ASHRAE Journal - September 2011 - 28

A
Cooling Tower No. 1 Cooling Tower No. 2 Cooling Tower No. 3

B
Cooling Tower No. 1 Cooling Tower No. 2 Cooling Tower No. 3

C
Cooling Tower No. 1 Cooling Tower No. 2 Cooling Tower No. 3

Figure 2: Cooling tower cell isolation options. Option A (left): Weir dams and/or low flow nozzles. Option B (center): Autoisolation valves on supply only. Option C (right): Auto-isolation valves on supply and suction. for most screw chillers and some hermetic centrifugal chillers, and very low for magnetic bearing chillers, which have no oil return considerations. There are two common reasons why low refrigerant head pressure can occur: • At start-up when water temperature in the cooling tower basins is cold. Some chillers can operate for a short period of time with low start-up head while others will trip on low head pressure safeties almost immediately. To determine if head pressure control is required, for cold starts, consult with the chiller manufacturer. • When integrated waterside economizers are used (discussed later). Head pressure control is almost always mandatory since cooling tower water temperatures are deliberately kept very cold for long periods. Options to avoid low head pressure problem include: • Tower three-way bypass valves. The bypass water is diverted around the tower fill into the cooling tower sump or into the suction piping, thus avoiding natural cooling that occurs across the tower fill even when tower fans are off. Piping the bypass to the suction line also avoids the mass of water in the basin for an even faster warm-up, but the design can be problematic: unless the bypass line is balanced to create a pressure drop equal to the height of the cooling tower, air will be drawn into the system backwards from the spray nozzles since piping above the basin will fall below atmospheric pressure. For staged or variable condenser water flow systems, the bypass must be balanced at the lowest expected flow rate. This creates a high pressure drop and reduced flow if more pumps operate, but reduced flow is acceptable when the intent of the bypass is to raise head pressure. The bypass valve is controlled by supply water temperature typically with a low limit setpoint well below the normal setpoint used to control tower fan on/ off and speed. Tower bypass is most commonly used where towers must operate in very cold weather to avoid freezing in the fill. The following two options are less expensive and, therefore, preferred in other applications. • For systems with dedicated condenser water pumps (Option A or C, Figure 1), variable speed drives on the pumps can be used to reduce water flow to the chiller. Head pressure can be maintained even with very cold supply water as long as the flow rate can be reduced so that the condenser refrigerant pressure can be high enough (head pressure depends on the
28 ASHRAE Journal

condenser water temperature leaving the chiller, not entering the chiller). Pump speed can be controlled by the temperature leaving the condenser at a setpoint that corresponds to minimum condenser pressure, or (preferably) by a signal from the chiller controller indicating head pressure needs; most chiller controllers have an analog output dedicated for this purpose. • For systems with headered pumps (Option B, Figure 1), the isolation valves can double as head pressure control valves by converting them from two-position to modulating. Valve position is typically controlled by the chiller controller head pressure control analog output, either directly or through the DDC system. This signal will close the valve when the chiller shuts off. The second two options mentioned previously reduce flow through the condenser. Many engineers are concerned that low condenser water flow will contribute to fouling of the condenser tubes, but there is little definitive evidence to support the concept that high velocity keeps tubes clean; strainers and sidestream filters that prevent particles from entering the condenser in the first place are preferred. But even if this is an issue, for most head pressure control applications there are few hours at reduced flow—only during cold starts—so the impact on tube fouling should not be significant. Low flow through the cooling tower may also be an issue (see discussion later) but, again, it should not be given the short duration.

