.
In the case of the screw compressor, a volume of refrigerant is sucked into the groove between the rotors and this volume decreases while pressure increases as the rotors rotate. The process up to this point is the same as for a reciprocating compressor. When the volume is decreases to the designed Vi, the groove is linked to the discharge port and the refrigerant is pushed out. The groove is linked to the discharge port according to the volume of the groove and is not dependent on internal pressure.
Vi (internal volumetric ration) is used to represent the value of the decreased volume of suction refrigerant when the groove aligns with the discharge port (or is discharged).

This can be expressed as follows:

Vi= Volume of suction refrigerant when compression begins .
Volume of same quantity of refrigerant at discharge port
In other words, Vi is the ratio of the groove volume after competition of suction to the volume when the discharge port opens.
Conventional screw compressors have three fixed Vi values, that is 2.63, 3.65 and 5.80, termed „L port,“ „M port“ and „H port,“ respectively.
The relationships are:

SEE ATTACHMENT!!!

Consequently, the Vi corresponding to the compression ration changes according to the refrigerant used.

With the fixed Vi of a conventional compressor, maximum efficiency can only be obtained when the system is operating at a pressure equivalent to the designed Vi. Unnecessary power is consumed, however, when pressure conditions diverge from the designed value. For example, if low compression ratio (high compression pressure or low discharge pressure) operation is carried out using a conventional M port compressor (designed for a medium compression ratio), compression will exceed discharge pressure and power will be wasted.
Conversely, if the same M port compressor is used under high compression conditions (high suction pressure or high discharge pressure), the discharge port opens before internal pressure has increased sufficiently, allowing refrigerant to flow back from the discharge port. Power is also wasted.
Obviously, if a compressor is expected to be operated for a long period of time under varying conditions, a variable Vi design is preferable to a fixed Vi type. For a conventional compressor with a high Vi, the discharge port can be machined to lower the Vi but a unit with a low Vi cannot be changed to a high Vi type. If a higher Vi is needed, the compressor must be replaced with a new one.

Please, can you describe little more what happen here?

Also the size of the motor can vary according to the Suction pressure. I recently replaced a 250HP with a 350HP motor because we raised the Suction pressure on the intermediate of two stage system.
NH3lvr

Best regards, Josip

[NH3LVR]

Josip;
What happen was this.
We installed a FES Model 270 in 1976. It has a Mycom Screw and and was used as a second stage machine running from, if memory serves, (I did not do the calculations) 9Lbs (o.6 BAR) Suction to 150Lbs (10.3 Bar) head (Nh3)
Due to installation of another first stage machine the Intermediate climbed to to over 20Lbs. This was predicated and not a surprise.
Looking at the FES supplied Charts it was determined that the Horsepower need was increased due to the higher suction. A decision was made to install the larger motor and starter, due to the changing conditions in the plant.
I think there is a bit of confusion on the power consumption effect of Suction Pressure as it is not usually a problem with smaller Hermetics. In my small experience with the Copelands in the 30 HP class they have more than sufficient Horsepower reserve to operate out of their design conditions. Copeland Discuses "Nominal Horsepower",as I recall. I work with motors that are rated as to HP and Service Factor.
Where the difference becomes apparent is in open drive compressors, which is almost all I work on. The exception being when I cover for our one Commercial guy. I might add that can be a humbling experience.

This effect is illustrated from Table 8 in this Carrier Document. (Page 7)
http://www.marinesystems.carrier.com...h_09rh-2xa.pdf

Another item is that the HP required by a system is often dictated by the startup requirements. If you have just enough HP to run at the design conditions it can be difficult to start a system that has been out of operation. As I mentioned before we have installed High Pressure Unloaders on Machines in order to achieve an initial pulldown.
In some cases I have had to start freezers from warm by partially closing the Suction Valve. Sufficient HP is not included on many skids because they will only be started at a high suction on rare occasions.
Screws of course will unload at high motor amperage so that is not a problem. You just let the Control Panel keep the machine unloaded until the Suction comes down. This is very common on Continuous freezers that are defrosted every day, or more.

[afeef]

dear all
thanks very much for your replays, you are realy interesting in this thread
now i confinced that there is no diffrence in pressurs(suction&discharge)for cooling and freezing comp.
i get this document(attchment) from bitzer comp. dealer
show obviously that there is diffrence, i.e. you can operate the same comp. for cooling or freezing purposes,( of course after checking its capacity),
god bless you all
afeef

[ameer]

dear sir

im engineer in iraq i want ur help to get a 6 compressor operate in our frezing store to reach atemp 25c under 0 temp where we have 4 store each one 36*30 meter 6meter heigh so i need ur help to advice me which kind i must use and its proprties

regards

[Peter_1]

What goods are stored and at what entering conditions?
With goods entering at -20°C, you will need at least 70 kW.
What's a 6 compressor?
Is the freezer room already made? What isolation was used? Is an underfloor heater installed?

[jim smith]

where can i find information concerning power consumption of large freezer compressors?

[jim smith]

our city is going to build a 4 millon pound cold storage for seafood at this time no one seems to know what it will cost to power it

[NH3LVR]

Welcome to the Forum Jim!
The basic answer to your question is not complicated. You simply multiply the Horsepower, converted to KW, times the power cost.
In reality you have to take several other things into account.
The Compressor HP has to be figured at the conditions the system will be run.
Although it may not be the case in Wrangell power costs are dependent on total consumption in many areas.
Seasonal loads vary and can change operating costs.
This is a legitimate question to ask of the Contractor if one has been selected, or to be asked for when bids are accepted.

[US Iceman]

Welcome Jim.

Operational costs due to the use of electricity are based on several things. The most important are: cost of energy (kWh) and demand charges (kW). Secondly, these often have different rates (charges) for what is considered ON-Peak and OFF-Peak.

The ON-Peak and OFF-Peak rates are considered time-of-use charges and can vary considerably. Your electrical utility can supply this information in something called a rate tariff sheet. This rate tariff will list the exact method for claculating the monthly charges and the costs for ON-Peak, OFF-Peak, energy and demand.

The last part that plays an important function in total energy use is related to the operation of the equipment. If screw compressors are used you do not want them to be operating at what we call part-load (partial capacity). Screw compressors are very energy inefficient at part load.

The way around this is to have the contractor calculate the energy use for seasonal usage. In the summer the capacity requirements will be much higher than the winter. So in cooler weather you might find a single compressor running at a very low capacity which is extremely costly.

One last thing I want to add to this discussion. Ask the contractor to design the rfrigeration system so that it works at 100 psig dicharge pressure (or lower) in the cooler weather.

Some system designers will design the system as if it operates in the summer all year round. That's not good.

[Peter_1]

Do you need anyhow cooling in Alaska
I should especillay take the suggestions serious of USiceman: try to condens as low as possible. Condensing high can double the total operating costs!!
And I suppose you have the possibilities to condens low in Alaska.

WhatI don't understand, they want to build such al rage store (what's the size of it) and they even don't know if they have the budget for it.

The normal way is that the idea is given to a engineering company who will make a rough estimate.

[Peter_1]

Some system designers will design the system as if it operates in the summer all year round. That's not good.

I see this more then systems trying to condens as low as possible.
Most supermarket systems I know have HP switches controlling the fans on a constant HP, winter and summer.
That's mostly because they don't know what they're doing.

[US Iceman]

That's mostly because they don't know what they're doing.

I totally agree. Most of the systems I see are designed for summer operation at full load and nothing else.