4. Refrigeration equipment selection Refrigeration equipment selection is done based on operating conditions of the system and a heat load calculation conducted for the environment we are aiming to cool and dehumidify. Equipment selection means relating product rating and specifications with cooling load and prevailing working conditions. Inputs or information from particular applications (cooling load and operating conditions) together with product performance data and specifications from manufacturers' literature will result in an informed selection decision. Different manufacturers' may use different rating and specification parameters; therefore, all the possible rating and specification parameters and their relation with refrigeration load and working condition parameters are to be seen. Operating conditions & Cooling load To design or select a system the heat load and operating conditions of the system must be known. The operating conditions of the system are determined from the application and working environment of the system. Operating condition parameters Operating (working) condition parameters are determined from application and prevailing environment of the storage. These are: Room air dry bulb temperature and relative humidity or wet bulb temperature Ambient air dry bulb temperature and relative humidity or wet bulb temperature Atmospheric pressure or altitude of the site Power supply Cooling load (refrigeration load) Refrigeration load is the sum of all heat load elements, per day, of the space to be refrigerated. This load is to be extracted by the refrigerating equipment during run time of the compressor – operation time of compressor per day. Here under, two ways of equipment selection are to be covered. These are: 1) Packaged equipment selection Packaged equipment selection means simply selecting factory assembled equipment which can yield the required refrigerating effect economically with in the prevailing working conditions. 2) Component based selection Component based selection means selecting components of refrigeration system based on the prevailing cooling load and working conditions and then system balance is to be conducted. In this paper selection of the main components is to be seen i.e.: • • • •
Evaporator selection Compressor selection Condenser selection Expansion valve selection
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4.1. Packaged refrigeration equipment selection Possible packaged unit performance data and specification parameters are: -
Nominal electric power Refrigerant Nominal absorption Rated voltage Cooling capacity Defrost type Discharge pressure (or SDT)
-
Suction pressure (or SST) Sub cooling Superheat Condenser air flow Evaporator air flow Air throw mass flow rate
Possible load and operating condition parameters are: Power supply Power supply is the available power input (Volt/Phase/Hz) Required condition of room air - determined from its application, these are: Room air dry bulb temperature (DB) Room air relative humidity (RH) or wet bulb temperature (WB) Conditions of ambient air – determined from site metrological data. These are: Ambient air temperature (DB) Ambient air relative humidity (RH) or WB Atmospheric pressure or altitude of the place Cooling load Heat load per day of the space to be refrigerated, see cooling load calculation above. To select packaged equipment all are some of the following parameters are important. These are: 1) Refrigerant Refrigerant selection depends on many factors, but the most factors which influence refrigerant selection are heat of vaporization of the refrigerant, ozone depilating potential and global warming effect. Currently, for commercial refrigeration the choice of the refrigerants is as follows: R404A and R134A the preference is according to the order. 2) Cooling capacity Cooling capacity is heat extraction capacity of the evaporator (cooling unit) per day based on running time of the compressor. It depends on saturated suction (evaporation) temperature (SST) which can be calculated by: SST = Room Temperature – TD
Where TD is temperature difference which depends on RH, see table 1 2
3) Run time and defrost operations Run time - is the maximum time that the compressor is in operation per 24 hours Defrosting operation – is a method of removing the frost formed on the evaporator coils Frost formation on the evaporator coils depends on saturated suction temperature (SST) which implies SST determines defrost operation needed and consequently run time of the compressor, which influences the required cooling capacity of the evaporator. When the design suction temperature is over 30°F, a defrost cycle is not normally required, and it is common practice to select equipment on a 20 to 22 hour compressor operation. a. Air defrost For suction temperatures below 30°F and room temperatures over 35°F, off-cycle (air defrost) can generally be used. This involves cycling the compressor off with a time clock while the evaporator fans remain in operation and room air melts the ice on the coil. For every two hours of compressor operation, one hour of air defrost time is needed. Therefore, compressor selection is based on 16 hours per day. For suction temperatures below 30°F