The Consultants Role
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The document discusses the role of the consulting engineer in ensuring temperature control systems work properly. It outlines 7 steps the engineer should take: 1) Define systems to control, 2) Define hardwired safeties, 3) Define sequence of operation, 4) Develop input/output list, 5) Define graphic requirements, 6) Identify component locations, 7) Write specifications. Taking these steps helps communicate design intent and ensures contractors implement systems correctly.
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The Consultants Role
- 1. Solutions: The Role of the Consulting Engineer Temperature Control Systems – Why Don’t They Work? ASHRAE CRC 2006
- 3. Why Temperature Control Systems?
- 12. Step 1: Define the System to be Controlled
- 14. Step 2: Define hard-wired control and safeties.
- 16. Step 2: Define hard-wired safeties.
- 17. Step 2: Define hard-wired safeties. SUPPLY FAN STATUS FREEZESTAT
- 18. Step 2: Define hard-wired safeties.
- 23. Step 3: Define the Sequence of Operation.
- 25. Step 4: Develop an Input / Output Summary (Points List) EXISTING POINTS TO BE NETWORKED Use Campus Sensor Network Input Outside Air Temperature OAT Use Campus Sensor Network Input Outside Air Relative Humidity OAH Contact Provided and Mounted by Div 16 Safety Fire Alarm System Input FIREALARM Manual Reset Low Limit Switch, Set at 40°F Safety Freezestat FRZSTAT Damper Endswitch. Set at approximately 75% Open Safety Damper Endswitch ENDSWITCH Manual Reset Air Press. Switch, Set at 4" Safety High Pressure Switch HIPRESS Manual Reset Air Press. Switch, Set at 2" Safety Low Pressure Switch LOPRESS Damper and 120V Actuator Safety Outside Air Damper OAD SAFETIES Field Device Type Description Point Name
- 26. Step 4: Develop an Input / Output Summary (Points List) Contact Closure from Damper Endswitch DI Outside Air Damper Position OAD Air Differential Pressure Sensor, 0 - 2" AI Filter Pressure Drop FILTERDP Flexible Averaging Thermistor AI Heating Discharge Temperature HEATTEMP MONITORING POINTS Relay (refer to interlock drawing) DI AHU Safety Status Alarm SAFETY Current Sensing Relay DI Supply Fan Status SFANST Relay Contactor DO Supply Fan Start/Stop Command SFANSS Electric 2-position Actuator DO Heating Valve Command HWV Electric Spring Return Actuators AO Face and Bypass Damper Command FBDMPR Valve with Electric Spring Return Actuator AO Chilled Water Valve Command CHWV Flexible Averaging Thermistor AI Supply Fan Discharge Temperature DAT POINTS REQUIRED FOR CONTROL Field Device Type Description Point Name
- 27. Step 4: Develop an Input / Output Summary (Points List) Fan Off and Damper Open for 4 Min X Outside Air Damper Position OAD 15 Min X Filter Pressure Drop FILTERDP 15 Min X Heating Discharge Temperature HEATTEMP MONITORING POINTS Change of State X 1 Min X AHU Safety Status Alarm SAFETY Change of State X Fan Failure, 1 Min X Supply Fan Status SFANST Change of State X Supply Fan Start/Stop Command SFANSS 15 Min X Heating Valve Command HWV 15 Min X Face and Bypass Damper Command FBDMPR 15 Min X Chilled Water Valve Command CHWV 15 Min X 5°F > or < than setpoint X Supply Fan Discharge Temperature DAT POINTS REQUIRED FOR CONTROL Period Trend? Alarm Rule and Delay Alarm? Description Point Name
- 29. Step 5: Define Graphics Requirements
- 31. Step 6: Identify Locations of Control Components
Notas do Editor
- Good morning! Thank you for coming.
