Microprocessor techniques for energy conservation in computer rooms Computer installations consume large amounts of energy and thus incur large capital and running costs. R S Hinsley breaks these costs down and describes an environmental control system for computers that is claimed to be an order of magnitude cheaper than conventional designs

Computers, particularly large ones used by finance bodies and government, are heavy users of electricity both to power the computer and to provide the environment recommended by the computer manufacturer. The power used by the environmental control equipment can be higher than that used by the computer itself. Both capital costs and running costs are high. Some cooling plants consume of the order qf a megawatt of power. Recently, very expensive systems have been developed to recover waste heat from computers for use elsewhere in buildings either for hot water supplies or for space heating.

This approach can only be easily applied to new custom designed installations, whereas in future most computers will be installed in existing buildings. Most heat is available from such systems in the summer when the demand is at a minimum. At best they are a palliative for an energy design which is wrong in concept.

This article advocates an integrated approach to the total computer system design where all the factors, including energy consumption, are considered from the outset and their interactions considered. A working example is described designed on these principles which uses a fresh air ventilation system controlled by a micro- processor. Other advantages include automatic condition monitoring of the equipment, improved security and fire protection. The microprocessor-based environment controller is an order of magnitude cheaper, both in capital and running costs, than conventional designs. There is no evidence to suggest that it is less effective, because the computer installations concerned show higher than average reliability. microprocessors environmental control computer rooms

Traditionally computers have been accommodated in large halls with air conditioning to very high standards Ip. Air is filtered to remove dust, the humidity is controlled and the temperature maintained within narrow limits. Because all the equipment and the human operators are housed in the same hall, the whole space and therefore all the equipment is maintained at this same very high standard. The conse- quence is that very large and expensive air conditioning plant is required. Furthermore special 'clean' electricity supplies are provided often by means of motor alternator

Building Research Station, Garston, Watford WD2 7J Rj U K

sets. The result is that the capital costs of electricity supplies, building and air conditioning plant can exceed by several times the cost of the computer system and furthermore involve high electrical cost for their operation.

These air conditioning systems can easily be specified to inappropriate standards and even if such systems are required by the computer it would be advantageous to redesign the computer system so that fine tolerances are not required and so that the energy consumption of the installation is significantly reduced.

If all the costs are considered together, a very different picture may emerge, particularly with respect to environ- mental control and energy consumption.

C O M P O N E N T S O F C O M P U T E R I N S T A L L A T I O N C O S T S

The costs of the project include

• computer hardware • systems software (supplied by manufacturer) • operations staff • applications software • applications programmers • environmental control • power supplies • buildings • communications • stationery and other disposables

All of these have capital and running costs. Very often the capital cost of the computer is the item looked at most critically (particularly in government). This can result in an overall increase in cost. For example, a large proportion of the cost of a computer system is spent on memory of various types. If the allegedly cheapest medium is used, magnetic tape, then human operators are needed to load and unload tapes, and tight temperature control is required to maintain the repeatability of tapes particularly if high speed high density tape decks are used. The tapes need to be rerecorded at regular intervals to maintain the integrity of the data. The magnetic tapes themselves are coated with an abrasive magnetic material which becomes detached and so causes a dust problem inside the computer room.

Because of the open plan layout of many computer installations, the temperature requirements of the tape decks become the temperature requirements of the whole installation. A similar situation occurs with humidity

vol 6 no 5june 1982 O141-9331/82/060219-05 $03.00 © 1982 Butterworth & Co (Publishers) Ltd 219

control. In this case peripherals using paper are involved, the most exacting being graph plotters.

SAVING ENERGY If the computer system is to be designed with energy conservation as an important criterion then the factors which have an influence on energy consumption must be examined critically.

These factors are

• the presence or absence of human operators in the computer room

• the use of temperature and humidity sensitive system components

Computer systems have, in the past, been labour intensive installations and people have shared the same environment as the equipment. As the computer produces a great deal of heat, air conditioning has been used to provide an acceptable standard of comfort for the operators. However, air conditioning for human comfort does not have to conform to the stringent requirements specified by most computer manufacturers. The reasons given for the tight tolerances on set point temperature control have been that the equipment will not work properly without it, that reliability will fall and in some cases catastrophic overheating would occur.

If a computer system is analysed from the environmental control point of view then it soon becomes apparent that set point temperature control is not always required and humidity control only rarely, particularly when the installation is in a part of the world with a cold or temperate climate.

The situation described earlier is that which is often thought to exist only in the field of mainframe computing. The minicomputer manufacturer and the suppliers of computer peripherals (both to the mainframes and minis) have taken the approach of protecting each cabinet of equipment separately. For example disc drives are cooled

by a supply of air, filtered and pumped inside the cabinet 3. Every computer system is a collection of cabinets and the same system components are often used by the manufac- turers of small and large systems with a surprising spread of tolerances in each case 6 .

