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McQuay International
Water cooledapplications with
Single ScrewTechnology andHFC 410A
-
PROXIMUS
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Contents
What is a refrigerant ................................................................... 3
What makes a good refrigerant .................................................. 5
Peculiarity for HFC 410A ............................................................ 8
Water cooled application with HFC 410A – Proximus ............... 12
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What is a refrigerant ?
Refrigerants are the working fluids in refrigeration, air-conditioning, and heat pumpingsystems. They absorb low-temperature heat energy from one source, for example an air-
conditioned space, and reject it into another, for example outdoor ambient, usually throughevaporation and condensation respectively.There are many substances classified as refrigerants by ASHRAE Standard 34, but only
few are used for commercial air conditioning.The most common refrigerants are the fluorinated hydrocarbons, but numerous othersubstances also function well as refrigerant, including many inorganic compound and
hydrocarbons.Following is quick a rundown of the common refrigerant groups that are either in use or
have been used in the past.
Water, R-718Most refrigeration processes use either an absorptioncycle or a vapor compression cycle. Commercialabsorption cycles typically use water as the refrigerant
with a salt solution such as Lithium Bromide as theabsorbent. Water is non toxic, non-flammable, abundantand of course it is a natural refrigerant.
Ammonia, R-717
Ammonia (NH3) is a natural refrigerant and considered to
be one of the most efficient. It is one of the only “original”refrigerants still in use today. It is used in vapor
compression processes, typically with positivedisplacement compressors. Ammonia is mostly used in industrial applications,although there are many large commercial air
conditioning installations that utilize it as well.
Hydrocarbons
Propane (R-290) and isobutane (R-600a), among other
hydrocarbons, can be used as refrigerants in the vaporcompression process. In Northern Europe, about 35% of
refrigerators are based on hydrocarbons. They can havelow toxicity and good efficiency but they are highlyflammable.
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Hydrochlorofluorocarbons (HCFCs)
Hydrochlorofluorocarbons (HCFCs) have been aroundalmost as long as CFCs. HCFC-22 is the most widely
used refrigerant in the world.They are used in vapor compression processes will alltypes of compressors.
These refrigerants are phased out, as required by theMontreal Protocol, in many countries. Others countries(extra EU) also have a phase-out schedule but on an
extended timeline.
Hydrofluorocarbons (HFCs)
Hydrofluorocarbons (HFCs) are relatively newrefrigerants whose prominence arose with the phase out
of CFCs. HFCs have no ozone depletion potential(ODP=0).They are used in vapour compression processes with all
types of compressors. The common HFCs are efficientand classified as A1 (lower toxicity – no flame
propagation) in the ASHRAE Standard 34.
Following is a table with some general information about these refrigerants.
RefrigerantNumber
Chemical NameChemicalFormula
SafetyGroup
ODP GWP
718 Water H2O A1 0 <1
717 Ammonia NH3 B2 0 0
290 Propane CH3CH2CH3 A3 0 0
600 Butane CH3CH2CH2CH322 Chlorodifluoromethane CHClF2 A1 0.055 1500
134a 1,1,1,2-tetrafluoromethane CF3CH2F A1 0 1300
407C 32/125/134a (23%/25%/52) A1 0 1530
410A 32/125 (50%/50%) A1 0 1730
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What makes a good refrigerant ?
At first glance properties such as lower toxicity, no flame propagation, efficiency, and costeffectiveness come to mind. Certainly these are important characteristics and ones that
are well advertised. But selecting a refrigerant for use in a refrigeration or air conditioningprocess goes far beyond just these properties. For example, “efficiency” can mean manydifferent things and can therefore cause misunderstanding and confusion.
Of all the properties of refrigerants, eff iciency is the most misunderstood and mostabused. To most people efficiency means “how much energy do I have to provide to get afixed amount of cooling”?
COP =
The more efficient the system, the less energy required to do the same amount of cooling.The issue is that eff ic iency is a sys tem pro perty not a refr igerant property .
Many parameters control how efficient a refrigeration system is, many of which have littleto do with refrigerants. The list includes motor efficiency, compressor efficiency (both at fulland part load), heat exchanger design and materials, operating conditions etc.
The refrigerant can affect the system efficiency in many ways including transfer properties(how easy is it to pump), heat transfer properties and so on.
Efficiency, however is a difficultparameter to comprehend. Carnotefficiency is the ultimate, but
doesn’t even take into account therefrigerant.Ideal cycles allow comparisons butagain, these cycles cannot beobtained in the real world.
Model l ing real world eff ic iencies
means taking into accou nt heat
transfer propert ies, transport
propert ies and refr igerat ion
sys tem des ign .
