Control by microprocessors and Building Management Systems (BMS and BEMS)

Instead of adopting the analogue philosophy of the foregoing an alternative approach to automatic control is one of calculation, making use of microcomputer technology. A microprocessor is then the main feature of the control system. Data on temperature, flow rates, pressures, etc., as appropriate, are collected from sensors in the system and the treated spaces and stored in the memory of the processor. Provided that equations defining the performance of the control elements, the items of plant and the behavioural characteristics of the systems controlled have been developed and fed into the micro-processor as algorithms, deviations from the desired performance can be dealt with by calculation, the plant output being varied accordingly. Mathematical functions replace control modes, such as P + I + D. For example, if room temperature rose in a space conditioned by a constant volume reheat system, the correct position of the valve in the LTHW line feeding the heater battery could be calculated and corrected as necessaiy, to bring the room temperature back to the set point as rapidly as possible, without any offset. Data can be stored to establish trends and anticipation can be built into the program so that excessive swings in controlled conditions may be prevented. Furthermore, self-correction can be incorporated so that the control system learns from experience and the best possible system performance is obtained. While this implies that commissioning inadequacies and possibly even design faults can be corrected it is a mistake to rely on this: optimum results are really only obtainable, and the cost of the installation justified, from systems that have been properly designed, installed and commissioned. Under such circumstances it is then feasible to extend the scope of microprocessor control to include the management of all the building services with an economic use of its thermal and electrical energy needs.

The functions of a building management system (BMS) or building energy management system (BEMS) are monitoring and control of the services and functions of a building, in a way that is economical and efficient in the use of energy. Furthermore, it may be arranged that one system can control a group of buildings.

There are three types of system:

(i) Central systems. These placed a heavy duty on communications, were unreliable and expensive. They are no longer popular.

(ii) Distributed intelligence systems. The outstations, where all data processing is done, have computing power, allowing local decisions and reprogramming to be made, with transmission to a central unit, if required.

(iii) Open systems (OSI). Different systems in different buildings (or even in the same building), including lifts, lights, security etc., may speak in different languages or use different procedures (protocols). This makes it difficult to harmonise monitoring and control. Translation devices, termed gateways, must then be used. The aim of these is to unify control of all the systems in a building and, as convenient, to unify the control and energy management of a group of buildings. See Scheidweiler (1992).

Building energy management systems have not always been a success in the past. A failure to understand the way a system worked has led to some of such systems being by­passed. Control systems and building energy management systems must be easier to understand; systems should be simple enough for the maintenance people to comprehend and be able to operate and override as necessary.

The communications network between computer terminals and related equipment within buildings is often based on the use of fibre optics. The dominant language used by such systems is called Transfer Control Protocol, or Internet Protocol (TCP/IP)—which is the basis of the Internet. This has been developed for use as a site communications backbone that connects information technology networks and the building services. The use of advanced digital signal processing can automatically adapt to a wide range of wire types and compensate for different methods of installation. The use of such open systems will allow users to control buildings through the Internet and will enable systems from different manufacturers to be dealt with.

1. (a) Explain what is meant by (i) proportional control and (ii) offset. Give an example of how offset is produced as a result of proportional action.

(b) Show by means of a diagram with a brief explanation, the basic layout and operation of a simple electrical or a simple pneumatic proportional control unit, suitable for use with a modulating valve and a temperature-sensitive element producing mechanical movement.

2. An air conditioning plant treating two separate rooms comprises a common chilled water cooler coil, constant speed supply fan, two independent re-heaters (one for each room) and a single dry steam humidifier (for one of the rooms only). A constant speed extract fan is provided and motorised control dampers arrange for the relative proportions of fresh, recirculated and discharged air to be varied, as necessary. Draw a neat schematic sketch of a system that will maintain control over temperature in one room and control over temperature and humidity in the other room, throughout the year. Illustrate the performance in summer and winter by a sketch of the psychrometry involved.

Notation

Symbol

Description

Unit

Av

Flow coefficient (capacity index)

A

Cross-sectional area of a valve opening

M2

C

Combined constant

Cl

Manual re-set constant

C2

Constant of integration

D

Diameter of a pipe or duct

M or mm

F

Dimensionless coefficient of friction (Fanning)

Fm

Dimensionless coefficient of friction (Moody)

G

Acceleration due to gravity

M s~2

HhH0

Head in a reservoir or sink, in m of fluid

M

H

Static head in m of fluid

M

Hi

Downstream static head in m of fluid

M

Hx

Head lost along a pipe in m of fluid

M

Hu

Upstream static head in m of fluid

M

Hv

Head lost across a valve in m of fluid

M

K, Ki

Constants of proportionality

Kd

Derivative control factor

H

Integral control factor

Proportional control factor

Z

Length of a pipe

In

P

Pressure

Pa

<?

Volumetric flow rate

3 -1

M s

Maximum volumetric flow rate

3 -1 m s

T

Time

S

V

Mean velocity of water flow or airflow

3 -1

M s

Z

Valve lift

M

Zo

Maximum valve lift

M

A

Valve or damper authority

L6fJK?/n2d5

System pressure drop excluding damper

 

Я

8

<t>

0

 

Fully open damper pressure drop potential correction in appropriate units deviation in appropriate units

 

Posted in Air Conditioning Engineering


Добавить комментарий

Ваш e-mail не будет опубликован. Обязательные поля помечены *