By: Allan Evora
Comparing DDC Controls to PLC at Hospital Energy Plants: PLC Wins Every Time
Unlike other mission-critical facilities that demand high utility availability, medical campus central energy plant controls traditionally have not utilized Programmable Logic Controllers (PLC) to manage the output and actions of boilers, chillers, and other fuel systems. Instead, these highly-industrial systems affecting hundreds of patients continue to be designed using the same Direct Digital Control (DDC) technology that dominates the non-mission-critical facility space, such as commercial office buildings, schools and shopping malls.
While it’s true that PLC and DDC controls both make decisions based off sensor data and subsequently write outputs, PLCs do it better, faster, and more reliably. PLCs provide a level of sophistication and industrial hardening that mission critical environments demand and DDCs can’t deliver.
So why aren’t more central utility plant operators (and data centers for that matter) replacing DDCs with PLCs? In short, they have a hard time seeing the value. Additionally, most design engineers believe that PLCs cost significantly more.
Nowadays, PLCs are much more competitively priced, with major brands like Allen-Bradley offering solutions at all price points (ControlLogix, CompactLogix, MicroLogix). If you would have told me 5 years ago that I could purchase an Allen-Bradley PLC for under $250, I would have said no way. Not so today.
If performance, reliability and lifecycle costs are your measuring stick in your healthcare central utility plant (CUP), then PLCs are worth their weight in gold. Any owner who values high system availability, the flexibility to use equipment from multiple manufacturers, and the ability to maintain and program their controls using their own staff needs a PLC-based system.
Comparing DDC to PLC: Pay Now or Pay Later
In general, PLCs are a better choice than DDC controls when high availability and high performance are the top priorities. Because PLCs are generally designed to meet industrial, or in some cases military specifications they have much higher mean time between failure (MTBF) when compared to DDC systems. When factoring in the costs associated with unplanned downtime or potential human risks, it is easy to see why PLCs have the lower total-cost of ownership (TCO) and are a perfect match for healthcare central utility plants.
Because DDC controls are used commercially, the higher failure rate is considered acceptable. After all, if a building automation system in a commercial office building fails due to a DDC component problem, human lives aren’t normally at risk. An equipment failure at a CUP can spell disaster as it is like the heart of a medical campus providing the lifeblood keeping all critical facilities, such as operating rooms and critical care units, operating 7x24 within industry regulated guidelines. This is why PLCs with their high MTBF and redundancy options are the best choice for this mission critical environment. It is also the reason why PLCs are almost exclusively used in emergency power supply system (EPSS) control.
The following are comparisons between DDC and PLC…and the reasons behind why PLC is the obvious choice for central energy plants.
Industrial vs. Commercial
Mission critical environments like manufacturing facilities and power plants use PLCs in combination with their SCADA monitoring system. Why? PLCs are robust, meant for dirty, hot, humid, electrically noisy environments. Larger temperature ranges, conformal coatings, and immunity to electromagnetic interference (EMI) are some of the characteristics that make the PLC the better choice for an industrial environment like a CUP.
PLCs can typically support a wider range of power supplies, which translates to fewer components and points of failure. Also, their power supplies are able to withstand a larger variation in the supply voltage. Additionally, PLCs can have a highly distributed, fast, and redundant input and output network, allowing for cost effective design when instruments and controls are distributed within the plant.
A DDC system is primarily suited for a commercial environment and created for building automation in non-mission critical environments like office buildings, schools, malls, warehouses or even non-critical medical office buildings. Primary applications for DDC include HVAC and lighting as well as control and monitoring of other non-mission critical systems such as irrigation, or energy measurement and verification. Its functionality was never meant for a high availability environment. As an example, with a DDC system you cannot generally hot swap components without first powering down the controller (and thus compromising control). This is easily done with a PLC.
Flexible, Customizable Integration vs. Pre-Programmed Installation
The integration differences between DDC and PLC are night and day.
Most DDC systems were developed to serve the HVAC market. There are some very distinct differences that stem from this origin:
- While DDC systems support interoperability with the support of many industry standard protocols and APIs, they are still not as open with respect to the sourcing of products and services. Most nationally recognized DDC systems, such as Johnson Controls, Trane, and Andover Controls, can only be sourced through an authorized, and often geographically exclusive, dealer or distributor. This can mean higher total cost of ownership since the owner becomes “locked in” to a sole supplier.
