ASHRAE Journal - Optimizing Design Control of Chilled Water Plants Part 2 Condenser Water Distribution System Design

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ASHRAE Journal
  26 ASHRAE Journal September 2011 T his is the second of a series of articles discussing how to optimize the design and control of chilled water plants. The series will summarize ASHRAE’s Self Directed Learning (SDL) course called Fundamentals of Design and Control of Central Chilled Water Plants and the research that was performed to support its development. See sidebar, Page 36 for a summary of the topics to be discussed. The articles, and the SDL course upon which it is based, are intended to provide techniques for plant design and control that require little or no added engineering time compared to standard practice but at the same time result in sig- nicantly reduced plant life-cycle costs. A procedure was developed to provide near-optimum plant design for most chill-er plants including the following steps: 1. Select chilled water distribution system.2. Select chilled water temperatures, flow rate, and primary pipe sizes.3. Select condenser water distribution system.4. Select condenser water tempera-tures, flow rate, and primary pipe sizes.5. Select cooling tower type, speed con-trol option, efficiency, approach tempera-ture, and make cooling tower selection. 6. Select chillers.7. Finalize piping system design, calcu-late pump head, and select pumps. 8. Develop and optimize control se-quences.Each of these steps is discussed in this series of five articles. This article dis-cusses Step 3: designing the condenser water distribution system. Steps 2 and 4 will be discussed in the next article. Three common piping arrangements for condenser water pumps are:  ã Option A: Dedicate a pump for each condenser ( Figure 1a );  ã Option B: Provide a common header at the pump discharge and two-way au-tomatic isolation valves for each con-denser ( Figure 1b ); and  ã Option C: Provide a common head-er with normally closed (NC) manual isolation valves in the header between pumps ( Figure 1c  ). The advantages of dedicated pumps for each condenser (Option A) include: About the Author Steven T. Taylor, P.E.,  is a principal at Taylor Engineering in Alameda, Calif. By Steven T. Taylor, P.E.,  Fellow ASHRAE Optimizing Design & ControlOf Chilled Water Plants   Part 2: Condenser Water System Design This article was published in ASHRAE Journal, September 2011. Copyright 2011 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. Posted at This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE. For more information about ASHRAE Journal, visit  September 2011 ASHRAE Journal 27 1. The pump can be custom-selected for the condenser it serves. Pump selection can then account for variations in condenser pressure drop and flow rates when chillers are not identical. This can reduce pump energy compared to Option B where the head of each pump must be the same and sized for the condenser with the highest pressure drop; balance valves at the other condensers must be throttled to generate this same pressure drop. 2. Controls are a bit simpler because the pump can be con-trolled using the contact provided with the chiller controller.  This ensures that the pump starts and stops when the chiller wants it to. With Option B, the control of the isolation valves and pumps is by the direct digital control (DDC) system and must be coordinated with the needs of the chiller controller to avoid nuisance trips. For instance, the pumps generally must run for several minutes after the command for the chiller to stop so that the chiller can pump down the refrigerant. 3. Pump failures do not cause multiple chiller trips. With dedicated pumps, if a pump fails, only the chiller it serves will see a flow disruption and trip. With Option B, all operating chillers will see a flow reduction when a pump fails, possibly causing more than one chiller to trip due to low flow or high refrigerant head. However if there is a lag or standby pump with Option B that can be started quickly, trips can usually be avoided because it takes some time for refrigerant head to rise. The advantages of headered (manifolded) pumps (Option B) include:1. Redundancy is improved. With Option A, if a pump fails and a chiller other than the one it serves also fails (albeit a rare event), then two chillers will be inoperative. With Option B, any pump can serve any chiller and under many conditions one pump can provide enough flow for two chillers to operate near full capacity. 2. Including a standby pump is much simpler. Adding a standby pump to Option A is cumbersome and expensive because it requires extensive piping and manual or automatic isolation valves. If standby pumps are desired, Option B is the best option. 3. Isolation valves can double as head pressure control valves. See discussion on head pressure control later. For Option A, head pressure control would require the addition of variable speed drives on condenser water pumps or tower bypass valves. 4. It is easier to integrate a water-side economizer. See discussion on waterside economizers below. Since waterside economizers are only operational in cold weather when loads are generally low, the condenser water side can use one (or more) of the condenser water pumps serving chillers rather than providing a dedicated pump. This reduces first costs. Headered pumps with manual isolation valves (Option C) can have the advantages of Option A (although it works best with identical chillers) and it overcomes the redundancy dis-advantage of Option A but accommodating a pump failure requires manual manipulation of valves vs. the automatic response in Option B. Including a standby pump is possible with Option C but it only works (depending on which pump fails) with the header isolation valves open and chillers must be staged by manually opening and closing their isolation valves. First costs are usually lowest with