- What do we mean by Temperature Control Systems? The industry has been around as long as buildings have been heated or ventilated. These systems started as simple a man with a shovel ~ shoveling coal or throwing wood onto a fire. This system is obviously not very sophisticated and is prone to failure and is expensive to operate (by today’s standards) Imagine the labor expense associated with keeping an office building warm if temperature is controlled by a man with a shovel. Control probably wouldn’t be very precise either… Unless the consultant was helping shovel coal, his role wasn’t very important. Fortunately, we’ve come a long way since then. Since these early days, control has become more sophisticated. Engineers (consultants) figured out that electromechanical controls could control mechanical systems safely and accurately, and as bonus, didn’t require a person to constantly watch and regulate the process. Using the boiler example mentioned before, if you look back in time you’ll find that boiler explosions were common. Oops, I shoveled too much coal… When was the last time you heard about a boiler explosion?
- A lot has happened over the past 100 years in terms of temperature control. The systems evolved from simple electromechanical temperature switches (such as on your household thermostat) to compressed air powered pneumatic controls to the digital era. As technology has become more affordable and more accurate, the HVAC industry has followed the technology. Coincidentally, with the increased capabilities of the newest temperature control systems, engineers are constantly pushing the envelope in attempting to design higher performance systems. Engineers attempt to control every parameter of operation at the critical edge in attempt to extract every possible BTU of heat from each unit of gas or to squeeze every possible ton of cooling from each KW of electricity. This technological shift has undoubtedly helped its users. Water chillers are a good example of this. Just 20 years ago, engineers, contractors and owners were deathly afraid of reducing water flow through the evaporator barrel of a chiller. This practice is becoming common place today. With high performance electronic controls, chiller manufacturers now offer significantly tighter control while using less energy and without freezing the evaporator.
- So, with all this technological improvement, what’s the problem. Why are owners frequently complaining that “the controls aren’t working” or “I’m hot” or “I’m cold” or “the doors are standing open” or “these controls cost too much”? Why do owner’s revert to manual control a week after sending the contractor final payment? Clearly there are many causes for this these problems, an likely, many of the problems are inter-related. So, where does the blame lie? Commonly it’s shared by the Owner, Contractor and Consultant. One of my clients frequently reminds his employees that temperature control systems aren’t any smarter than people, the controls just happen to have a lot of time on their hands.
- Owners frequently are their own worst enemy. Clearly an engineer or control contractor’s best work can be rendered useless by an Owner who indiscriminately overrides control points or modifies programming. For the cost of the components that make up the temperature control system, it is in the Owner’s best interest to be trained on their system. ~ By system I mean the control system and the mechanical system that is being controlled. Contrary to popular belief, you can’t do everything with controls. Defining site standards are important so that engineer working on high project understands if the site has settled on a particular controls manufacturer and has any other standard that would affect the specification. At WUSM for instance, there are only two manufacturer’s controls that are welcome on site. One last important point to make in reference to this slide, engineers and contractors are quick to point out that “The Owner screwed it up by overriding all the points”. Likely, 6 times out of 10, the owner used overrides because the system didn’t work ~ whose fault is that?
- The other favorite whipping boy… The Control Contractor frequently is also blamed for many of the problems experienced with control systems. As part of my work, I frequently am able to review installations and controls programming. In this work, I see that contractors fail to implement SOO’s as specified. There are times I’d swear that the contractor never looked at the specified sequence. I see control valves and dampers that move backwards. I also see that control loops are not tuned properly such that response times are very slow or are out of control. It is not uncommon to find the controls technician on the job site preparing the programming and graphics while sitting on a makeshift shelf built from electric cable spools. Clearly the human mind is not at it’s fullest potential in this arrangement. An important reminder though, just as often as not, the control contractor does it right and your system still doesn’t work. Whose fault is it now?