The main purpose of refrigerationis to move heat from where it is notwanted to where it is wanted (or atleast not a problem). Heat transfer
properties are key to this process.It is possible for a refrigerant to
have run of the mill theoreticalefficiency yet be an excellent performer because of its strong heat transfer performance.Good heat transfer allows close approaches in the heat exchangers. This in turn means
smaller lifts for the compressor and improved efficiency.Many factors affect heat transfer. Several have to do with the refrigeration system itself
such as piping design and material and flow rates (Reynolds numbers).
Useful refrigerating effectNet energy supplied from external
COMP
WORK
COOLINGEFFECT
O
T R
T O
3
1
ENTROPY, S
A B S O L U T E
T E M P E R A T U R E , T
Carnot Ideal Cicle on T,s diagram
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Three key properties of the refrigerant also affect the overall heat transfer capability of the
system. These are viscosity (µ), specific heat (Cp) and thermal conductivity (k).The factors are used to calculate Prandtl numbers (Pr= µ·cp/k) which are used in the
design of the heat exchangers. The goal is to work with substances that can carry a lot ofenergy (high specific heats) and can transfer the energy easily (high thermal conductivity).
It is also desirable to have low viscosity to enhance turbulence and reduce the workrequired to move the fluid .
With these important considerationtaken in consideration, let’s see aquick comparison between HFC
134a, that is, from athermodynamic point of view, one
of the most efficient refrigerantavailable on the market, and another refrigerant, HFC 410A that at
this time is known only in efficientsplit system application.
Fig 2 shows an ideal refrigerantcycle reported on ( T,s ) diagram fora water cooled application with
R410A. The cycle parameters areshowed in the following table:
Evaporating Condition 3 °C
Condensing Condition 40 °CSuperheat 5 °C
Subcooling 5 °C
DP evaporator 0,0 bar
DP condenser 0,0 bar
Suction piping lenght 0 m
Discharge piping lenght 0 m
η compressor 1
As you can appreciate by the values reported in table, the cycle shown is an idealrefrigerant cycle (no Entropy increase).Comparing the same cycles, with the same parameters, for HFC 134a, HFC 407C andHFC 410A we can found:
COPwith HFC 134a = 6,40
COPwith HFC 407C = 6,24
COPwith HFC 410A = 6,02
R - 410A
Fig. 2
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and looking at the “volumetric cooling capacity” parameter we found:
Volumetric Cooling Capacity with HFC 134a = 2429 kJ/m3
Volumetric Cooling Capacity with HFC 407C = 3629 kJ/m3
Volumetric Cooling Capacity with HFC 410A = 5599 kJ/m3
From the figures above we cam assert :
• HFC 401A, in an ideal refrigerant cycle for a water cooled application, has a COP
almost same of HFC 134a that is the best on the market from a thermodynamic
efficiency point of view ;
• Utilizing the same compressor size, with the same swept volume capacity, we can
improve the chiller cooling capacity by more than 200 percent.
This is one of the great advantage of R410A: the extremely high refrigerant effect perswept volume which can be up to 150% compared to R407C or up to 200% compared to
R134a.In this way smaller plant and components may be used, allowing more compact units withsmaller footprint be developed in comparison to HFC 134a.
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Peculiarity of HFC 410A
This hydro fluorocarbon refrigerant is a blend of HFC 32 and HFC 125, both theserefrigerants are at concentration of 50%.
Even if it is a blend, its temperature glide is negligible: only 0,2 K .For this reason the general behaviour of this refrigerant is almost identical to a pure one,as HFC134a for example.
No glide means much easier refrigerant re-charging in case of refrigerant loss, and noneed to take any particular attention during refrigerant charge.
As very well know, on a chiller working with a refrigerant blend presenting a “strong”temperature glide a leakage results in a change of components proportions in the mixture,because the relative losses of its most volatile components will be disproportionately high.
If a simple standard refilling is adopted, there is no guarantee that the new refrigerantmixture has the same proportions as it had before the leakage. For this reason to guarantee the unit performance, it is preferable to remove the remaining
charge, to make the vacuum in the circuit and re-charge it with new refrigerant (necessarilyin liquid state).
Of course only skilled personnel can perform this service activity.R-407C and R-410A are both zeotrope, however the glide for R-407C is 5 K while the glidefor R-410A is only 0,2 K.
Also the high glide for HFC 407C means that it can be practically used only in DX systemwhile HFC 410A can be also used in flooded system, without problems.
50% 50%
Glide temperature
0,2 KNO
GLIDE
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Another strength point of HFC 410A is that heat transfer propert ies [thermal
conductivity (k)] and transport propert ies [viscosity (µ)] are very goo d for th isrefrigerant.
These properties aren’t the refrigerant’sthermodynamic characteristics but are
very important to realize an efficientsystem, an efficient chiller from a “real”point of view.