Option A if the chiller and pump pairs are close-coupled and the manual isolation valves between the two are eliminated (each chiller-pump pair is iso-lated for service as a pair). Option C is usually less expensive than Option B, but Option B is usually the best choice where head pressure control and standby pumps are required. Refrigerant Head Pressure Control All chillers will require a minimum refrigerant head (lift) between the evaporator and condenser. This can be quite high Figure 1:  Condenser water pump piping options. Option A (left): Dedicated pumps. Option B (center): Headered pumps with con-denser auto-isolation valves. Option C (right): Headered pumps with manual isolation valves. Cooling Tower No. 1Cooling Tower No. 2Cooling Tower No. 3Chiller No. 1Chiller No. 2Chiller No. 3CHW Pump No. 1CHW Pump No. 2CHW Pump No. 3Cooling Tower No. 1Cooling Tower No. 2Cooling Tower No. 3Chiller No. 1Chiller No. 2Chiller No. 3CHW Pump No. 1CHW Pump No. 2CHW Pump No. 3Optional Standby PumpCooling Tower No. 1Cooling Tower No. 2Cooling Tower No. 3Chiller No. 1Chiller No. 2Chiller No. 3CHW Pump No. 1CHW Pump No. 2CHW Pump No. 3N.C.N.C.  ABC  28 ASHRAE Journal September 2011 for most screw chillers and some hermetic centrifugal chill-ers, 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:  ã 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. ã When integrated waterside economizers are used (dis-cussed later). Head pressure control is almost always manda-tory since cooling tower water temperatures are deliberately kept very cold for long periods. Options to avoid low head pressure problem include:  ã  Tower three-way bypass valves. The bypass water is di-verted around the tower fill into the cooling tower sump or into the suction piping, thus avoiding natural cooling that oc-curs 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 pres-sure. 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. ã For systems with dedicated condenser water pumps (Op-tion 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 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 mini-mum 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. ã 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 con-trol 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 con-denser 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 con-denser in the first place are preferred. But even if this is an is-sue, 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 uni-formly across the fill through spray nozzles via a piping head-er 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 be-tween 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. Op-tions 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 Figure 2:  Cooling tower cell isolation options. Option A (left): Weir dams and/or low ow nozzles. Option B (center): Auto- isolation valves on supply only. Option C (right): Auto-isolation valves on supply and suction. Cooling Tower No. 1Cooling Tower No. 2Cooling Tower No. 3Cooling Tower No. 1Cooling Tower No. 2Cooling Tower No. 3Cooling Tower No. 1Cooling Tower No. 2Cooling Tower No. 3  ABC September 2011 ASHRAE Journal 29 tower cells, this can eliminate the need for isolation valves, which cost much more than the weir dams and nozzles. This option is also the most efficient; tower energy use is mini-mized by operating as many cells as possible, particularly when tower fans are controlled by variable speed drives.  This is because fan speed is reduced (reducing fan power by almost the cube of the speed) and cooling is achieved through tower cells even when fans are off. With most man-ufacturers and tower types, nozzles and dams are available to reduce flow by 50%, and many can go down to 33% or even 25% depending on the selection and design flow rate. Because of low cost and high efficiency, this option should always be the first choice. When a plant has many tower cells and automatic isolation valves are unavoidable, the dams and nozzles should still be selected to allow as many cells to operate as possible. Option B: Install automatic isolation valves on supply lines only. This option uses the equalizer to keep basin levels between overflow and fill lines and will require that equal-izers be oversized from that required by normal duty. For example, assume there are three tower cells, and only one is active; supply flow to the others is shut off. But water is drawn out of all three cell basins since the suction lines have no automatic isolation valves. The water level in the basin of the cell that is supplied will rise while the other two ba-sin levels will fall. The difference in the two elevations must provide enough head for water to transfer from the supplied cell to the others through the equalizer. If the equalizer is undersized, water will overflow in the supplied cell, and the others will be drawn so low that makeup water valves open, wasting water and water treatment chemicals. There are only a few inches of elevation difference between the overflow and fill lines, so it is imperative that the equalizer be properly sized for this option to work. Another approach is to elimi-nate the basins at each tower and use a common sump, often located indoors in cold climates. This avoids the need for equalizer lines entirely but is much more expensive.Option C: Install automatic isolation valves on both sup-ply and suction lines. This is usually the most expensive option since automatic valves are expensive relative to an incremental increase in equalizer size. This design also in-creases exposure to a valve failure; an oversized equalizer line has no failure modes. It also increases the risk of freez-ing (or increases the energy used by basin heaters) in the basins of inactive cells in systems that must operate in cold weather. But this is often the best option when there are many tower cells that are not located close together (long equalizer lines).
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