ASHRAE Journal - August 2011 - 37

liquid flow to individual evapoHigh Pressure Gas rators throughout the system. The hand-expansion valves are adjusted to achieve appropriate Evaporative Evaporative liquid overfeed rates as-required Condenser Condenser for given evaporator designs. King Leaving each unit will be a mixValve ture of saturated vapor produced High Pressure Liquid High by absorbing heat from the refrigPressure Receiver eration loads and saturated liquid that was overfed. The two-phase Dry Suction mixture is carried back to the same pumped recirculator vessel through the “recirculated liquid return” or Pumped Overfed “wet suction return.” The recirculaRecirculator Evaporator(s) tor vessel separates overfed liquid from vapor and liquid falls to the T bottom of the vessel to be pumped Compressor(s) back out to the evaporators while Centrifugal Liquid Refrigerant Pump the saturated vapor is directed to the compressors through the “dry suction” line. Liquid is made up to Figure 1: Mechanically pumped liquid overfeed system arrangement. the recirculator vessel from a high pressure part of the system to replace the liquid that is evaporated to meet the refrigeration loads; the recirculator vessel separates mass flow rate of vapor-phase refrigerant leaving the evaporathe flash gas formed as a result of this throttling process just as it tor in lb/min [kg/s]. separates the two-phase mixture returning from the evaporators. The total liquid refrigerant mass flow rate the pump needs to deliver is then:
Equalizer Line

How Much Liquid Refrigerant Needs to Circulate?

For industrial refrigeration systems using ammonia as the refrigerant, the flow rate of liquid refrigerant delivered by the pump to connected evaporators is relatively low due to the refrigerant’s high heat of vaporization. The total pump flow rate will depend on the minimum mass flow rate required to meet the aggregate capacity of connected evaporators and the recommended overfeed rate for the evaporators. The minimum mass flow rate of liquid required to meet the aggregate refrigeration load is given by:

  mrefrigerant , pumped = mrefrigerant , min ( OR + 1)

Re ci rc

 mrefrigerant, min =

Qload, total h fg

(1)

 where mrefrigerant,min is the minimum liquid refrigerant that must be supplied to all connected evaporators to meet their aggregate load in lb/min (kg/s), Qload,total is the aggregate refrigeration load in Btu/min (kW), and hfg is the enthalpy of vaporization for the refrigerant at the operating pressure of that suction level in Btu/lb (kJ/kg). The overfeed rate (OR) is the ratio of liquid mass flow rate to vapor mass flow rate leaving the evaporator:1
m  OR =  liquid m   vapor     wet return
(2)

 where mliquid is the mass flow rate of liquid-phase refrig erant leaving the evaporator in lb/min [kg/s] and mvapor is the
August 2011

 where mrefrigerant,pumped represents the mass flow rate of liquid refrigerant the pump must supply to the connected overfeed evaporators expressed in lb/min (kg/s). The term (OR+1) is commonly referred to as the circulating rate, Nr. The circulating rate represents the mass ratio of liquid pumped to the evaporators to the amount of vaporized liquid in the evaporators.1 Table 1 shows the required recirculating liquid flow rate expressed in gallons per minute of liquid flow for each ton of refrigeration load over a range of saturation temperatures and circulating rates (OR+1). What overfeed rate is required for an individual evaporator? Evaporator manufacturers typically publish recommended overfeed rates based on the unit’s specific design. Deviating significantly from the manufacturer’s recommended overfeed rate can lead to lower operating capacity of the evaporator. Too low of a refrigerant flow rate will starve the unit but excessive liquid supply will cause the evaporator to brine. Excessive supply of liquid refrigerant flow also increases the likelihood of pump cavitation. Finally, excessive overfeeding of liquid to evaporators increases the difficulty associated with returning the unused liquid to the recirculator when the return path involves a vertical riser. In this case, there is an increased tendency for liquid to accumulate or log up in evaporators.
ASHRAE Journal 37

ed Li qu Re id ci rc Re (P u tu um la rn te pe d (W d Li et Li qu qu id Su id S ct up io Li n) ne p ) ly

ul at

(3)



