Controls

1. Discuss two ways to effectively control a medium sized heating system that has a heat demand of 150 kW and a hot water requirement of 20 kW.

Show your proposals on suitable drawings.

The heating system comprises 2 boilers and 4 heating zones, one of which is the primary flow and return to the hot water cylinder.

Single, in-line pumps are to be used.

2. Discuss some of the advantages and disadvantages of using a BEMS system in a large hospital complex.

3. Draw, on a suitable schematic diagram, a control system for a large multi-storey office building.

The control system is for the heating installation.

Duplicate pumps should be utilised.

Two boilers should be used.

The control system should have at least four pipe circuits or zones.

At least one circuit should be for convective heating.

At least one circuit should be for heating with radiators.

At least one circuit should be to an indirect coil in a hot water cylinder.

The main purpose for the drawing in this task is to enable commissioning engineers to balance the systems.

The drawing should show all necessary items to automatic control and commissioning of the heating system.

Make assumptions where necessary.

4. Compare the system of control in the experiment with three term fully modulating control systems.

What are the main differences between these types of control?

What type of system do you think is best for most building services applications and why do you think it is best?

5. Compare the system of control in the experiment with a pneumatic system.

What criteria would guide you in the choice between a pneumatic control

system and an electrical control system?

MARKING CRITERIA

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Mark Criteria

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Pass Most calculations are correct.

All definitions are generally correct

Discussion covers all main topics.

Demonstrate correct use of formulae

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Merit All calculations are correct.

All definitions are correct

Appropriate methods are discussed in sufficient detail.

Diagrams are used to back up discussion.

Diagrams are presented neatly.

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Distinction All calculations are correct.

Appropriate methods are discussed in detail giving

examples and typical methods.

All definitions are correct

Correct choice of equipment.

Relevant research is demonstrated in answers.

Diagrams are neatly presented and fully annotated.

Answers demonstrate understanding of processes.

Appropriate depth of understanding of topic.

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Answer by G. Goodfellow

1. Discuss two ways to effectively control a medium sized heating system that has a heat demand of 150 kW and a hot water requirement of

20 kW.

Show your proposals on suitable drawings.

The heating system comprises 2 boilers and 4 heating zones, one of which is the primary flow and return to the hot water cylinder.

Single, in-line pumps are to be used.

Task 1

Part “A” – Effective control of medium sized heating system utilising conventional controls.

For this mode of control also refer to Figures 1, 2 & 3.

Each circuit, whether radiator or DHW, has a master timeswitch for selection of start/stop times as required.

In these days of energy conservation, the amount of system equipment required is virtually the same regardless of the types of HVAC controls selected as the same job can be done by the differing systems. As it will be observed, a Motor Control Panel (MCP) has been utilised for the connection of cables, neatness, and the incorporation of interlocks etc.

With a conventional control system there are a lot more hardwired connections and interlocks incorporated into the MCP and these will be described later. It is assumed that each boiler safety circuit is incorporated i.e. limit stat and lock-out.

With the presence of a DHW primary pump and diverting valve, I have assumed that this will also act as a boiler shunt system to prevent rear end thermal shock as it is a Constant Temperature (CT) type circuit.

For this type of control, I have selected a proportional controller with the main temperature detector located in the common return header.

Heating zones 1 – 3

Each zone has a master timeswitch which can be set to whatever schedule is deemed necessary for heating occupancy. Each system should be set to switch on at least 1 hour before occupancy to allow for initial warm-up period.

Each timeswitch controls a heating pump with an internal electrical interlock to operate the boiler sequencing system.

Each compensator controller has the following devices connected.

1. Outside temperature detector

2. Immersion flow temperature detector

3. Actuator on 3-port mixing valve.

The compensators would have the following settings

1. Origin – 25’c

2. Ratio – 2.5

The origin is the temperature of the water in the radiator circuit when the outside air temperature is 20’c.

The ratio is the amount by which the temperature of the water in the radiator circuit will increase or decrease if the outside air temperature rises or falls by 1’c. Therefore at an outside air temperature of 10’c, the flow temperature would be 25 + (10*2.5) = 50’c

DHW system

In this system there are 3 no. circulating pumps utilised.

1. Primary pump – to circulate the boiler water either through the heating coil or diverted straight into the boiler return header.

2. Secondary return pump – to circulate temperature controlled water throughout the secondary pipework system.

3. De-strat or de-stratification pump – to pump secondary water from the top of the cylinder to the bottom. This has a two-fold effect, to prevent legionella bacteria build-up in the lower cooler water and to provide a fuller cylinder of equally temperature controlled water.

The DHW primary pump also acts as a heat dissipation pump. Heat dissipation runs for approx. 30 mins after heating and DHW timeswitches have switched off.