However, it may be a mistake to assume that mini- computers 'do not need air conditioning'. In some cases they can be more difficult to protect because of an excessively high packing density inside the cabinets.

In the traditional mainframe computer hall, cool (refrigerated) air is usually blown from the ceiling to the floor where it is extracted. This is often in opposition to the air flow in the cabinets, when the air inlets are at the bottom of the cabinets and the exhausts at the top. This produces a thermal environment very comfortable for human beings, with a very even temperature distribution throughout the computer hall. It does not produce efficient cooling inside the cabinets, because it induces uncontrolled recirculation, mixing the expensively cooled air with the hot exhaust from the cabinets.

As a consequence, the air entering the room via the ceiling must be cooled to a temperature below that required for effective cooling of the computer equipment in order to reduce the average temperature of the computer hall. If cold air is blown into the base of the cabinets and the hot exhaust is extracted then the cabinets are cooled efficiently but an uncomfortable environment for people results, i.e. the feet can be icy whilst the head is hot. This does not matter if the computer system has been designed to be operator-free, for example, by replacing high density tape decks with disc drives.

The result of designing an operator-less computer system is that direct staff costs are saved, together with correspond- ing drop in overheads, and a floor to ceiling ventilation system can be used, with controlled recirculation, which does not require refrigeration cooling of the input air. Such a cooling system is described later in this paper. It requires detailed examination of each piece of equip-

J Alarm be Tope J VDU J and warning J

recorder [ J lights J

• . __. I .~ Environment I ~ , . i J ton/off} [ J monitor [ A = J PDP-II/45 ~ - '" c°mputer col~net

,//\ ';-' , -. Intake duct I-------- ~ I ~ap~o~ J / / \ " ~ Thermistor temperature J

.!.- i ".._ .. / \ prab,,; v,uuy..., :. / ~ , • room/disc drive outslaeolr / ~ [ environment temperature temperature / \ //

I 3OOMbY, I 300 e r I I

/ I

Probes 2~ 3: disc drive exhaust temperature

Figure 1. Computer room schematic diagram

220 microprocessors and microsystems

ment cabinet by cabinet to identify whether a zoned approach is required with the equipment separated by walls or partitions.

This can be useful in the case of noisy equipment which needs to be visited, such as line printers. In the case des- cribed below a zoned approach was not necessary but the disc drives posed the largest problem.

E X A M P L E OF T H I S A P P R O A C H

A computer system cooled by open circuit ventilation has been built at the Building Research Establishment, UK. It is controlled by a 6800 microprocessor which allows the use of more sophisticated control functions.

Microprocessor-based environment monitor and controller

A schematic diagram of the computer, its air conditioning system and the environment monitor is shown in Figure I. The computer comprises a PDP-I 1/45 processor, two 300 Mbyte disc drives and a variety of other storage media not shown. Altogether, these have a heat output of about 7 kW. The air conditioning system consists simply of a pair of 300 mm diameter intake fans, which draw in filtered fresh air from outside the building, and an extractor fan. Room temperature is controlled by using a flap valve in the intake duct to mix recirculated room air with the fresh air.

Data ond oddrsss buses

Motorola 6800

microprocessor

128 byte RAM 4 kbyts EPROM

VDU interfoce

Serial interface (6850)

Proo~ommoble timer (6840)

Porallel interfoce (~z 6e2o)

8-bit A/D converter 16 chonnel multiplexer

PoroUel interfoce (i 68201

Parallel interface

(I/t 68201

Memory

9-in VDU for display of: • temperatures in bar formot • system stotus • alarm messages dialogue

between micro ond mini

V24 link to minicomputer

Baud rote generator I Hz interrupt for dote-time clock

8-bi t porollel link to tope recorder/winter

16 onologue inputs for meosurement of temperature and humidity

Digital OUtl~uts to: eolarm bell and warning lights • fon control relays e flap valve motor drive unit edisc drive on/off relays • thermistor switches • wotshdog timer

Dioitol inputs from: • smoke detectors • flap valve switches • override switch

Figure 2. Environmental monitor hardware

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The environment monitor's functions are, in summary: to operate the air conditioning system, to monitor the environment and computer hardware, to warn of faults and to automatically switch of f faulty equipment. The monitor not only attempts to maintain a constant room temperature, but it also limits the rate of change of temperature (for example, on start-up) to a level that ensures reliable operation of the disc drives. By measuring temperatures in and around equipment, the monitor detects a variety of faults both in the computer hardware and in the air conditioning system; and by monitoring the output of the two smoke detectors, it is also able to warn of fire.