All refrigerants, whether in vapor or liquidform, suffer pressure drop as they flow
through the refrigeration circuit: both in thepiping and in the components themselves(i.e. a compressor passageway). As thepressure changes, the temperature of therefrigerant changes as well, with aproportion which depends on the
refrigerant; there temperature variationsadversely affect system performance.
Consider the pressure drops in the suctionand discharge lines: starting from the actual pressure (and temperature) in the evaporator
and in the condenser, the pressure drops (and hence temperature changes) will have tobe added to the total lift that the compressor provides. The type of refrigerant used impactson the temperature change relative to the pressure change.
A quickly comparison between HFC134a liquid viscosity and HFC410A liquid viscosity canshow that the visc os ity fo r HFC410A is abou t 40% less than HFC134a visco sity ,consequently the pressure drops described above and their effect on chiller efficiency is
reduced.This advantage is especially very important for Dry-eXpansion evaporators, where therefrigerant flows as a mixture liquid-vapour (saturated) and the refrigerant pressure dropscan reach considerable values.
Another important physical parameter to consider for an efficient system is the heattransfer coefficient in both the evaporator and condenser. Higher is this parameter, higheris the potential for increased efficiency.
As sh own in the above f igures thermal con duct iv i ty for HFC410A, both for saturated
l iquid and saturated vapour, is much high er than HFC134a thermal con duc t iv i ty: for
satu rated v apo ur is 17,9% hig her , fo r sat ur ated l iq ui d i s 3,6% hi gh er (at 25 °C).
0,08
0,081
0,082
0,083
0,084
0,085
0,086
0,087
0,088
0,089
0,09
[ W / ( m K ) ]
R-410AR-134a
Thermal conductivity of saturated liquid (25°C)
+ 3,6%
0,0125
0,013
0,0135
0,014
0,0145
0,015
0,0155
0,016
0,0165
0,017
[ W / ( m K ) ]
R-410AR-134a
Thermal conductivity of saturated vapor (25°C)
+ 17,9%
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Good refrigerant thermal conductivity helps achieve good heat transfer and this allows
close approaches in the heat exchangers. This in turn means smaller lifts for thecompressor and improved efficiency.
Thanks to these good refrigerant characteristics McQuay is able to offer two efficiency line
for PROXIMUS, its water cooled unit equipped with single screw compressor:
• SE with a COP up to 4,3
• XE with a COP up to 5
COP of 5 means fast pay back time for the investment and big savings in the future chiller
operating costs.
The HFC410A advantages described and
the exceptional volumetric cooling capacityallows us to obtain very high performance
from each single screw compressor. As shown on Fig. 3, the R410A specificvolumetric cooling capacity is more than
200% greater then HFC134a. This meansthat with the same compressor swept
volume the unit can give a availablecooling capacity more then twice greaterthan that with a HFC134a.The main advantage of that is the
extremely compact dimensions of unitsand a highly competitive cost perrefrigeration ton.
Small footprints and reduced weight obviously facilitate installation and leave more space
for other equipment. A compact design also helps installation through most standard industrial doorwayswithout modification, saving introduction time and costs
.
0
1000
2000
3000
4000
5000
6000
k J / m 3
R134a R410A
Volumetric Cooling Capacity [kJ/m3] Fig. 3
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Higher working pressure.
We have seen that HFC410A have some advantages like good heat transfer properties,low viscosity, very high specific volumetric cooling capacity; the only disadvantage is that
operating pressures are higher.The internal mechanical loads in the compressors are obviously proportional to the
pressure, and therefore compressors presenting relevant axial and radial thrusts arepenalized in the use of this kind of refrigerant, and require redesign of some components(such as bearings).
McQuay Single Screw technology presents perfectly balanced internal loads, so to workwith higher pressures does not pose any problems.
Whilst it should be noted that the maximum pressure inside the refrigerant circuits is higher
compared to R134a and R407C, it is also important to understand that this could be a limitonly in air-cooled units. On water cooled units this problem basically don’t exist, becausewater cooled condensers can keep the condensing temperature considerably lower
compared to air cooled condenser working, for example, with R407C.Let’s have a look to the following table:
As you can appreciate the highest pressure levels (condensing section) are almost thesame. This means that work with HFC 410A in water cooled applications, from amechanical point of view, is equivalent to work with HFC 407C in air cooled applications.
Is important take in consideration that also ∆P between suction and discharge compressor
side is almost the same for two refrigerants and it allows the compressor to work with a
volumetric efficiency that is the same, both in the case of R407C and of R410A. This isbecause the internal leakage of the compressor, between discharge and suction side, areequal for both refrigerants.
McQuay has also obtained the full PED (Pressure Equipment Directive) certification for
R410A heat exchangers (both evaporator and condenser); PED certification, establishedby the European Community in May 2002, is one of the most restrictive standards onpressure equipment and requires high safety margins on pressure calculations and tests.