Heat dissipation is to prevent the boilers entering a high limit situation after shutdown of the heating circuits.

The secondary and de-strat pumps are interlocked with the primary pump to provide an over-run period of 30 mins after the primary pump has switched off. This is for cleaning staff to have hot water after occupancy.

The control of the water temperature is achieved by means of an immersion thermostat in the DHW cylinder that is connected to the actuator on the 3-port diverting valve. The interconnections for this system are done locally to the cylinder.

Boiler sequencing.

As detailed above, the boilers are always on stand-by for whatever heating or DHW circuit is programmed to operate.

The controller used is a proportional type that in turn is operating a 2-stage step controller.

With boiler sequencing, the lead boiler back-end valve is always open to provide a circulation of water. This interlock is done through the boiler sequence selection switch mounted on the MCP.

With a 2-stage step controller, the 2^nd stage back-end valve and lead boiler is controlled.

The step controller has settings that is voltage related and the stages can be switched off at various voltages from the main controller.

The mode of operation is as follows.

When the boilers are not required and the flow temperature drops, the controller will open both back-end valves for the next days operation.

Upon activation of the boilers, both boilers will switch on due to the back-end valves being opened. After a period of time the flow temperature will rise significantly enough for the controller to start “ramp down”.

With the control detector being located in the return header, we are assuming that the controller has a setpoint of 85’c and a Proprtional band (Pb) of 10’.

When the flow temperature reaches 75’c, the controller will start to ramp down.

In this example, what happens is that the controller will have an output of 10vdc at flow temperatures less than 75’c, once this temperature is exceeded, the voltage starts to drop.

A 2-stage step controller will do the following.

1. At approx 5 volts, the lag boiler back-end valve will be switched to close. Within the back-end valve actuator there is an auxilliary switch that will disable the burner control circuit. The time taken for the valve to close down will allow for a degree of heat dissipation from the lag boiler.

2. After the lag boiler has switched off, the lead boiler still continues to fire and therefore increase the flow temperature. At approx. 1.5 – 2 volts, the lead boiler burner circuit is disabled switching the boiler off. The boiler will stay off until the voltage reaches approx. 2.5 – 3 volts. This will cut down on dry-cycling.

Frost protection

In a conventional controls system, it is advisable to incorporate hard-wired frost protection. In this example, 2-stage frost protection has been catered for.

Stage 1 occurs when the outside air temperature drops below 0’, upon this occurrence all heating zone valves open and all pumps bar the de-strat pump switch on to circulate water only. The DHW valve may or may not open due to the water temperature in the cylinder.

Stage 2 occurs when the water in the return header falls below 6’c, upon this occurrence, both back-end valves open and both boilers fire to increase the return water temp. Stage 2 should always open valves and switch on pumps even if stage 1 frost has not occurred.

Part “B” – Effective control of medium sized heating system utilising BEMS controls.

For this mode of control also refer to Figures 4, 5 & 6.

Each heating circuit is controlled via a software optimiser for selection of start/stop times as required.

The DHW system is controlled by a software timeschedule.

The items of plant are similar to the conventional controls scheme except for the addition of room temperature detectors for the optimiser and internal frost protection. As with the conventional system, there is an MCP, however, with BEMS all cables are terminated in the MCP and controlled by the BEMS. There are no local DHW circuits in this example.

All interlocks are done through software programming and not hardwired. The only hardwired links are the back-end valve/burner control circuit and the boiler safety circuits.

With BEMS, there are a lot less hardwired connections as interlocks are incorporated into software programming as will be described later.

For this type of control, I have located the main temperature detector located in the common flow header.

Heating zones 1 – 3

Each zone has a master timeschedule with accompanying optimiser programme which can be set to whatever schedule is deemed necessary for heating occupancy. The optimiser will automatically calculate the run-up period depending on the initial parameters and maximum period allowed for run-up.

Each optimiser controls a heating pump with an internal software interlock to operate the boiler sequencing system.

Each zone is weather compensated with circuit flow temperature being dependent upon the outside temperature. The external temperature detectors are located on a North facing wall to minimise solar gains and therefore depressing the heating system capabilities unnecessarily. With BEMS, room influence can be added in that if the room setpoint is not achieved, the system will increase the flow temperature and inversely, if the room temperature is above the setpoint, the system will decrease the flow temperature.

The origin and ratio settings are similar to those used in conventional controls except that they are now located within software programes.

DHW system

In this system there are 3 no. circulating pumps utilised.

1. Primary pump – to circulate the boiler water either through the heating coil or diverted straight into the boiler return header.

2. Secondary return pump – to circulate temperature controlled water throughout the secondary pipework system.

The DHW primary pump also acts as a heat dissipation pump. Heat dissipation runs for approx. 30 mins after heating and DHW optimisers have switched off.

