Staging Chiller Plants for Maximized Efficiency and Reliability

Last month's Engineering White Paper covered the pros and cons of both air and water cooled chillers as applied to mission critical duty. This month we take the topic one step further and discuss various methods of staging and controlling chillers to provide not only maximum reliability but efficiency as well.

Mission critical is all about simplicity and redundancy. The rules are to keep it simple, provide multiples and keep them running. Don't shut things off because they may not start again and if something does fail, get another running as quickly as possible. Some mission critical designs revolve around a "run to fail" philosophy which simple means run it, don't turn it off and if it doesn't want to run, run it anyway….even if it fails. A single chiller is probably not best suited to mission critical duty as it provides little redundancy. If a single chiller is used be sure that you at least incorporate multiple compressors into the design. This requirement may eliminate centrifugal chillers on some smaller water-cooled projects as most manufactures don't offer them with multiple compressors until you get into rather large tonnages.

One exception is the McQuay "Magnitude" WMC magnetic bearing, frictionless, oil-less centrifugal chiller with 2-compressors.

McQuay WMC Centrifugal Chiller, Multiple Compressors with Magnetic Bearings from 145-400 Tons

Reliability in mission critical facilities is as much about redundancy as anything else. Designs typically revolve around "multiples" of components and may include 100% back-up should any single unit fail. This redundancy is typically indicated as n+1 or n+ 2 etc., meaning "n" the number of base units plus the number of redundant unit's desired. It is unusual to see less that two of any critical component on projects of this nature and with large tonnage jobs multiples may be the only way to meet load requirements anyway, based on the maximum single chiller tonnage offered by manufactures.

When multiple chillers are used it is the type of staging strategy employed that can define the reliability and efficiency of the chiller plant. One common approach implemented is "staged-unloading" and with two chillers this strategy would result in a sequence as follows. At building loads between 50%-100% both chillers would operate (together) while at loads between 0%- 50% one chiller is shut off and only one chiller would be left running. With building loads between 50%-100% both chillers run together and modulate capacity, at lower loads load the single running chiller modulates capacity.

With this method we "turn chillers on and off" as required to meet load requirements. Each chiller could have its own evaporator pump, cooling tower and condenser pump so that when an individual chiller is on or off so are its ancillaries. Simple in theory, staged unloading can be an efficient strategy when constant speed chillers are used. While staging is one thing, control is altogether another and even a strategy as simple as staged-unloading can get tricky. When we turn on-or-off a chiller we must also turn on-or-off water flow through the chillers evaporator. Flow must be isolated from any "off" chiller or warm return water will pass through it, mixing with water that has been cooled by the active chiller(s). This mixture results in warmer than design water being supplied to the loads resulting in a loss of building temperature control.

Chiller Flow Isolation Valves

We need to add shut-off isolation valves to each chiller which can respond (open or close) as required to eliminate flow through the inactive chiller(s). Designed, installed and controlled properly valves will do the trick but there are some additional considerations that need to be addressed as well.

Modern water cooled chillers have proof of flow and minimum and maximum evaporator flow requirements that must be adhered to. A lesser understood requirement is that of the chillers "flow turn-down ratio" which must also be taken into account. What this means is that evaporator flow can only increase or decrease in a controlled and prescribed manor. We can't just open or close the isolation valve as doing so will cause a rapid rate of change, also known as flow transients. When we start a second chiller the first thing that happens is we open its isolation valve. If the valve opens instantaneously flow through the active chiller will drop by approximately 50% and if the chillers unloading cannot respond properly the compressor will continue to produce the same capacity with half the flow, doubling the Delta-T. If the chiller was making 42-degree water it could now be making 28-degree water which would produce evaporator suction temperatures below freezing.

Rapid increases in evaporator flow can cause loss of leaving water temperature control while rapid decreases can cause low temperature shutdown or "freeze-stat" lockout. This occurs in just about every parallel chiller system, both primary secondary or variable primary flow. A chiller manufacture may stipulate something like; "Chiller evaporator flow rate change to be no greater than 30% per minute" This limitation is placed on the chiller to allow for stable adjustment to rate of change within the limits of its controllability and it must be recognized and adhered to. Chillers with less sophisticated control capabilities may be limited to flow rate changes as small as 2% per minute. In this case it would take over 25-minutes to properly transition a flow change of 50%, far too long to allow stable and reliable chilled water temperature control. Note: Small and constant flow fluctuations are typical of normal operation in most all chilled water systems, even constant flow designs.