and rooms below 35°F, electric defrost, hot gas defrost or water defrost is required. With these positive methods of defrost, equipment selection can be based on longer compressor operation, with 18 to 20 hours most common. However, this depends on the type of equipment used and the latent load in the storage. A modern unit cooler or product cooler in a tight room with average latent load can be selected on 20 hour operation. The type of defrost used is generally a matter of either contractor or owner preference. Different geographic regions tend to use one particular type of defrost more frequently. As a rule, electric defrost is more common than hot gas, and hot gas more common than water defrost. b. Electric Defrost Electric defrost is the most common method in use today. Equipment cost is about the same as with hot gas but installed cost can be lower. Operating cost is about 15% higher with electric defrost than with hot gas and a fair amount of heat and moisture is released in the room during defrost. c. Hot Gas Defrost Hot gas defrosting is still the most efficient method of defrosting regardless of storage temperature but, unfortunately, most contractors are reluctant to use it. Defrost is very quick with minimum room temperature rise. Hot gas defrost, however, requires care to ensure that the compressor is protected against liquid slugging. d. Water Defrost While not very common, water defrost can be used on both medium and low temperature storages. Water must be at least 50°F and is sprayed on the coil at a rate of about 3gpm/square foot of coil for five to 15 minutes, depending on severity of frosting. Water defrost is fast and efficient but some moisture is re-released into the room. These systems also require more maintenance than electric or hot gas systems.
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e. General Defrost Considerations Because of high suction pressure (and high load) after defrost, compressor selection must be checked to see that it can operate in a higher range than the actual design point. If not, a crankcase pressure regulator may be required to keep suction pressure down to acceptable values. If this is the case, an accumulator should also be used. This is very important for a blast freezer. On large air defrost systems (gravity coil, for example) it is a good idea to have solenoids in the liquid and suction lines so refrigerant will not migrate during defrost. In addition, large fin coil installations are often split into sections with a thermostat for each section to compensate for uneven room loading. It is also recommended that a pump down system be used for both off- cycle and all defrost periods. f. Frost and dust (fin spacing) reduction factor (table 2) While high fin density gives increased coil capacity, it also increases the problem of dirt and frost collection, which results reduction in cooling capacity. Cooling capacity formula Required cooling capacity =
Cooling Load per 24 hours Run Time F
4) Evaporator fan air quantity (or Evaporator coil face velocity) In air cooled direct expansion (DX) systems refrigeration is achieved by passing air over an evaporator coil surface which is directly cooled by an evaporating refrigerant flowing inside a tube, or tubes, over which the air passes. Therefore; the quantity of air per KW of cooling capacity that has to pass over the evaporator coil surface to achieve the required cooling can be approximated by: . R V = KW P C 1 Texit p Tin Where . V KW R P Cp Tin
Texit
is evaporator air quantity (m3/s) per KW of cooling capacity is gas constant, for air = 277.7kJ/kg is pressure of entering air (atmospheric pressure) in KPa is average specific heat capacity of air = 1.005kJ/kg. K is absolute temperature of entering air is absolute temperature of leaving air
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5) Evaporator coil face velocity Evaporator coil face velocity ( v ) can be calculated by calculated by: .
V v= A Where A is evaporator coil face area = Fin height x Finned length 6) Suction pressure ( Ps ) Suction pressure of compressor is given by: Ps = Psat - PD
Where Psat PD
is saturation pressure @ SST is evaporator pressure drop
7) Discharge pressure ( Pg ) Discharge pressure is the pressure at the exit of the compressor. Discharge pressure for a given refrigerant is the saturation pressure corresponding to condensing temperature of the refrigerant. The condenser is cooled by the ambient air; therefore, condensing temperature depends on the ambient air temperature. Entering temperature difference (ETD) of Condensers ETD is the temperature difference of entering (ambient) air dry bulb temperature and condensing temperature. ETD for air condensers is in the range ( ). Condensing temperature ( Tc ) = ETD + Tatm
From refrigerant properties table of the selected refrigerant: Discharge pressure = saturation pressure @ Tc
8) Power consumption Power consumption is the power to be absorbed by the system to extract the cooling load. The power consumption of the system is calculated as follows: Power transferred to the refrigerant ( Win ) can be calculated by: Win =
Cooling Capcity COP
Where COP is coefficient of performance of a system with sub cooling & superheating 5
i.