- Many times, the consultant screws it up too… Two weeks ago I visited a job site where the engineer specified a low pressure switch to be installed on three air handling units. The purpose of this safety device is to stop the unit fan so that ductwork isn’t collapsed if a damper fails to open. The consultant’s drawings clearly indicated that the sensor be located downstream of the unit filters. Guess what, the switch tripped as the filters loaded. I think I was able to talk the control contractor into fixing the mistake without charge… I won’t make that mistake again… In reviewing the work of others, I find that the temperature control systems are generally poorly defined. The sequence of operation is poorly written or doesn’t exist. Also common are sequences that can not possibly work. Harry D. will speak to this in his talk. Out of the realm of mistakes and onto the realm of consultant decisions that are made that contribute to poor performance or lack of owner satisfaction ~ The consultant gold-plates the job making future maintenance more expensive or fails to specify components that make maintenance operations and diagnostics more difficult. Finally, out of pure misunderstanding of the bid process or ignorance of the work required to integrate control systems, the consultant specifies interoperability that is expensive and difficult to implement (and usually doesn’t provide a value equal to the first cost). Mention Aspen Rec Center. So now we understand how it all gets screwed up. How do we work to prevent us from being the source of the problem? How do we best serve our client while achieving the level of control necessary for the high performance system that we are designing? How do we end up with a project that the Owner, Controls Contractor and Consultant are proud to call their own? As we continue this presentation, I will be describing the steps I believe are important for the consultant to take to achieve success in specifying controls. If the consultant does his or her job successfully, the chance that the control contractor will be able to successfully implement the system and that your Client will achieve the benefits of their investment improve significantly. We certainly don’t want the Owner to feel the need to shovel coal after spending thousands and thousands of dollars on a state-of-the-art control system…
- I learn best by looking at examples. With that is mind, we are going walk-through the preparation of construction documents for a temperature control system. As I and others see it, the steps to perform a temperature control design are listed on the slide here. It is also important for me to mention that ASHRAE Guideline 13, Specifying Temperature Controls is a decent guideline to follow for this process. This document also has a guide specification that is relevent (albiet a little behind the times in the changing world of temperature controls)
- The example we are going to be working through is a fraternity house project. This 24,000 sqft facility serves 65 students. There is a full kitchen, a dining hall, recreation rooms and 25 sleep/study rooms. The basement and first floor are conditioned with a VAV system with a relief fan. Stairwells are heated (but not cooled). The 2 nd and 3 rd floors are served by fan coil units and a heating and cooling makeup air system for ventilation. On the second and third floor there is also a central study room that has no exposure and will require year round cooling. These rooms are served by VAV boxes attached to the makeup air unit. The building will be maintained by college maintenance staff. Monitoring must be conducted remotely.
- Starting from the beginning, what systems need to be controlled: Heating water system Chilled water system Domestic hot water system Unit Heaters Fan coil units VAV boxes Air handling units Exhaust fans Sump Pump Exterior lighting Somewhere in the contract documents, a control system needs to be defined. The two system we are going to concentrate on are the unit heaters and the makeup air unit that serves the upper floors of the building. 1 st ~ here’s a flow diagram of the unit heater. Simple system. It has a fan and a control valve. Next ~ Here is a simplified flow diagram of the makeup air system. Note that I’ve included the exhaust fan that serves the 2 nd and 3 rd floors on the same diagram. We haven’t included the fan coil units on this drawing because I don’t intent to utilize the makeup air unit to attempt to control space temperature (50 cfm only goes so far). A secondary goal of this exercise is to create a drawing that would be appropriate on the operator workstation graphics. Of course we’ve only gone through two of the systems we identified. Clearly this needs to be done for every single system.
- READ THE SLIDE Somewhere in the contract documents, a control system needs to be defined. The two system we are going to concentrate on are the unit heaters and the makeup air unit that serves the upper floors of the building. The unit heater is hardly worth the flow diagram. What you have is a fan and heating coil to control. Next ~
- Here is a simplified flow diagram of the makeup air system that serves the sleep/study rooms. WALK THROUGH THE SYSTEM A secondary goal of this exercise is to create a drawing that would be appropriate on the operator workstation graphics. We’ll get to that later. Of course we’ve only gone through two of the systems we identified. But this work needs to be done for every single system.
