ice_e info pack 2 operation and choice of compressors

8/13/2019 ICE_E Info Pack 2 Operation and Choice of Compressors

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Compressors

In the recent years scroll compressors have

been more widely used especially for relatively

small chilled stores. If the reciprocating

compressors technology can be considered

as rather mature, on the other side, in the last

decades there has been a strong research

activity for improving the efficiency and the

reliability of scroll and screw compressors.

However, if compared to the widespread use

of reciprocating and screw compressors, the

application of scroll machines in cold stores

can still be considered a niche and so this

technology is not investigated in this info pack.

Reciprocating vs screw compressors.

As mentioned, because of its technological

maturity, it seems difficult to implement

innovation strategies that could markedly

improve the energetic efficiency of open

reciprocating compressors during full load

operation. However, improvements are still

possible by achieving higher efficiency of the

electric motors especially in small

semihermetic compressors. As a general

statement, the efficiency of screw and

reciprocating compressors at full load are

similar, whereas part load operations with

variable pressure ratios of reciprocating

compressor are somehow higher than screw

ones.

Reciprocating compressors are more suitable

for single stage compression with high

pressure ratios.

In the recent years the reliability of screw type

compressors has been improved so that

nowadays it can be considered quite similar to

the reciprocating technology.

Regarding the cooling capacity of a singleunit, referring to -10/+25 C

evaporation/condensation saturation

temperatures, the reciprocating compressor

can reach up to 1200 kW, whereas the screw

compressor can reach up to 5 times higher

cooling capacity.

With reference to the lowest cooling capacity

achievable, the reciprocating compressors

present no limitations, as it is well known. On

the contrary, the screw type is usually not

available for cooling capacities below 150 kW,

in refrigeration applications (lower cooling

capacities are available for screw

compressors for air conditioning applications).

Some models of ammonia compressors by

different manufacturers have been considered

in Cavallini et al., 2007. Swept volume [m

3

/h]data sets, taken from company web sites,

have been compared. The difference between

screw and reciprocating compressor

displacement is remarkable. The reciprocating

compressor swept volume can reach, at most,

3000 m3/h, even for two-stage machines.

Screw compressors, instead, can display a far

larger swept volume, which can reach 10000

m3/h. Also single-screw compressors are

available on the market: they display compact

dimensions and low displacement. Ammonia

screw compressors can reach very high

refrigeration power and there is a wide

selection of models. Except for single-screw

ones, all the models have a nominal cooling

capacity higher than 100 kW; some of them go

over 1000 kW. On the contrary, nominal

refrigeration powers of reciprocating

compressors are far lower and only few of

them have a cooling capacity higher than 300

kW.

Basing on previous considerations, one can in

general state that reciprocating compressorsare more widespread for relatively small cold

stores.

The most used typeof compressors inrefrigerated storesare reciprocatingand screw ones.

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Figure 2: typical values of the volumetric efficiency for reciprocating and screw

compressors, as a function of pressure ratio.

Figure 1 and 2 report the volumetric efficiency

and the discharge temperature, respectively,

as a function of pressure ratio. Accordingly,the reader can appreciate the different

attitude of screw and of reciprocating

compressor when working under high

pressure ratios.

The screw compressor displays an

higher volumetric efficiency for all the

pressure ratios thanks to the more

favorable fluid-dynamics of the refrigerant

(lower drag), to the lower vapour heating

during the suction phase and, above all,

thanks to the missing penalization (that is onthe contrary remarkable in reciprocating

compressors) effect caused by the expansion

before the suction phase, of the refrigerant

vapor trapped in the dead volume.

Screw compressors show lower

discharge temperature thanks to

the vigorous cooling effect

promoted by the lubricant injection

in the compression chamber during the

refrigerant compression. This aspect is very

advantageous with ammonia: the high

discharge temperature for high pressure

ratios makes mandatory the use of two-

stage reciprocating compressors, whereas

single stage screw compressor are normally

use (Note: the choice of the designer ofinstalling two-stage screw compressors with

ammonia is usually based only on energetic

Figure 1: typical values of the discharge temperature for reciprocating and

screw compressors, as a function of pressure ratio.


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considerations, not to technologicalaspects, such as the high discharge

temperature).

Given the particular thermodynamic and

thermophysical properties of ammonia, a

dedicated paragraph can be found in the

present info-pack, after some general

(applicable both to synthetic refrigerants

and to ammonia) consideration on

energetic aspects of screw compressors.

Screw compressor energetic

efficiency

Screw compressor do not have discharge

or suction valves: this peculiarity improves

reliability and reduces pressure losses

during refrigerant flow inside the machine.

Furthermore, thanks to the valve-less

design, screw compressor can bear the

suction of small amount of liquid

refrigerant.

The main drawback of the screw

compressor is linked to thepresence of a discharge port:

when the compression chamber reaches

the discharge port, the discharge

pressure, for a given suction condition,

attains a set value that could not fit the

pressure level in the discharge manifold.

If this occurs, the indicator diagram area

(that, as well known, corresponds to the

compression work) increases, in

comparison with the ideal case of

pressure at the discharge port identical to

the discharge manifold pressure. As a

consequence, the nominal design

conditions for a screw compressor are

strongly dependent on the ratio vi of thevolume of the vapour at the beginning of

compression process and the volume of

the same amount of gas at the end of the

discharge process. In general it is

possible to choose between a limited

number of values (three or four) between

2.5 and 5.5 of the above mentioned

volumetric ratio vi. Accordingly, when the

screw compressor works in off-design

conditions it easily shows energetic

penalizations.