Power in put at the compressor shaft ( W shft ) can be calculated by: W shft =
Where
com
com is compressor efficiency
ii.
Required electric power supply (W) will be: W=
Where
Win
Wshft
mec em
mec is mechanical efficiency em is electric motor efficiency
9) Condenser heat rejection (THR) and condenser fan air quantity The heat load absorbed at the evaporator and heat of compression has to be rejected at the condenser. To absorb this heat sufficient air should pass over the air cooled condenser coil. a. Total heat rejection (THR) at a condenser can be calculated by: THR = Q + Win Where Q Win
is cooling capacity is power transferred to the refrigerant
b. Condenser fan air quantity (m3/s) per KW of heat rejection can be calculated by: . R V = KW P C 1 Texit p Tin
6
Where . V KW R P Cp Tin
Texit
is condenser air quantity (m3/s) per KW of heat rejection is gas constant, for air = 277.7kJ/kg is pressure of entering air in KPa is specific heat capacity of air is absolute temperature of entering air is absolute temperature of leaving air
10) Condenser coil face velocity Condenser coil face velocity ( v ) can be calculated by: .
V v= A Where A is condenser coil face area = Fin height x Finned length .
11) Refrigerant mass flow rate ( m )
.
m=
Cooling Capacity h1 h4
Where
h1 = h @ Ps and T s = SST + Superheat h 4 = h f @ T L = Tc - sub cool Where Superheat and liquid temperature (sub cool) are to be taken from product rating 12) Required Air throw Air throw can be estimated roughly as follows: Air throw should be, at least maximum of (width, height or length of the storage room)
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4.2. Packaged unit selection form I.
Inputs
i)
Cooling load & Operating Conditions -
-
-
ii) II.
Available power supply (Volt/Phase/Hz): _____/____/____ Room air: DB _____ o C or _____ o F WB _____ o C or _____ o F , or RH _____ % Ambient air: DB _____ o C or _____ o F WB _____ o C or _____ o F , or RH _____ % Site condition: Atmospheric pressure ( Patm ) ______Pa, or Altitude above sea level (H) ______ m Cooling load per 24hrs: ___________KW = __________tons = ________BTU/hr Product specification (manufacturers catalog)
Liquid temperature: ______ o C Superheat: ______ o C Temperature difference (TD) ______ o C Coil face area: ______ m2, or Fin height: ______m & Finned length: ______m Leaving air: DB ____ o C & WB _____ o C Leaving air velocity: ____m/s or _____CFM, or Air throw: _____m Fixing and relating the parameters
1. Refrigerant selection - Order of preference: R404A, R134a, according to heat of vaporization requirement and evaporation temperature 2. Cooling capacity calculation 2.1 Fix SST ( saturated suction temperature) SST = Room Air Temperature (DB) – TD TD – use table1 or product catalog
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2.2 Fix defrost method For SST > 30 o F , normally no need of defrost cycle For SST < -1 o C or 30 o F and room temperatures > 2 o C or 35 o F : • Air defrost – off cycle defrost For SST < 30 o F and room temperatures < 35 o F : a positive defrost method • Order of preference: Electrical, Hot gas and Water defrost respectively 2.3 Fix run time (compressor operation time per day) • For no defrost cycle – 20 to 22 hrs • For off cycle (air) defrosting - 16 hrs • For a positive defrost method - 18 to 20 hrs 2.4 Fix fin spacing • 8fins/inch- down to 32 o F • 6fins/inch- hold freezers • 4fins/inch- blast freezers Cooling Capacity formula Cooling Load per 24 hours Run Time F
Required cooling capacity = Where
F is fin spacing reduction factor (see table 2) 3. Evaporator air quantity per KW of Cooling Capacity . R V KW = P C 1 Texit p Tin
Where P = Patm in KPa C p = average specific heat capacity of air Texit = Leaving air temperature (DB) Tin = ambient air temperature (DB) 3.1 Evaporator coil face entering air velocity .