- Hard-wired controls ~ what do I mean by this. This is basically a way of saying that the control function is not subject to override by an operator and will function no matter how the operator chooses to manually override the DDC controls. We use hard-wired controls for many reasons. Two reasons we will explore here is cost and safety. For our example of the unit heater, I choose to control this system by an electromechanical thermostat (nothing any more complicated than your household thermostat. To accomplish this, we required that the control contractor to provide a 120V thermostat and control valve for the electrical contractor to install. To properly specify this, we put a simple wiring diagram on the temperature controls drawing and on the electrical drawings for the project. This way all parties are able to identify scope. So ~ why control this hard-wired instead of using a building automation system? Cost and no need for monitoring. These systems serve space that are not at risk for freezing. Additionally, the Owner wasn’t interested in central monitoring for these spaces. Now let’s take a look at the makeup air unit. Here we’ve got a fan rated at 4 inches pressure with deadhead capability of 6 inches. We’ve got an inlet damper that opens whenever the unit is online. Some other facts, the ductwork is specified to be 2” pressure class at the inlet of the unit and 4” pressure class on the outlet. The heating water system and chilled water system are water based systems. Protecting this system from catastrophic failure would include hard-wired safeties for high pressure (at the fan discharge), low pressure on the inlet ductwork, coil freeze protection and fire alarm system failure (which is code mandated). Additionally, do you want to give the system the chance of starting with the inlet damper closed? My vote would be no. So for this, we need to interlock fan control (in automatic control and in hand) with the damper control. Here’s a copy of the flow diagram we put together in step 1. I’ve added requirements for the safety switches and components required to implement the intended function.
- Here’s a copy of the flow diagram we put together in step 1. I’ve added requirements for the safety switches required to implement the intended function.
- There are some who are comfortable specifying this hard-wired controls with words. A picture is worth a thousand words. For this reason I like to show the safeties on the flow diagram like was shown on the previous slide. Also, I prefer to describe the operation of the safety with a diagram. Here’s one of two wiring diagrams that were used to specifiy these interlocks.
- There are some who are comfortable specifying this functionality with words. However, in this instance, I believe a picture is worth a thousand words. Here’s one of two wiring diagrams that were used to specifiy these interlocks… Step through slide Aside from the occasional mistake in setting the normal open position of a damper actuator, I’ve had good luck getting this implemented. Here’s the second wiring diagram used for this makeup air unit. Like the unit heater example we already went through, this system also requires coordination between the electrical contractor and the controls contractor. The intent of this drawing is that is be placed on the electrical drawings and on the mechanical / temperature controls drawing.
- There are some who are comfortable specifying this functionality with words. However, in this instance, I believe a picture is worth a thousand words. Here’s one of two wiring diagrams that were used to specifiy these interlocks… Step through slide Aside from the occasional mistake in setting the normal open position of a damper actuator, I’ve had good luck getting this implemented. Here’s the second wiring diagram used for this makeup air unit. Like the unit heater example we already went through, this system also requires coordination between the electrical contractor and the controls contractor. The intent of this drawing is that is be placed on the electrical drawings and on the mechanical / temperature controls drawing.
- There are some who are comfortable specifying this functionality with words. However, in this instance, I believe a picture is worth a thousand words. Here’s one of two wiring diagrams that were used to specifiy these interlocks… Step through slide Aside from the occasional mistake in setting the normal open position of a damper actuator, I’ve had good luck getting this implemented. Here’s the second wiring diagram used for this makeup air unit. Like the unit heater example we already went through, this system also requires coordination between the electrical contractor and the controls contractor. The intent of this drawing is that is be placed on the electrical drawings and on the mechanical / temperature controls drawing.