ASHRAE Journal - August 2011

Table of Contents for the Digital Edition of ASHRAE Journal - August 2011

ASHRAE Journal - August 2011
Contents
Commentary
Industry News
Letters
Meetings and Shows
Feature Articles
2011–12 Presidential Address: Sustaining ASHRAE Through Leadership
Retrocommissioning Older Buildings
Liquid Refrigerant Pumping in Industrial Refrigeration Systems
Technology Award Case Studies:
Hospital Upgrade: Heat Recovery, Geothermal Save Energy
Cool Weather Savings: Using Hybrid Refrigeration Systems for Chiller Retrofit
Special Section
InfoCenter
Standing Columns
Solar NZEB Project
Emerging Technologies
Special Products
IAQ Applications
Washington Report
People
Products
Classified Advertising
Advertisers Index
ASHRAE Journal - August 2011 - Intro
ASHRAE Journal - August 2011 - ASHRAE Journal - August 2011
ASHRAE Journal - August 2011 - Cover2
ASHRAE Journal - August 2011 - 1
ASHRAE Journal - August 2011 - 2
ASHRAE Journal - August 2011 - Contents
ASHRAE Journal - August 2011 - Commentary
ASHRAE Journal - August 2011 - 5
ASHRAE Journal - August 2011 - Industry News
ASHRAE Journal - August 2011 - 7
ASHRAE Journal - August 2011 - 8
ASHRAE Journal - August 2011 - 9
ASHRAE Journal - August 2011 - Letters
ASHRAE Journal - August 2011 - 11
ASHRAE Journal - August 2011 - 12
ASHRAE Journal - August 2011 - 13
ASHRAE Journal - August 2011 - 14
ASHRAE Journal - August 2011 - 15
ASHRAE Journal - August 2011 - Meetings and Shows
ASHRAE Journal - August 2011 - 17
ASHRAE Journal - August 2011 - 2011–12 Presidential Address: Sustaining ASHRAE Through Leadership
ASHRAE Journal - August 2011 - 19
ASHRAE Journal - August 2011 - 20
ASHRAE Journal - August 2011 - 21
ASHRAE Journal - August 2011 - 22
ASHRAE Journal - August 2011 - 23
ASHRAE Journal - August 2011 - 24
ASHRAE Journal - August 2011 - 25
ASHRAE Journal - August 2011 - Retrocommissioning Older Buildings
ASHRAE Journal - August 2011 - 27
ASHRAE Journal - August 2011 - 28
ASHRAE Journal - August 2011 - 29
ASHRAE Journal - August 2011 - 30
ASHRAE Journal - August 2011 - 31
ASHRAE Journal - August 2011 - 32
ASHRAE Journal - August 2011 - 33
ASHRAE Journal - August 2011 - 34
ASHRAE Journal - August 2011 - 35
ASHRAE Journal - August 2011 - Liquid Refrigerant Pumping in Industrial Refrigeration Systems
ASHRAE Journal - August 2011 - 37
ASHRAE Journal - August 2011 - 38
ASHRAE Journal - August 2011 - 39
ASHRAE Journal - August 2011 - 40
ASHRAE Journal - August 2011 - 41
ASHRAE Journal - August 2011 - 42
ASHRAE Journal - August 2011 - 43
ASHRAE Journal - August 2011 - Hospital Upgrade: Heat Recovery, Geothermal Save Energy
ASHRAE Journal - August 2011 - 45
ASHRAE Journal - August 2011 - 46
ASHRAE Journal - August 2011 - 47
ASHRAE Journal - August 2011 - Cool Weather Savings: Using Hybrid Refrigeration Systems for Chiller Retrofit
ASHRAE Journal - August 2011 - 49
ASHRAE Journal - August 2011 - 50
ASHRAE Journal - August 2011 - 51
ASHRAE Journal - August 2011 - InfoCenter
ASHRAE Journal - August 2011 - 53
ASHRAE Journal - August 2011 - 54
ASHRAE Journal - August 2011 - 55
ASHRAE Journal - August 2011 - 56
ASHRAE Journal - August 2011 - 57
ASHRAE Journal - August 2011 - 58
ASHRAE Journal - August 2011 - 59
ASHRAE Journal - August 2011 - 60
ASHRAE Journal - August 2011 - Solar NZEB Project
ASHRAE Journal - August 2011 - 62
ASHRAE Journal - August 2011 - 63
ASHRAE Journal - August 2011 - 64
ASHRAE Journal - August 2011 - 65
ASHRAE Journal - August 2011 - Emerging Technologies
ASHRAE Journal - August 2011 - 67
ASHRAE Journal - August 2011 - 68
ASHRAE Journal - August 2011 - 69
ASHRAE Journal - August 2011 - 70
ASHRAE Journal - August 2011 - 71
ASHRAE Journal - August 2011 - 72
ASHRAE Journal - August 2011 - 73
ASHRAE Journal - August 2011 - 74
ASHRAE Journal - August 2011 - Special Products
ASHRAE Journal - August 2011 - IAQ Applications
ASHRAE Journal - August 2011 - 77
ASHRAE Journal - August 2011 - 78
ASHRAE Journal - August 2011 - 79
ASHRAE Journal - August 2011 - Washington Report
ASHRAE Journal - August 2011 - 81
ASHRAE Journal - August 2011 - People
ASHRAE Journal - August 2011 - 83
ASHRAE Journal - August 2011 - Products
ASHRAE Journal - August 2011 - 85
ASHRAE Journal - August 2011 - Classified Advertising
ASHRAE Journal - August 2011 - 87
ASHRAE Journal - August 2011 - Advertisers Index
ASHRAE Journal - August 2011 - Cover3
ASHRAE Journal - August 2011 - Cover4
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