Heat dissipation is to prevent the boilers entering a high limit situation after shutdown of the heating circuits.

The secondary and de-strat pumps are interlocked via software with the primary pump to provide an over-run period of 30 mins after the primary pump has switched off. This is for cleaning staff to have hot water after occupancy.

The control of the water temperature is achieved by means of an immersion temperature detector in the DHW cylinder connected to the outstation, in turn, the actuator on the 3-port diverting valve is also connected to the outstation.

Boiler sequencing.

As detailed above, the boilers are always on stand-by for whatever heating or DHW circuit is programmed to operate.

The control mode being used is proportional + integral and all stages are connected to the outstation.

With boiler sequencing, the lead boiler back-end valve is always open to provide a circulation of water. This interlock and boiler rotation is also catered for through software.

Control is similar to the 2-stage step controller, except that with BEMS, it becomes a 3-stage system, however all 4 components can be controlled, i.e 2 x burners and 2 x back-end valves.

The mode of operation is as follows.

When the boilers are not required and the flow temperature drops, the controller will open both back-end valves for the next days operation.

With the control detector being located in the flow header, we are assuming that the outstation has a setpoint of 75’c and a Proprtional band (Pb) of 10’.

When the flow temperature reaches 65’c, the BEMS will start to ramp down.

In this example, what happens is that the software programme will decide at which point the sequencing will occur.

1. At approx 66 – 68’c, the lag boiler will be switched off.

2. At approx 69 – 71’c, the lag boiler back-end valve will close, please note, in the back-end valve actuator there is an auxilliary switch that will disable the burner control circuit.

3. At approx 73 – 75’c, the lead boiler will be switched off.

4. The boiler will stay off until the temperature drops to 72’c. This will cut down on dry-cycling.

Frost protection

In a BEMS controlled system, it is advisable to incorporate hard-wired frost protection but this is not always the case. In this example, 2-stage frost protection has been catered for.

2. Discuss some of the advantages and disadvantages of using a BEMS system in a large hospital complex.

Task 2

Advantages and disadvantages of BEMS installed in a large Hospital complex.

Advantages

1. The central supervisor may be located in an office remote from the site. This supervisor, provides the means by which to monitor various elements of heating, ventilation and air conditioning utilised throughout the whole site.

2. Various setpoints for heating, cooling and humidity can be altered via the supervisor without the need to visit different plantrooms.

3. The supervisor provides on-screen graphics of all the various plant which can assist in diagnosing system faults.

4. Maintenance personnel by interrogating the supervisor can save valuable time by not going to various plantrooms if the problem can be remedied from the central supervisor.

5. Sequencing of boiler and pump duties can be programmed for daily/weekly/monthly/hours run basis dependent on plant use.

6. Greater control can be achieved for areas that have abnormal occupancy hours.

7. Various alarms can be generated through the controllers back to the supervisor. These alarms may include fire, flood, boiler lockout, high/low limit, high/low pressure and pump/fan failure to mention a few. These alarms can also be logged for record purposes.

8. Plant logs can be set up and manipulated for energy conservation purposes.

9. Proper management of BEMS can provide substantial financial savings on fuels.

10. Plants can be reprogrammed in various ways to achieve the best mix of comfort and energy conservation.

11. Optimiser logs can be set up and reviewed for plant performance analysis.

12. Changes in BEMS can only be achieved by 1^st logging on with a defined authorised users password. Any changes are then recorded for review.

13. The site can be remotely view from an Engineers home via modem link during out-of-hours calls.

14. Usage of utilities such as gas, water & electricity can be monitored for billing purposes.

15. Load shedding can be programmed in should the mains electricity fail and the stand-by generator starts to operate. Load shedding is a means whereby the generator is not overloaded.

16. The BEMS usually incorporates a back-up battery system in each controller to maintain programmed functions whilst the stand-by generator is starting up after mains electricity failure.

Disadvantages.

1. The initial financial outlay on the installation of a large BEMS can be quite substantial restrictive.

2. Only approved agents/contractors are capable of working at the controllers/software.

3. Approved agents/contractors can be expensive on an hourly basis with some rates being up to £50.00/hr.

4. Maintenance agreements can be expensive to operate depending on the agreed coverage.

5. Continual training is essential in the ever changing worlds of computer operated BEMS.

For a large Hospital site, the advantages of BEMS outweighs the disadvantages, however, a BEMS must be properly managed to provide the advantages as detailed above, if not, the system can become a glorified timeswitch.

On a hospital site, there are so many important departments such as X-ray, operating theatres, pharmacies & CSSD. These areas depend on steady temperatures and humidities and the installation of a BEMS can ensure the safe and steady operation of plant.