V v= A Where Coil face area (A) is to be taken from product specification 4. Required Air throw (X) X should be, at least maximum of (width, height or length of the storage room)
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5. Suction pressure Suction pressure of compressor is given by: Ps = Psat - PD
Where Psat PD
is saturation pressure @ SST is evaporator refrigerant pressure drop
6. Discharge pressure From refrigerant properties table of the selected refrigerant: Discharge pressure = Saturation pressure @ Tc Where Condensing temperature ( Tc ) = ETD + Tatm ETD = () or specified by the manufacturer
.
7. Refrigerant flow rate ( m ) .
m=
Cooling Capacity h1 h4
Where
h1 = h @ Ps and T s = SST + Superheat h 4 = h f @ T L = Tc - sub cooling Where Superheat and sub cooling temperatures are to be taken from product rating 8. Compressor power absorption ( W shft ) and Electric power supply (W) Power input at the compressor shaft ( W shft ) W shft =
Win
com
Where Cooling Capcity COP COP is coefficient of performance of a system with sub cooling & superheating given by: Win
=
COP = (1 Y1 Y3 Y4 )COPb
10
Where
COPb is coefficient of performance of the system with out sub cooling and super heating Y1 is percentage influence of sub cooling on COP Y3 is percentage influence of internal super heating on COP Y 4 is percentage influence of external super heating on COP (See refrigerant cycle data table)
Required electric power supply (W) will be: W= Where
Wshft
mec em
mec is mechanical efficiency em is electric motor efficiency
9. Selection Knowing the above parameters, from manufacturers catalog packaged unit which best full fills the parameters is to be selected.
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4.3. Evaporator selection To absorb the total heat loads at the given operating conditions the system must: Have refrigerant in the evaporator at a sufficiently low Temperature to enable it to absorb heat from the storage room air and Have a sufficient quantity of refrigerant flowing through the evaporator. Therefore let's work from the evaporator to determine system capacities.
The evaporator is the basis for capacity calculation - it is the component directly responsible for absorbing heat energy from the storage.
The main factors which influence evaporator selection are: • Room size, shape, orientation and application • System temperature difference (TD) • Refrigerant type • Saturated suction temperature (SST) - Evaporating temperature (Te) • Air velocity (coil face velocity), evaporator air quantity or evaporator fan capacity • Leaving air velocity (air throw) Specification and rating parameters of DX coil evaporators - Refrigerant - Refrigerant Flow - Evaporating Temperature (SST) - Refrigerant PD - System temperature difference (TD) - Leaving Air DB/WB - Fin spacing - Maximum Air Pressure Drop - Capacity - Leaving Vapor Velocity - Coil face area (Fin Height & Fined Length) - Coil (Tub) Type - Evaporator fan air quantity - No of Rows - Super heat - No of Circuits - Liquid temperature ( T L ) - Fin material Relation of specification and rating parameters with cooling load and operating conditions Relation of cooling load and operating conditions with specification and rating parameters are shown above in packaged unit selection. Accordingly, DX fin and tube evaporator is to be selected.
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Evaporator Selection Form
I.
Inputs
i)
-
Cooling load & Operating Conditions Room air: DB _____ o C or _____ o F WB _____ o C or _____ o F , or RH _____ % Ambient air: DB _____ o C or _____ o F WB _____ o C or _____ o F , or RH _____ % Atmospheric pressure ( Patm ) ______Pa, or Altitude above sea level (H) ______ m
-
Cooling load per 24hrs: ___________KW = __________tons
-
ii) -
II.