Minimum Flow Rates
When water enters the cooling tower, it is distributed uniformly across the fill through spray nozzles via a piping header or gravity distribution basin. Each cell has a minimum flow rate to ensure that tower fill is fully wetted along the face of the air entering the fill. If there are dry spots along this face, air will bypass the wetted fill due to lower pressure drop and, more importantly, cause scale to build up at the boundary between the wet and dry fill as water is evaporated and dissolved solids remain. So it is important to maintain minimum tower cell flow rates, particularly in areas with hard makeup water. In plants with multiple cooling towers and chillers, it is desirable to operate one condenser water pump at low loads, which will reduce the flow rate through cooling towers. Options for maintaining minimum flow rates (Figure 2) include: Option A: Select tower weir dams and/or nozzles to allow one pump to serve all towers. For systems with two or three
ashrae.org September 2011



ASHRAE Journal - September 2011

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

Contents
Commentary
Industry News
Meetings and Shows
Feature Articles
Adsorption Refrigeration: New Opportunities for Solar
Optimizing Design & Control of Chilled Water Plants—Part 2: Condenser Water System Design
Designing for Comfort: Selecting Air-Distribution Outlets
Technology Award Case Studies
Library Sets Example
Sustainable Campus
Standing Columns
Building Sciences
IAQ Applications
Emerging Technologies
Products
Classified Advertising
Advertisers Index
Special Supplement: AMCA International inmotion Magazine
ASHRAE Journal - September 2011 - Intro
ASHRAE Journal - September 2011 - Cover1
ASHRAE Journal - September 2011 - Cover2
ASHRAE Journal - September 2011 - 1
ASHRAE Journal - September 2011 - 2
ASHRAE Journal - September 2011 - Contents
ASHRAE Journal - September 2011 - Commentary
ASHRAE Journal - September 2011 - 5
ASHRAE Journal - September 2011 - Industry News
ASHRAE Journal - September 2011 - 7
ASHRAE Journal - September 2011 - 8
ASHRAE Journal - September 2011 - 9
ASHRAE Journal - September 2011 - Meetings and Shows
ASHRAE Journal - September 2011 - 11
ASHRAE Journal - September 2011 - 12
ASHRAE Journal - September 2011 - 13
ASHRAE Journal - September 2011 - Adsorption Refrigeration: New Opportunities for Solar
ASHRAE Journal - September 2011 - 15
ASHRAE Journal - September 2011 - 16
ASHRAE Journal - September 2011 - 17
ASHRAE Journal - September 2011 - 18
ASHRAE Journal - September 2011 - 19
ASHRAE Journal - September 2011 - 20
ASHRAE Journal - September 2011 - 21
ASHRAE Journal - September 2011 - 22
ASHRAE Journal - September 2011 - 23
ASHRAE Journal - September 2011 - 24
ASHRAE Journal - September 2011 - 25
ASHRAE Journal - September 2011 - Optimizing Design & Control of Chilled Water Plants—Part 2: Condenser Water System Design
ASHRAE Journal - September 2011 - 27
ASHRAE Journal - September 2011 - 28
ASHRAE Journal - September 2011 - 29
ASHRAE Journal - September 2011 - 30
ASHRAE Journal - September 2011 - 31
ASHRAE Journal - September 2011 - 32
ASHRAE Journal - September 2011 - 32A
ASHRAE Journal - September 2011 - 32B
ASHRAE Journal - September 2011 - 33
ASHRAE Journal - September 2011 - 34
ASHRAE Journal - September 2011 - 35
ASHRAE Journal - September 2011 - 36
ASHRAE Journal - September 2011 - 37
ASHRAE Journal - September 2011 - Designing for Comfort: Selecting Air-Distribution Outlets
ASHRAE Journal - September 2011 - 39
ASHRAE Journal - September 2011 - 40