Warnings are given by soundind an alarm bell, by display- ing alarms on a VDU and by sending messages to the minicomputer, which passes them on in turn to users who are logged on. This last feature gives users the opportunity of finishing up tidily, of overriding false alarms from their terminal or of taking other appropriate action before the monitor shuts down the computer. Figure 2 shows a block diagram of the monitor hardware.

TEMPERATURE MEASUREMENTS The following temperatures are measured once a second

• the room temperature in the region of the disc drives • the temperature of the air exhausted by each disc drive • the temperature around the processor cabinet • the outside air temperature

The transducers are thermistor probes, each one forming half a potential divider network to generate a voltage related to temperature. An 8-bit A/D converter interfaces the thermistor to the microprocessor, and voltages are converted to temperature with the aid of a lookup table in ROM. A calibration offset, determined at 25°C, is added in software to each thermistor measurement to compen- sate for differences in the characteristic between individual thermistors. This results in a maximum error of +0.2°C at 25°C and +1 °C at 0 and 50°C, the range limits. The thermistors are shorted with transistor switches when not in use in order to minimize setfheating.

The temperatures are displayed in bar format on the VDU. Resolution is 1°C and they are updated each second.

TEMPERATURE CONTROL The environment monitor operates the pair of two-speed intake fans and the flap valve in the intake duct to control temperature by allowing the warm air already in the room to mix with the intake air. The minimum temperature set point is 20°C, although once the outside temperature rises above 18°C set point control is lost and the rate of change of temperature is limited to 6°C per hour which is the maximum rate which can be tolerated by the disc drives.

This is more important at evening time when the outside temperature can drop rapidly. Control is normally achieved just by varying the amount of recirculation with the flap valve. The fan speed is initially set to 50 per cent to keep noise down to a minimum, but it is raised to 100 per cent when the outside temperature rises to a high enough value to make it necessary (i.e., when the recircu- lation reaches 0 at 50 per cent fan speed). In order to maintain the computer room at a positive pressure and prevent the entry of dust, the extractor fan is only turned on when the fan speed is 100 per cent and the recirculation is zero. If the flap valve stops working, the fans will continue to regulate temperature, albeit rather coarsely.

Alarm conditions Of the computer system, the disc drives are the components which have the tightest specification for environmental control. The manufacturers specify that the drives should operate at an ambient temperature between 15 and 32°C and that the rate of change of temperature should not exceed 6°C per hour. This is because the disc drives are constructed to very fine mechanical tolerances and although they are temperature compensated, differential expansion and contraction will cause registration errors. The room temperature probe is therefore placed close to the air intake of the drives and an alarm is triggered if the room temperature ever deviates outside the specified limits. This would happen if the cooling system failed, or if the outside temperature rose above about 30°C.

The disc drive exhaust temperature normally exceeds its input temperature by about 13°C. If this difference ever exceeds 20°C - perhaps because of a blocked air filter or a motor bearing failure - an alarm is triggered. For greater reliability, however, the alarm is triggered only if the exhaust temperature is also greater than 35°C.

A probe has been placed close to the processor cabinet, in such a position as to detect temperature rises caused by a cooling fan failure or an electrical fault. The alarm is triggered if this temperature rises above 40°C.

Out-of-range temperatures are indicated by sounding an alarm bell and by making the appropriate temperature bar on the VDU flash red or blue. If a temperature sub- sequently returns within limits as a result of action taken automatically by the environment monitor, the bell is switched off - but the bar continues to flash as a warning until the monitor is reset manually.

The environment monitor checks the output of two smoke detectors - an ionization detector for small particles and an optical one for large particles. If either detector is triggered, the alarm bell sounds and a fire alarm message is flashed on the VDU. The bell can only be switched off by manually resetting the smoke detector.

The monitor can detect a certain number of faults in its own operation. For example, the flap valve has two optical limit switches: if both are triggered at the same time or if either one can not be triggered, a hardware fault message is flashed on the VDU. If either of the disc drive exhaust temperatures drops below 4°C, it indicates a disconnected probe and the hardware fault message is again flashed on the VDU. A hardware fault does not set of f the alarm bell unless it also causes an out-of-limit temperature. An override switch has been installed for test purposes. When set, all alarm conditions are ignored and a warning message is flashed on the VDU.

DISC DRIVES Each disc drive has been fitted with a relay-driven off switch. The relay contacts are in series with the drive's start]stop switch, so that a drive is always switched off in the normal way rather than by interruption of the mains supply. Once the alarm bell has sounded, there is a two minute delay before the relay is operated in order to give users time to finish off their work tidily. To reduce the risk of drives being switched off accidentally - for example, when the environment monitor is undergoing maintenance - the relays incorporate a hardware timer, which delays the opening of the contacts for a minute after power is applied to the relay coil.