- Identify how each component of the system is controlled. Only describe how something is controlled once. Chilled water valve control shouldn’t show up five times in a sequence. This makes for messy programming and less predictable performance. It’s also very difficult for anyone else to interpret your intent. Do not confuse hard-wired logic with programmed logic. By splitting hard-wired logic out of the sequence, the logic is simplified and more focused for the technician or engineer who is programming your system. Identify time schedule for operating equipment. Determine trigger point for starting and stopping logic (local project loops started based on command ~ fan started without command caused freezestat failure) Consider what happens when equipment is offline. (local campus ~ all AHU control valves continue to operate when unit is offline ~ screws up chilled water system) Consider part load scenarios Attempt to not rely solely on outside air temperature for decision making. The last thing an owner wants is every piece of equipment turning off when someone lifts a wire during the middle of the summer. For most functions, logic can be implemented without relying on this piece of information. Keep it simple (supply fan pressure reset, equal runtime, supply temperature reset all sound like great ideas in optimizing system performance. However, for a system with 100 VAV boxes, the odds that supply fan pressure reset will do something good is pretty small (especially if interior zones are served by the system). The increased complexity of the programming only muddies the water. Sequence to eliminate possibility of simultaneous heating and cooling. With pneumatic controls, it was common to control mixed air temperature by modulating the economizer dampers. The heating and cooling valve would be tied to another controller that would sequence both valves together (hopefully such that both valves weren’t open at the same time). The contractor would set the setpoint on the economizer receiver/controller to 50°F so that the supply air wouldn’t be overheated. This controlled assured that the heating water valve was open (trimming the control) for half the year (even though if properly controlled, the economizer would be able to maintain the exact temperature required). Because of some of the limitations of proportional only pneumatic controls, controlling the economizer in sequence with the heating and cooling valves was rarely (if ever) done. With DDC, the logic is often times implemented is much the same way. And like the pneumatic controls that proceeded DDC, the heating valve spends much of the year heating when heat truly would not be required if the control was set up properly. Setting it up to prevent this inefficiency requires sequencing the heating valve, economizer and cooling valve together electronically on one control loop. Avoid control modes (ie summer / winter modes ~ if used, a definition must be provided to tell the programmer when this mode applies. It must tell the programmer when to start the mode and when to stop the mode.
- Identify how each component of the system is controlled. Only describe how something is controlled once. Chilled water valve control shouldn’t show up five times in a sequence. This makes for messy programming and less predictable performance. It’s also very difficult for anyone else to interpret your intent. Do not confuse hard-wired logic with programmed logic. By splitting hard-wired logic out of the sequence, the logic is simplified and more focused for the technician or engineer who is programming your system. Identify time schedule for operating equipment. Determine trigger point for starting and stopping logic (local project loops started based on command ~ fan started without command caused freezestat failure) Consider what happens when equipment is offline. (local campus ~ all AHU control valves continue to operate when unit is offline ~ screws up chilled water system) Consider part load scenarios Attempt to not rely solely on outside air temperature for decision making. The last thing an owner wants is every piece of equipment turning off when someone lifts a wire during the middle of the summer. For most functions, logic can be implemented without relying on this piece of information. Keep it simple (supply fan pressure reset, equal runtime, supply temperature reset all sound like great ideas in optimizing system performance. However, for a system with 100 VAV boxes, the odds that supply fan pressure reset will do something good is pretty small (especially if interior zones are served by the system). The increased complexity of the programming only muddies the water. Sequence to eliminate possibility of simultaneous heating and cooling. With pneumatic controls, it was common to control mixed air temperature by modulating the economizer dampers. The heating and cooling valve would be tied to another controller that would sequence both valves together (hopefully such that both valves weren’t open at the same time). The contractor would set the setpoint on the economizer receiver/controller to 50°F so that the supply air wouldn’t be overheated. This controlled assured that the heating water valve was open (trimming the control) for half the year (even though if properly controlled, the economizer would be able to maintain the exact temperature required). Because of some of the limitations of proportional only pneumatic controls, controlling the economizer in sequence with the heating and cooling valves was rarely (if ever) done. With DDC, the logic is often times implemented is much the same way. And like the pneumatic controls that proceeded DDC, the heating valve spends much of the year heating when heat truly would not be required if the control was set up properly. Setting it up to prevent this inefficiency requires sequencing the heating valve, economizer and cooling valve together electronically on one control loop. Avoid control modes (ie summer / winter modes ~ if used, a definition must be provided to tell the programmer when this mode applies. It must tell the programmer when to start the mode and when to stop the mode.