ASHRAE Journal - September 2011 - 41
ASHRAE Journal - September 2011 - 42
ASHRAE Journal - September 2011 - 43
ASHRAE Journal - September 2011 - 44
ASHRAE Journal - September 2011 - 45
ASHRAE Journal - September 2011 - 46
ASHRAE Journal - September 2011 - 47
ASHRAE Journal - September 2011 - Library Sets Example
ASHRAE Journal - September 2011 - 49
ASHRAE Journal - September 2011 - 50
ASHRAE Journal - September 2011 - 51
ASHRAE Journal - September 2011 - 52
ASHRAE Journal - September 2011 - 53
ASHRAE Journal - September 2011 - Sustainable Campus
ASHRAE Journal - September 2011 - 55
ASHRAE Journal - September 2011 - 56
ASHRAE Journal - September 2011 - 57
ASHRAE Journal - September 2011 - 58
ASHRAE Journal - September 2011 - 59
ASHRAE Journal - September 2011 - 60
ASHRAE Journal - September 2011 - 61
ASHRAE Journal - September 2011 - 62
ASHRAE Journal - September 2011 - 63
ASHRAE Journal - September 2011 - Building Sciences
ASHRAE Journal - September 2011 - 65
ASHRAE Journal - September 2011 - 66
ASHRAE Journal - September 2011 - 67
ASHRAE Journal - September 2011 - 68
ASHRAE Journal - September 2011 - 69
ASHRAE Journal - September 2011 - 70
ASHRAE Journal - September 2011 - 71
ASHRAE Journal - September 2011 - 72
ASHRAE Journal - September 2011 - 73
ASHRAE Journal - September 2011 - 74
ASHRAE Journal - September 2011 - 75
ASHRAE Journal - September 2011 - IAQ Applications
ASHRAE Journal - September 2011 - 77
ASHRAE Journal - September 2011 - 78
ASHRAE Journal - September 2011 - 79
ASHRAE Journal - September 2011 - 80
ASHRAE Journal - September 2011 - Special Supplement: AMCA International inmotion Magazine
ASHRAE Journal - September 2011 - AMCACover2
ASHRAE Journal - September 2011 - AMCA3
ASHRAE Journal - September 2011 - AMCA4
ASHRAE Journal - September 2011 - AMCA5
ASHRAE Journal - September 2011 - AMCA6
ASHRAE Journal - September 2011 - AMCA7
ASHRAE Journal - September 2011 - AMCA8
ASHRAE Journal - September 2011 - AMCA9
ASHRAE Journal - September 2011 - AMCA10
ASHRAE Journal - September 2011 - AMCA11
ASHRAE Journal - September 2011 - AMCA12
ASHRAE Journal - September 2011 - AMCA13
ASHRAE Journal - September 2011 - AMCA14
ASHRAE Journal - September 2011 - AMCA15
ASHRAE Journal - September 2011 - AMCA16
ASHRAE Journal - September 2011 - AMCA17
ASHRAE Journal - September 2011 - AMCA18
ASHRAE Journal - September 2011 - AMCA19
ASHRAE Journal - September 2011 - AMCA20
ASHRAE Journal - September 2011 - AMCA21
ASHRAE Journal - September 2011 - AMCA22
ASHRAE Journal - September 2011 - AMCA23
ASHRAE Journal - September 2011 - AMCA24
ASHRAE Journal - September 2011 - AMCA25
ASHRAE Journal - September 2011 - AMCA26
ASHRAE Journal - September 2011 - AMCACover3
ASHRAE Journal - September 2011 - AMCACover4
ASHRAE Journal - September 2011 - Emerging Technologies
ASHRAE Journal - September 2011 - 82
ASHRAE Journal - September 2011 - 83
ASHRAE Journal - September 2011 - 84
ASHRAE Journal - September 2011 - 85
ASHRAE Journal - September 2011 - 86
ASHRAE Journal - September 2011 - 87
ASHRAE Journal - September 2011 - 88
ASHRAE Journal - September 2011 - Products
ASHRAE Journal - September 2011 - 90
ASHRAE Journal - September 2011 - 91
ASHRAE Journal - September 2011 - 92
ASHRAE Journal - September 2011 - 93
ASHRAE Journal - September 2011 - Classified Advertising
ASHRAE Journal - September 2011 - 95
ASHRAE Journal - September 2011 - Advertisers Index
ASHRAE Journal - September 2011 - Cover3
ASHRAE Journal - September 2011 - Cover4
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