Controlling the rate of change requires properly sized, installed and controlled "slow acting" isolation valves, tuned to the requirements of the chiller and maintaining a linear relationship between valve position and flow. The slower valve reduces the flow rate of change and helps provide stability. Another strategy we could use to help the chillers through these unstable flow transients would be to partially unload the active chiller (demand limit) or adjust the leaving water set point (up) before opening the valve. Once the system is stabilized all set points could be returned to design but this is a rather complex control task and may be difficult to properly implement.

Let's take a closer look at how we might go about designing the best staging strategy to be implemented with multiple chillers on a mission critical application. Redundancy aside for now (and keeping it simple) let's assume we have a 1000-ton load requirement. With an eye on efficiency, let's specify two 500-ton variable-speed, water cooled, R-134a centrifugal chillers to be piped in parallel with one another. We are going to design the plant around variable primary flow (VPF) with manifolded pumps to provide first-cost savings and a simplified piping/pumping scheme.

(2)-500Ton Variable-Speed Chillers with VPF Water Distribution System

A plant utilizing variable speed chillers should attempt to keep as many chillers operating as possible, as long as they are loaded somewhere above 20%. In general it is more efficient to run two variable speed chillers at 45% than one chiller at 90% with the ultimate efficiency points being dependent on how the chillers and ancillary devices interact. It is also true that running two cooling tower fans at half speed is more efficient than running one tower fan at full speed and shutting the other fan off, the two fan approach will typically consume about 1/3 the power. This is a very important point to remember and implement into the staging of variable speed chiller plants for maximized efficiency.

Let's look at a very simple and efficient chiller staging strategy known as "equal percentage unloading" which works like this. All chillers would run together, regardless of building load and they load and unload (together) through their full range of capacity. We try not to turn chillers on and off but rather turn them on and "modulate" them (together) so their combined output meets the capacity requirements of the load. In our design the two chillers would both be operating at 100% capacity to meet the full building load (just like with staged unloading) but when the building load is reduced to 50% (or less) both chillers continue to run together and simply unload capacity to meet the building requirements. We still adhere to all proof of flow, minimum and maximum flows and flow rate change requirements but we are not opening and closing isolation valves or turning on and off pumps as much. This reduces the possibility of flow transients and safety trips and results in a simple, efficient approach to staging our variable speed chiller plant.

We haven't yet addressed redundancy so what if we were to use three chillers instead of two, with the third chiller provided as a back-up (n+1) in case of failure. Let's run all three chillers to meet the building load so that if one chiller does fail the back-up is already operational. The system doesn't have to recognize a failure and start the spare chiller or turn on pumps and open isolation valves and we don't have to worry if the back-up chiller will start. We turn all chillers on and leave them on and by equally unloading all chillers they run together and efficiently modulate capacity to meet the required load. Known a "running redundancy", this approach is perfect for mission critical duty.

The cooling towers and condenser pumps could run all the time as well (turn them on and leave them on) and by utilizing VFD's on these pumps we could reduce flow to each tower as the load drops resulting in condenser pump energy savings. By leaving the tower fans on and reducing water flow over the tower fill it maximizes tower heat transfer and produces cooler leaving tower water. Chillers love cooler condenser water, it reduces lift and saves on compressor horsepower. With VFD's on the condenser pumps we can provide low ambient control of the leaving tower water temperature which means no tower bypass line or bypass valve is required. We get additional first cost savings as well as exceptional efficiency.

Mission critical design isn't typically as much about saving energy as it is providing reliability. Staging chillers with equal percentage unloading may be the ultimate win-win scenario; we provide running redundancy, reduce the potential for flow transients, help avoid low temperature shutdown or freeze-stat lockout and at the same time produce nice energy savings. We also abide by the mission critical rule that says to keep it simple, providing multiples and keeping them running.

The McQuay "Magnitude" magnetic bearing, frictionless, oil-less centrifugal chiller not only offers unparalleled reliability but provides industry leading part-load efficiencies as well. With part-load performance as low as 0.31 kW /ton IPLV it leads the industry in sustainability. The WME is capable of flow changes up to 50% per minute, as long as minimum and maximum tube velocities are enforced and is capable of operating with entering condenser water temperatures as low as 48-degrees (consult us for details).

McQuay WMC/E Centrifugal Chiller with Magnetic Bearings, Frictionless, Oil-less, 145-700 Tons

"Tower Tech" cooling towers can provide the lowest flow turn-down rates of any tower available. The combination of these two products is ideal for applications using equal percentage unloading of chillers and where the utmost in reliability and energy efficiency is required.

"Tower Tech" Cooling Tower with Patented Low-Turndown Spray Nozzle

Let HTS assist you with your next mission critical HVAC design. Whether air or water cooled chillers are required we have the product to meet your needs. We also represent a vast array of other products and services that can assist in providing efficient, reliable and cost-effective mission critical HVAC systems. Contact us today!

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