- Commercial HVAC systems are a highly competitive industry. If you think about it, every building requires a HVAC system. To drive costs as low as possible, DDC manufacturers supply hardware and software that for the most part is pre-configured/pre-programmed for the equipment it will be controlling. This works great for a majority of commercial HVAC systems, but is problematic with customized control sequence required in large medical campus central energy plants requiring control of chillers, boilers, compressors, fuel systems, emergency, and normal power systems.
PLCs are broadly supported by systems integrators who use industry accepted programming languages and tools to configure PLC systems. By using PLCs as opposed to DDC controls, the owner can not only source multiple suppliers, but also opt to maintain and program the system themselves, which is usually not an option with DDC.
Redundant Power and Data vs. Singular Systems
PLC redundancy helps reduce the possibility of downtime, but rarely do you see DDCs with “true” redundancy options. For example, many PLC manufacturers have offerings within their line of products for hot failover. This means when it comes to control, the process will not be impacted by the failure of the primary controller as the transition to the backup controller is “bumpless”. Control processor is just one of the forms of high availability that a PLC can offer. Redundant power supplies and redundant input/output systems are also options in some of the recognized brands of PLCs.
Another large advantage of PLC systems is the ability to distribute input and outputs. This can greatly reduce the amount of field wiring required to interface to instruments and control devices. Not only can the inputs and outputs be distributed, but they can be distributed using a redundant data network that is deterministic. This translates to high availability with known performance; a highly desirable requirement for any mission critical sequence of operation.
Fast Accuracy vs. Imprecise Sluggishness
PLCs are designed for real-time control, which means they have greater speed and communications abilities. PLCs are also designed to be more precise. They have greater resolution (14-bit vs 10-bit) which translates to more accurate temperature, flow, pressure, voltage, and current measurement. They’re designed to avoid electrical interference that could affect data accuracy.
There’s a reason PLC actions are measured in milliseconds or program cycles, while DDC is only measured in seconds, or even minutes. While it is true that not every application requires the performance and accuracy that a PLC provides, my experience has been that most mission critical control operations that can impact life safety and/or reduce costs due to faster reaction time, such as protective load shed will always be the domain of the PLC.
Here are some examples of the speed differences between PLC and DDC controls that illustrate network communication and program execution rates.
|Refresh Rate||<1 second||>30 seconds|
|Operator Command Response Rate||Immediately||Up to 1 minute|
|SCADA Alarms||<5 seconds||Up to 1 minute|
It’s obvious that operator response to system errors can have seriously different results between PLC and DDC. It’s also safe to say that central utility plants utilizing measurements from PLC devices would have more accurate and more consistent Joint Commission EPSS test reports than those using DDC.
Medical Central Utility Plants – Start Integrating PLCs
There may be a bit of overlap on the capabilities of DDC and PLC, but when specifically considering a central utility plant environment, the choice is obvious. DDC controls were built for commercial building automation. PLCs were built for industrial and mission critical process control. With a PLC, operators will receive more accurate data on which to base future decisions and react quicker to safety-threatening or economically-threatening situations in the plant.
Allan Evora is a leading expert in control systems integration and president of Affinity Energy with over 20 years of industry experience working in every capacity of the power automation project life cycle. With a background at Boeing Company and General Electric, Allan made the decision to establish Affinity Energy in 2002. Allan is an alumnus of Syracuse University with a B.S. in Aerospace Engineering, graduate of the NC State Energy Management program, and qualified as a Certified Measurement & Verification Professional (CMVP).
Throughout his career, Allan has demonstrated his passion for providing solutions. In 1990, he developed FIRST (Fast InfraRed Signature Technique), a preliminary design software tool used to rapidly assess rotary craft infrared signatures. In 2008, Allan was the driving force behind the development of Affinity Energy's Utilitrend; a commercially available, cloud-based utility resource trending, tracking, and reporting software.
Allan has been instrumental on large scale integration projects for utilities, universities, airports, financial institutions, medical campus utility plants, and manufacturing corporations, and has worked with SCADA systems since the early ‘90s. A passion for data acquisition, specialty networks, and custom software drives him to incorporate openness, simplicity, and integrity into every design in which he is involved.