In the case of an overheated disc drive the affected

222 microprocessors and microsystems

unit only is deenergised. Both drives are switched off for all other alarm conditions eg if a smoke detector is triggered. A message is displayed on the VDU to show that a drive is off, and the drive can be switched back on again only by a hardware reset. Later an electrically operated mains isolator will be installed so that the whole computer system can be disconnected.

COMMUNICAT ION WITH THE MINICOMPUTER

The environment monitor communicates with the PDP-11/45 minicomputer along a V24 line. It is able to warn users of faults via a command file in the minicomputer, which transmits alarm messages to all logged on terminals. Users are allowed to return messages to the monitor indicating what action it should take.

Any alarm condition results in a message being sent from the monitor to the minicomputer indicating that either the room is too hot or too cold, a disc drive is overheating, a smoke detector has been triggered, or that there is a hardware fault. If there is a hardware fault, the monitor does not wait for a reply and it takes no action other than to keep on looking for further faults. For all other faults, the monitor waits up to two minutes for a reply. If no reply is received, then the monitor takes independent action, for example, switching off a disc drive. If a reply is received, the monitor takes the action indicated in the reply.

This might be to switch off one or both drives, to clear all alarms as the alarm was false, or to ignore the alarm condition for half an hour before looking again to see if it still exists. In the last case, the alarm bell is switched off for the half hour period and only switched on again if at the end of that time the alarm condition is still present. If a different alarm condition is detected during the half hour, then another message is sent to the mini- computer. This reply gives users more time to put right a fault or to finish off their work when the fault is not serious, for example, slight overheating of the room on a hot day. Once a message has been received, acknowledged and implemented by the minicomputer, that message is not repeated.

All messages that pass between the monitor and the computer are displayed on the bottom six lines of the VDU, which are scrolled upwards as new text arrives. As each character is received from the minicomputer, an interrupt routine is called to display the character in the next free space on the VDU screen. Transmission errors are detected by comparing the transmitted character with that echoed back by the minicomputer. If an error occurs in any character, the whole message is repeated, up to four times if necessary. If satisfactory communication is not established then the monitor takes independent action as though it were not connected. The monitor only transmits and receives codes correspond- ing to alarm and action signals. These are translated by a command file in the minicomputer into text. A software real-time clock has been incorporated and the date and time are continuously displayed on the VDU. The date and time are reset whenever the monitor logs on to the minicomputer.

SOFTWARE

Figure 3 demonstrates the principal features of the soft- ware. After being reset the program normally loops looking for alarm conditions, recording data and operating the

Six characters at 1200 baud from minicomputer

Reset - /

° E Character Update display and received ~100 ms date-time cbck interrupt

a. I Hz [ '~ 11111"='3 I--I Check a l a r m s interrupt ,, [ UU~ U k /andrecorddata

Mo~ loop ,, 250/~.s

[ I I I

Figure 3. Program timing

cooling system. Once a second an interrupt routine is called to measure the temperatures and to update the clock and the display. Whenever a character is received from the minicomputer a second, higher priority interrupt routine is called to display the character on the VDU. The 1 Hz interrupt routine continues, if necessary, during the 8 ms gaps between the characters of a string.

CONCLUSIONS

This system demonstrates how a microprocessor can take over the functions normally carried out by human operators and also control the cooling of a large computer system using equipment costing a few hundreds of pounds Sterling, which is also an energy conservation system because the heated air is blown into the rest of the building in cold weather and no refrigeration is needed even in the hottest weather that our climate provides. An open circuit system of this type appears to be safer than the closed circuit type in the event of fire because the smoke is immediately extracted consequently reducing this hazard.

For the system described, the tolerances set out by the component manufacturers have not been exceeded. Two years of operation have produced no evidence to support the claim that the absence of fixed point control decreases the reliability of a computer system.

REFERENCES 1 Broadgate, N 'Cooling your assets' Comput. Talk

(14 November 1979)

2 Nelson, G 'Energy banks'J. Chart. Inst. Build. Serv. (August 1979) Vol 1 No 12

3 Proposal for the supply ofa NCR Elliot 4100 data processing system to the Building Research Station Elliot Automation Computers Ltd, (1965)

4 Product Specification for the 9766 Storage Module Drive Control Data Corporation, Memory Products Engineering Division

5 Bromley, A K R 'The design of a microprocessor-based environment controller for computers' BRE Internal Note

6 Longbottom, R Computer system reliability J ohn Wiley & Sons (1980)

7 Hinsley, R S 'An integrated approach to environmental control for minicomputers' Environmental services for computers Conference Chart. Inst. Build. Serv. London (March 1981 )

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