BUILDING ENERGY MANAGEMENT SYSTEM - BEMS
PLANT CONNECTIONS AND CONTROLS:
Single or multiple boilers should always be connected to either a mixing header or separate flow and returns headers. This allows heating circuits to be independently connected to the boiler plant and facilitates the maintenance and breakdown of individual circuits and boilers. It is for this reason and indicator of good engineering practice.
Heating systems with CVVT (Constant Volume Variable Temperature) control are particularly subject to low return temperatures in mild weather. This can have a detrimental effect on the boiler by inducing condensation in the flue gas. Low return temperatures can also cause differential expansion within some boilers and consequent stress in the boiler metal. A by-pass protection is necessary so when the thermostat senses low temperature water in the boiler return, it energizes the pump and cuts it out on a predetermined higher temperature.
Condensing boilers are specifically designed to operate on low return water temperature. In the process, further sensible and latent heat of condensation from the flue gas are given up, therefore increasing the thermal efficiency of the boiler.
Temperature controls
In HVAC systems the heat emitter must be sized to meet the peak heating load (at design conditions) of the room. This involves determining the design steady-state heat loss rate and applying a plant ratio factor to allow for the thermal capacitance of the structure where intermittent heating is used.
The emitter will be required to provide full heat output at start-up, but solar gains and internal gains due to occupants, lighting and equipment will quickly reduce the heat load.
It is essential that the output rate of the emitter can be efficiently controlled both to ensure thermal comfort and to reduce energy consumption.
The output rate of any heat emitter can be achieved in one of two ways: modulating the water flow rate whilst maintaining the flow water temperature or modulating the flow water temperature whilst maintaining the water flow rate.
Modulation of water flow rate:
This is usually achieved by means of a two-port or three-port valve which throttles the water flow rate in response to a room temperature sensor. For radiators and radiant panels, thermostatic radiator valves (TRV) are invariably used. These are low cost and self-actuating valves requiring no wiring. Be aware that the response curve of the system is highly non-linear and such non-linearity requires a valve characteristics which will produce a large reduction in flow rate for a small valve stem movement, but they are relatively expensive.
Flow temperature modulation:
Flow temperature modulation is achieved by the blending of return water with the flow using a three-port mixing valve. The response of this control is very nearly linear, so good control is achieved. Unfortunately, such valves and control systems are relatively expensive so this system is used for controlling large groups of emitters.
Combined flow rate and temperature modulation:
Good control is achieved by combining the two methods. Flow temperature modulation can be carried out centrally by scheduling it to outdoor air temperature. Local two-port valves, controlled by the local room temperature, then modulate the water flow rate to each emitter in response to changing solar and internal heat gains. This combination of central and local control is specified in the Building Regulations.
Control systems in building energy management systems
Traditionally many building services systems are controlled using either pneumatics or electric/electronic and mechanical devices such as the five elements in CVVT control: valve body, valve actuator, immersion thermostat, outdoor detector and controller.
Direct digital control and supervisory control can be more user-friendly and can give the user more control over the building services systems either locally or remotely via a modem to a building energy management system (BEMS). The capital costs and advantages of a BEMS depend upon whether the user has the time and commitment to use this facility and take full advantage of the technology.
The local area network (LAN) might include BEMS outstations or original equipment manufacturer's (OEM) outstations. This would be linked to a modem if the final control and monitoring location is remote. Software is generated and dedicated to operate the controls and relay system conditions such as temperature, relative humidity, pressure, pressure drop, and status such as duty plant operation, standby plant operation. These conditions can be called up on a visual display unit (VDU) or monitor, and will include system logic diagrams. The way a BEMS is connected to a LAN is called the topology, of which there are basically three: bus, star and ring.
The keyboard and central processing unit (CPU) complete with visual display unit (VDU), collectively called a HMI Central Station, are connected to the LAN via modem.
Outstation functions:
These are split in three levels:
1. High Level: remote communication, user interface, optimizer control, cascade control, maintain trend logs, maintain event logs.
2. Mid Level: proportional plus integral plus differential control (PID), main data control (MD), alarm check, alarm communication, calendar/clock control, program defined interlocks.
3. Low level: hard-wired interlocks, timer control, check input limits, sensor interface, scan inputs.
Interlock systems:
Interlocks can be considered as "don't until" statements, and are of importance in defining the control strategy and in detailing the schematic.
An example of a system of interlocks of plant operation might be:
- Don't start primary heating pump until the time is right.
- Don't start the lead boiler unless primary pump is energized.
- Don't start secondary pump on zone 1 until boiler primary circuit is at 80 degrees Celsius ...
Supervisor station and central station functions:
- Plant supervision.
- Maintenance supervision.
- Security supervision.
- Energy monitoring.
- Environmental monitoring.
- System development.
- Plant executive control.
- Reporting.
- Data archival.
- Design evaluation.
Control strategies for heating systems:
These are a few recommendations relating to controls for heating systems:
1. The boiler primary circuit should be pumped at constant volume and be hydraulically independent of the secondary circuits.
2. Domestic hot water (DHW) should be provided by a separate system.
3. The temperature to each zone should be compensated according to outdoor temperature (clearly this cannot apply to fan convectors or unit heaters).
4. The zones themselves should be selected on the basis of:
- Solar heat gain (building orientation).
- Building exposure (multi-storey buildings, horizontal zoning..).
- Occupancy times.
- Building thermal response.
- Types of heating appliance.
5. Frost protection during off periods.
6. Sequence control of multiple boilers.
7. Control of demand.
8. Summer exercising of pumps.
9. Flue gas monitoring and alarm.
10. Graphic display of operating times and off times.
System operation method statement:
An important part of the schematic or logic diagram control is a written statement that explains how you intend the systems to operate.
The Method Statement will include:
- Details of zoning by time scheduling and control of temperature.
- Description of plant and circuits, which would include space heating, ventilation, hot water supply.
- Description of interlocks.
- Client interface (how the client can use the system).
- Specialist interface (adjust/monitor/refine and maintain).
Further reading and information in:
CIBSE Guide H: Building control systems
CIBSE Guide B1 Heating
