ASHRAE Journal - August 2011 - 38

Anatomy of a Centrifugal Pump

At a fundamental level, the centrifuLiquid Saturation Temperature (°F) Circulating gal pump used for circulating refrigerRate –60 –40 –20 0 20 40 ants is similar to the centrifugal pump 2:1 0.1117 0.1164 0.1216 0.1274 0.1340 0.1415 used to move water or other secondary fluids. Figure 2 shows a single-stage 3:1 0.1676 0.1746 0.1824 0.1911 0.2009 0.2122 centrifugal pump common for moving 4:1 0.2235 0.2328 0.2431 0.2548 0.2679 0.2829 liquids. The main feature of a centrifu5:1 0.2793 0.2910 0.3039 0.3185 0.3349 0.3537 gal pump is the impeller, which rotates 3/h per kW . T within the pump casing creating a low Multiply by 0.0646 to convert table values to m pressure zone near its center (the eye). Table 1: Recirculated gallons per minute per ton of refrigeration. This area of low pressure draws liquid into the pump where the rotating impeller increases the kinetic energy of the fluid by accelerating the liquid outward radially to the impeller tips. As the liquid leaves the impeller tips, its kinetic energy is at a maximum. The pump housing or volute surrounding the impeller then takes over to orderly collect the liquid leaving the impeller. The process of “gathering” liquid in the volute converts the kinetic energy of the fluid to pressure (potential) energy. The higher pressure fluid then leaves Impeller Eye the pump through the discharge line. Because a refrigerant pump is moving a volatile fluid, it is highly susceptible to cavitation during operation (see What is Cavitation? sidebar). To reduce the likelihood of cavitation, Suction liquid refrigerant pumps include design details that differ from ordinary water or secondary fluid pumps to decrease the pressure loss through the pump suction.

Recirculated Liquid Flow Rate (gpm/ton)

Liquid Refrigerant Pumps
Before discussing the operating details of centrifugal refrigerant pumps, it is important to consider a few concepts fundamental to their successful operation. One of the most important concepts is net positive suction head (NPSH). Quite simply, “suction head” represents the pressure at the pump’s suction. The term “net positive” is intended to account for the balance of positive pressures (static, i.e., height, and absolute pressure above the vapor pressure of the fluid) and negative pressures (losses) attributable to fluid flow. Effectively, NPSH is the difference in pressure of refrigerant at the pump suction and the refrigerant’s saturation pressure. An NPSH of 0 for a pump attempting to move a volatile liquid refrigerant indicates that the liquid will flash to a vapor state as it moves into the pump. Two types of NPSH that need to be considered to ensure proper pump operation: net positive suction head required or NPSHr and net positive suction head available or NPSHa. NPSHr is the minimum net positive suction head required to prevent the liquid refrigerant from flashing to a vapor. It is a characteristic of each given pump and varies with the pump’s operating point (head and flow) as provided by the pump manufacturer. NPSHa is the available net head at the pump suction accounting for those factors that effectively increase head (static head to the liquid elevation ahead of the pump suction, subcooling of liquid refrigerant within the vessel) and decrease
38 ASHRAE Journal

Impeller Volute

Figure 2: Single-stage centrifugal pump.3 head (frictional losses, heat gains, and form losses). To prevent pump cavitation, the NPSH available to the pump must be greater than the minimum required by the pump.

Discharge

NPSH a > NPSH r

(4)

If a refrigerant pump is cavitating, several options can remedy the situation. First, determine the pump impeller diameter from original equipment installation documentation or from the data tag on the pump. Next, read the pressure on the discharge side of the pump during operation and compare with the vessel’s pressure to determine that the pump is developing a pressure rise and note the magnitude (estimated by difference between the gauge reading and the vessel pressure). Then obtain the pump curve for that specific model pump and impeller diameter (see the next section). With this information, look at the manufacturer’s pump curve and determine the pump’s flow rate and the required net positive suction head (NPSHr) corresponding to that operating point on the pump curve. If the pump is cavitating due to operating “out on the curve” (toward the right side of the pump curve), one approach to cure this cavitation is to
ashrae.org August 2011



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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