Product specification (manufacturers catalog) Liquid temperature: ______ o C Superheat: ______ o C Temperature difference (TD) ______ o C Coil face area: ______ m2, or Fin height: ______m & Finned length: ______m Leaving air: DB ____ o C & WB _____ o C Leaving air velocity: ____m/s or _____CFM, or Air throw: _____m
Fixing and relating the parameters
1. Refrigerant selection - Order of preference: R404A, R134a, according to heat of vaporization requirement and evaporation temperature 2. Cooling capacity calculation a. Fix SST ( saturated suction temperature) SST = Room Air Temperature (DB) – TD TD – use table1 or product catalog 13
b. Fix defrost method For SST > 30 o F , normally no need of defrost cycle For SST < -1 o C or 30 o F and room temperatures > 2 o C or 35 o F : • Air defrost – off cycle defrost For SST < 30 o F and room temperatures < 35 o F : a positive defrost method • Order of preference: Electrical, Hot gas and Water defrost respectively c. Fix run time (compressor operation time per day) • For no defrost cycle – 20 to 22 hrs • For off cycle (air) defrosting - 16 hrs • For a positive defrost method - 18 to 20 hrs d. Fix fin spacing • 8fins/inch- down to 32 o F • 6fins/inch- hold freezers • 4fins/inch- blast freezers Cooling Capacity formula Cooling Load per 24 hours Required cooling capacity = Run Time F Where F is fin spacing reduction factor (see table 2) 3. Evaporator air quantity per KW of Cooling Capacity . R V KW = P C 1 Texit p Tin
Where P = atmospheric pressure ( Patm ) in KPa C p = average specific heat capacity of air Texit = Leaving air temperature (DB) Tin = ambient air temperature (DB)
Evaporator coil face entering air velocity .
V v= A Where Coil face area (A) is to be taken from product specification 4. Required Air throw (X) X should be, at least maximum of (width, height or length of the storage room) 14
.
5. Refrigerant flow rate ( m ) .
m=
Cooling Capacity h1 h4
Where
h1 = h @ Ps and T s = SST + Superheat h4 = h f @ TL Where Superheat and liquid temperature are to be taken from product rating
6. Selection Knowing the above parameters, from manufacturers catalog DX fin and tube evaporator which best full fills the parameters is to be selected.
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4.4.Compressor selection The compressor has 2 functions in respect to the circulation of refrigerant. It must: Discharge refrigerant into the condenser against the head pressure (high side pressure) (determined in principle by the ambient temperature) and Pull refrigerant through the evaporator (via the suction) to provide a nominated saturation temperature / pressure at an adequate flow rate (i.e. to pull the refrigerant temperature down) The flow rate varies depending on the conditions in which the compressor is operating. These conditions are often specified in the compressor ratings or performance graphs, which must be referred to in the selection process of the compressor. The following parameters are important for compressor selection: i) Compressor capacity Since the capacity and power requirement of a compressor and so of the system, vary with changes in evaporating and condensing temperatures (pressures), liquid sub cooling and super heating of the refrigerant, these conditions must be specified on selecting a compressor for an application. ii) Operating conditions of the compressor The maximum operating conditions of the compressor (suction & discharge pressures) can be determined from the ambient temperature and required evaporation temperature as follows: P iii) Suction pressure ( s ) Suction pressure is the pressure at the inlet of the compressor. Suction pressure of compressor is given by: Ps = Psat - PD
Where Psat PD
is saturation pressure @ SST is evaporator pressure drop
If PD is not known, then: When selecting compressor, use Suction Temperature 5°F below evaporating temperature to allow for suction line Pressure Drop and 10°F when suction accumulator is not used.
iv) Discharge pressure ( Pg ) Discharge pressure is the pressure at the exit of the compressor. Discharge pressure for a given refrigerant is the pressure corresponding to condensing temperature of the refrigerant. From refrigerant properties table of the selected refrigerant: Discharge pressure = Saturation pressure @ Tc 16
Where
Condensing temperature ( Tc ) = ETD + Tatm ETD is the temperature difference of entering (ambient) air dry bulb temperature and condensing temperature. ETD for air condensers is in the range ( ). .
v) Compressor mass flow rate ( m comp ) Mass flow rate of compressor is given by: .