All McQuay heat exchangers are PED approved, another warranty for safety, reliability andefficient chiller operation.
with HFC 410A LCWT [°C] Tcond [°C] Pcond [bar]
Proximus SE 35 40 24,4
Proximus XE 35 38 23,2
with HFC 407C OAT [°C] Tcond [°C] Pcond [bar]
McPower SE ST 35 55 24,7
McPower SE XXN 35 65 28,3
LCWT = Leaving Condenser Water Temperature
OAT = Outside Ambient Temperature
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Water cooled application with HFC 410A - Proximus
Proximus is the new water cooled chiller equipped
with McQuay Single Screw compressor andworking with HFC 410A.
The evaporator is a Dry-eXpansion type with adiffuser at inlet to allow a perfect refrigerant fluiddistribution to all tubes inside the evaporator schell.With Proximus McQuay continues using a DX,single pass, counter flow evaporator that
guarantees an excellent heat transfer propertiesand a perfect oil circulation.
We have do not forget that in a single pass evaporator there are any vertical tube part that
can cause, during partial cooling loads and so with small refrigerant velocity inside tubes,an little oil stagnation inside the evaporator.
In other words with a DX-double-passevaporator the oil circulations can be not sogood during partial loads and so is not
guarantees a perfect oil return to thecompressor.
Proximus is equipped with an electronic expansion device, the
only device that can match the infinitely variable cooling load
compressor system and the PID controller to achieve a perfectleaving evaporator water temperature control.
The set-point for the chilled water that the user sets on the controlpanel will be always respected, in all operating conditions within +
0,1°C.Electronic EXpansion Valve (EEXV) is a device that gives thepossibility to reach, if the chiller is “well managed”, a bigeconomical saving in the operating unit cost. As you know, in these years the energy saving problems are evenmore actual, important for all the communities both in theEuropean area and outside it.
All the system that allow a energy saving are promoted and often there are publiceconomical helps to push the adoptionof these efficient plant solution.In a water cooled chiller is possible toreach very high EER values when agood condensation’s control is
adopted, in other words, when theminimum Entering Condenser Water
Temperature (ECWT) allowed by unitsis reached.
Double-pass evaporator
3
4
5
6
7
8
25 30 35 40 45 50
Condensation Temperature [°C]
E E R
Evaporator conditions: IN 12°C – OUT 7°C
Fig. 4
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This is possible of course when spring, river, lake water is available to cool the condenser
or when the cooling tower fan control is implemented to supplies the chiller’s condenserwith the coolest water.
Take in consideration that during all the year the Outside Ambient Temperature (OAT) iswell below 20°C for the 65% of the time (this is true in the South-Europe, in the North the
percentage can increases). With this temperature is possible supply to unit an ECWTbelow 30 °C with a relevant EER increases. (see Fig. 4)In a chiller supplied with Thermostatic Expansion Valve (TEXV) is not possible to reach
these energetic benefits because the system is obligated to operate with highercondensation pressure to ensure correct working condition to the lamination device.
Proximus, for sizes with acooling capacity more than1000 kW, is equipped with two
Single Screw McQuay Frame 4compressor that working in two
“real” independent circuits: twocompressors, two refrigerantspiping lines, two electronic
expansion valves (EEXV), twoexternal oil separator.
This means a more reliableunit because in case oftechnical problems for a
components, you can supply HVAC plant with more then 50% of cooling capacity (morethan 500kW) and so can satisfy the major necessities.In a chiller equipped with only one compressor and with a very big cooling capacity (morethan 1000 kW) the part load efficiency is made bad by the strongly unloaded condition forthe electric motor that works with low cosϕ values.
If this chiller is equipped with a thermostatic expansion device, or with a fixed orifice, alsothe refrigerant flow control became no so good because of the unstable behaviour of this
lamination device at strongly unloaded conditions. That can give problems in EvaporatorLeaving Water Temperature (ELWT) control.
In a chiller equipped with two “real” independent compressors, at part load conditions, youcan have:
• a compressor at nominal working condition, with the nominal cosϕ value ;
• more than 50% of the total unit cooling capacity ;
• always an optimum EWLT control thanks the electronic expansion valve that worksat nominal condition too.
McQuay International is the first manufacturer in “European HVAC Market” to offer a water
cooled chiller working with HFC 410A: an ecological, efficient, safe, easy to managerefrigerant.Thanks this HFC is possible to make, and so to offer, a unit with the best compromise
among: efficiency, compactness, reliability and price.
Proximus is the best choice both to make a new HVAC system and to replace an oldexisting refrigerator.
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We reserve the right to make changes in design and construction at any time without notice, thus the cover picture is not binding.
McQuay Italia S.P.A.S.S. Nettunense, km 12+300 – 00040 Cecchina (Roma) Italia – Tel. (06) 937311 – Fax (06) 9374014 – E-mail: [email protected]