- Identify how each component of the system is controlled. Only describe how something is controlled once. Chilled water valve control shouldn’t show up five times in a sequence. This makes for messy programming and less predictable performance. It’s also very difficult for anyone else to interpret your intent. Do not confuse hard-wired logic with programmed logic. By splitting hard-wired logic out of the sequence, the logic is simplified and more focused for the technician or engineer who is programming your system. Identify time schedule for operating equipment. Determine trigger point for starting and stopping logic (local project loops started based on command ~ fan started without command caused freezestat failure) Consider what happens when equipment is offline. (local campus ~ all AHU control valves continue to operate when unit is offline ~ screws up chilled water system) Consider part load scenarios Attempt to not rely solely on outside air temperature for decision making. The last thing an owner wants is every piece of equipment turning off when someone lifts a wire during the middle of the summer. For most functions, logic can be implemented without relying on this piece of information. Keep it simple (supply fan pressure reset, equal runtime, supply temperature reset all sound like great ideas in optimizing system performance. However, for a system with 100 VAV boxes, the odds that supply fan pressure reset will do something good is pretty small (especially if interior zones are served by the system). The increased complexity of the programming only muddies the water. Sequence to eliminate possibility of simultaneous heating and cooling. With pneumatic controls, it was common to control mixed air temperature by modulating the economizer dampers. The heating and cooling valve would be tied to another controller that would sequence both valves together (hopefully such that both valves weren’t open at the same time). The contractor would set the setpoint on the economizer receiver/controller to 50°F so that the supply air wouldn’t be overheated. This controlled assured that the heating water valve was open (trimming the control) for half the year (even though if properly controlled, the economizer would be able to maintain the exact temperature required). Because of some of the limitations of proportional only pneumatic controls, controlling the economizer in sequence with the heating and cooling valves was rarely (if ever) done. With DDC, the logic is often times implemented is much the same way. And like the pneumatic controls that proceeded DDC, the heating valve spends much of the year heating when heat truly would not be required if the control was set up properly. Setting it up to prevent this inefficiency requires sequencing the heating valve, economizer and cooling valve together electronically on one control loop. Avoid control modes (ie summer / winter modes ~ if used, a definition must be provided to tell the programmer when this mode applies. It must tell the programmer when to start the mode and when to stop the mode.
- It is hard me to convey how important it is to have a sequence of operation that works. Although it is technically very easy to change programming, the time and effort required to get all the right players at the same place at the same time to work out the problems is worth avoiding. To this end, for some projects, we have begun to simulate control sequence’s. The goal of this work is to be able to more exactly describe the sequence and know that it will work. With the software that we use, we are able to construct logic blocks that mimic the logic that most control contractors are able to utilize in building programming. We are able to test what happens as temperatures rise and fall and are able to see if actuators will sequence as intended. We used this to develop lead/lag logic for a chiller plant. You’d be surprised how complicated it can get. Another benefit of this technique is that you are limited to implement logic that available. Soft and fuzzy general statements will not work. In addition gaining a better understanding of how the logic will function, we’ve been successful selecting control loop tuning parameters. This software enables the user to build a simulation of the system being controlled as well as building the control system. The advantage of this is time. The time required to properly tune a control loop in the field is significant.
- Here’s a sample of the output from Simulink. If your brave enough, you can issue this with your specifications as the sequence of operation. After the control contractor determines what block of theirs is equal to our blocks, he / she is able to implement the logic easily.
- In the instance of the hard-wired safeties, these are shown only so that I can communicate to the contractor what kind of field device is required. If specified like this, it is important to note that these individual points are not monitored by the DDC system. In order to accomplish that functionality, more relays would need to be added to the wiring diagram I already displayed. Bear in mind that doing so will increase first cost. I’d only monitor every point if an Owner requested it.
- If you look hard at your own projects, you would be surprised how many operator workstation graphics do not display the correct system. Imagine the confusion the operators would feel if they trusted the graphic and it was patently wrong.