A common practice is the use of a part

loading slide: this device moves along the

screws vane and opens a by-pass lane

that permits a recirculation of part of the

vapour previously sucked that is sent

back to the inlet port, before the suction

line. As a consequence, it occurs a

reduction of vi, under the hypothesis of a

uniform shape of the discharge port. The

compressors producers try to optimize the

design of the discharge port by combining

an axial port at the end of the screws anda radial port that is extended to the side of

the screws. It is not the target of this info

pack to give detailed description of the

ports design procedure: what is crucial is

to highlight that the variation of vi (linked

to the particular design of the screw

compressor) affects the isoentropic

efficiency of the compressor, especially at

part load operation and with high pressure

ratios. This aspect is clearly depicted in

figure 3. For pressure ratios higher than

10, even if the cooling capacity is reduced

by some 70%, with respect to full load,

Figure 3: part loading efficiency of compressors, case of ammonia. For screw type, two

different pressure ratios (rp=9.4 and 3.8) and variable or fixed vi are considered.


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the compression power is reduced by less

than 40 %.

Furthermore, figures 4 and 5 show the

isoentropic and volumetric efficiency for

screw compressor with fixed and variable

vi (this latter technology is usually

achieved through a suitable design of the

radial port shape) at full load operation.

As expected, similarly to reciprocating

compressors, both the isoentropic and the

volumetric efficiency decrease with

increasing the pressure ratios.

Nevertheless, the use of screw two-stagecompressors is limited even for low

temperature freezing applications: on the

contrary, more and more widespread is

the use of economizers that permits

rather good system efficiency with

relatively low increase of investment

costs. It is worth noting that the use of the

economizer is effective only close to full

load operation (with the possibility of

excluding the economizer for operation

below 75% of the nominal cooling

capacity) as a consequence of the shifting

of the part loading slide, that reduces the

head pressure on the economizer port.

On the basis of the previousconsiderations, it appears that part

loading of screw compressors at high

Figure 4: typical value of isoentropic and volumetric efficiency of screw compressors, withvariable vi and for different constant values of vi, as a function of pressure ratio. Ammonia.

Figure 5: typical value of isoentropic and volumetric efficiency of screw compressors, with

variable vi and for different constant values of vi, as a function of pressure ratio. R404A.

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3.5

3.6

3.7

3.8

3.9

4

4.1

4.2

10 100 1000 10000

cooling capacity [kW]

COP[-]

screwcomp.rec.

Figure 6: comparing screw and reciprocating compressors at full load for large machines,

with ammonia. Ref. conditions -10/+35C.

pressure ratios (i.e. frozen cold stores)

could be a source of energetic

penalization. One must also consider that

in case of cold stores for frozen food are

usually characterized by relatively long

time constants, depending on the amount

of frozen foodstuffs loaded. As an

example one can consider the

temperature profile during compressors

off period considered in one of the auditscarried out during within ICE-E project.

Accordingly, the cold store refrigerating

system operator should consider the

possibility of installing more than one

compressor, storing the cooling effect

during low cost electric energy tariff time

and then running a limited number of

compressors during high energy cost

periods. In this way, it is possible to have

rather efficient operation of the store, with

screw compressor, without needing

compressor part loading.

AMMONIA COMPRESSORS

Ammonia compressors efficiency

analysis

In the literature it is hard to find papers

which report the efficiency of a single

compressor; more often the COP of the

whole cycle is provided. In figure 6

system COP evaluated for large capacity

ammonia compressors (according to

manufactures calorimetric data) is

reported.

In Cavallini et al. 2007, a survey on data

available in the open literature was

carried out. The results are reported in

figure 7. The first data set by Bowater

(2004) refers to experimental tests on a

reciprocating, two-stage compressor. It is

compared to data for some different

screw compressors: single-stage with

super-feeding, two-stage with super-

feeding and single-stage without super-feeding. There are not significant

differences between reciprocating and

screw compressors and between systems

with or without super-feeding;

considerable improvements can be

observed if a two-stage compressor is

used instead of a single-stage one.

Rivets data points (2004) refer to screw

compressors used in ammonia cascade

systems. Data of Korfitsen and Kristensen

(2005) is taken from experimental tests on

a reciprocating compressor; it is used

both in a refrigerant cycle and as a heat

pump. As expected, heat pump COP is

higher than refrigeration cycle one, but it

is calculated only in a narrow pressure

ratio range. All this data shows that the

COP decreases when pressure ratio

increases.

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Pressure ratio [-]

COP[-]

Bowater (2004)

Rivet (2004)

Korfitsen and Kristensen(2005) Refrigeration

Korfitsen and Kristensen(2005) heat pump

Figure 7: Ammonia compressor COP (from Cavallini et al. 2007)..

-Rivet P., 2004, Green solutions for

freezing applications, 6th

IIR G. LorentzenConf., 4b: 1-8.

-Korfitsen E., Kristensen A. P. R., 2005,

Ammonia high pressure heat pumps in

food refrigeration applications, Int. J.

Refrigeration, 21(3): 212-218.

-Bowater F. J., 2004, Selection of

compressors for low temperature

For more information, please contact: Claudio Zilio ([email protected])

ammonia refrigeration systems, 6th IIR G.

Lorentzen Conf., 4c: 1-12.-Cavallini A, Chiarello M, Del Col D, Zilio

C (2007). Compressors for natural

refrigerants: state-of-the-art review. In:

The 22nd International Congress of

Refrigeration. Beijing (PRC), August 21-

26, 2007, ISBN: 978-2-913149-59-5.

ice_e info pack 2 operation and choice of compressors

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