.
m comp = suc V act
Where .
m comp is compressor mass flow rate suc is density of suction vapor .
V act is actual compressor volume flow rate
Density of suction vapor is given by:
suc =
1 v suc
Where v suc
is specific volume of suction vapor which can be read from refrigerant properties table of the selected refrigerant at suction pressure and temperature i.e.
v suc = v @ suction pressure ( Ps ) and suction temperature ( T s ) = SST + Superheat
Actual volume flow rate of compressor is given by: .
.
V act = v V
Where
v is volumetric efficiency of compressor .
V
is theoretical volume of compressor
Theoretical volume of compressor is given by: D 2 L V= N 4 .
Where N D
is number of cylinders is cylinder bore diameter 17
L
is stroke length
Compressor capacity ( Q comp ) can be calculated by: .
Q comp = m comp ( h1 - h 4 )
Where
h1 = h @ Ps and T s = SST + Superheat h 4 = h f @ T L = Tc - sub cooling vi) Power consumption (BHP/ton) Brake horse power, power required at the compressor shaft, per ton of compressor capacity is given by: 4.72 Bhp = Ton COP Where COP is system coefficient of performance, see above vii)
Required electric power supply (W)
Required electric power supply (W) can be calculated by: W=
Where
Wshft
mec em
mec is mechanical efficiency em is electric motor efficiency
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4.5.Forced convection type air cooled condenser In commercial refrigeration forced circulation type air cooled condensers are used. An air-cooled condenser is required to reject the heat load as well as the power drawn by the compressor. The air-cooled condenser, installed outside, rejects the heat to outdoor ambient air. The selected air-cooled condenser must meet the capacity requirements at the maximum outdoor ambient air temperature of the region where the air-cooled condenser is installed. Condenser Selection Factors There are factors that are required to enable the selection of a suitable condenser. These are: - Application (low, medium, high temperature), approximate evaporating temperatures are: • High +30°F to +50°F • Medium -10°F to +30°F • Low 40°F to -10°F - Total heat rejection - Condenser fan air quantity Selection Procedure 1. Maximum ambient operating temperature should be slightly greater or equal to ( Tatm ) 2. Total heat rejection (THR) Medium temperature refrigeration applications - use the following formula: THR = Compressor capacity x THR factor x 1.05 Low temperature applications - use the following formula: THR= Compressor capacity x THR factor x 1.1
3. Condenser fan air quantity Condenser fan air quantity (m3/s) per KW of heat rejection can be calculated by: . R V = KW P C 1 Texit p Tin Where . V is condenser air quantity (m3/s) per KW of heat rejection KW R is gas constant, for air = 277.7kJ/kg P is pressure of entering air in KPa C p is specific heat capacity of air Tin
Texit
is absolute temperature of entering air is absolute temperature of leaving air
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Condenser coil face velocity ( v ) can be calculated by: .
V v= A Where A is condenser coil face area = Fin height x Finned length 4.6.Expansion valve The function of an expansion device in a refrigerating system is first to maintain the pressure differential between the low pressure side (evaporator) and the high pressure side (condenser) for a compressor driven refrigerating process. The second purpose is to regulate the refrigerant flow to match the heat flux in the heat exchangers. If the heat flux in the evaporator increases, the mass flow through the evaporator should be increased accordingly. Expansion devices can be divided into eight basic types: 1. Hand expansion valve 5. Electronic expansion valve 2. Capillary tube 6. Low pressure float valve 3. Automatic expansion valve 7. High pressure float valve 4. Thermostatic expansion valve (TEV) 8. Constant level regulator The first two are non-regulating expansion devices, while the other types adjust the flow based on different means of signals. In commercial refrigeration capillary tube and TEV are commonly used. Selection Using inputs from the selected evaporator, compressor and condenser suitable expansion valve from manufacturer's catalog is to be selected. The inputs are: - Refrigerant type - Compressor mass flow rate or capacity - Pressure drop ( P ) 4.7. System balance
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