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MASTERCLASS - AIR CONDITIONING TECHNOLOGY

PART 28

In last months article we continued with our study of low ambient controlmethods under the current category of our review of Control, Safety &

Protection Devices used in the Vapour Compression Cycle in which wecovered plant room damper control, multiple fan and fan speed control. Thismonth we complete this element by looking at Head Pressure Control of WaterCooled Systems.

Head Pressure Control for Water Cooled Systems:

Water cooled system are probably the most efficient in their ability to removeheat from a refrigeration plant and maintain low condensing temperatures. It isfor this reason that they are (or were) first choice for use on larger refrigerationplant. Due to concerns about the spread of Legionnaires disease many usershave reservations about using them and those that do use them are faced with

considerable maintenance costs in ensuring that they comply with local WaterAuthority regulations. However if good practice is complied with, then they canbe used safely and their efficiency used to reduce operating costs.

J ust as air cooled systems are affected by the temperature of the air onto theirfin & tube condensers, water cooled system are affected by the temperature ofthe water entering their condensers. It makes no difference if the water cooledcondenser is shell and tube or a plate heat exchanger or an evaporativecondenser. It is therefore necessary to keep the temperature of the waterflowing over the condensing tubes within system requirements.

At one time a water cooled system may have used mains water or well waterwhich would flow through the condenser and then to waste. Naturally, thismethod would no longer be allowed by any Water Authority. All water cooledsystems therefore work with recycled or recirculated water.

In this article we will cover the fundamental differences between EvaporativeCondensers and Cooling Towers. Both are illustrated in Figures 1 and 2respectively.

Figure 1 - Evaporative Condenser

Evaporative Condensers

Evaporative condensers can be described as being essentially the same as anair-cooled system but with the addition of water being sprayed onto the tubescontaining discharge vapour. The other difference with air cooled condensersis that the condenser comprises of bare tubes without any fins. They have theadvantage of being able to use the latent heat of vaporisation of water atambient wet bulb temperature therefore their heat of rejection per meter ofcondenser tube is significantly greater than air cooled condenser tubes.

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Typically an evaporative condenser will occupy 1/3rd the floor area of an aircooled condenser although it will be taller.

Water is sprayed onto the pipe coils through a system of sparge pipes andthen falls into a sump from which it is pumped up to the sparge pipes. Toassist the rate of evaporation of the water, air is forced through the water

spray by centrifugal or axial flow fans.

Controlling condensing temperature with an evaporative condenser is primarilyachieved by controlling the volume of air being circulated. On condensersfitted with centrifugal fans, pressure or temperature controlled dampers arefitted to the air outlet of the condenser. As the condensing pressure falls, thedamper blades are rotated and close the air outlet. This reduces the air flowand capacity of the condenser. The reduced capacity of the condenser resultsin the condensing pressure stabilising or causes it to increase.

Evaporative condensers fitted with propeller fans use fan speed as the methodof controlling air volume rate. The simplest type of speed control is to fit a

three phase two speed motor. The normal full speed operation of this motor iswhen the windings are connected in Delta. Changing the windings to Starconnection reduces the speed between 20% and 30% depending on thenumber of poles within the motor. If this level of speed reduction is insufficientto maintain the discharge head pressure at the required level, then liquid back-up will have to be used in conjunction with fan speed control. Alternatively aninverter controlled fan can be fitted and this would allow speed variations toless than 20% of full speed.

It should be noted that attempting to control condensing pressure by switchingthe water spray pumps on and off is considered bad practice. The repeatedwetting and drying of the condenser tube surface will result in theaccumulation various deposits on the coil block and the resulting scale willreduce the efficiency of the condenser to the extent it will not have sufficientcapacity at full load.

In addition to the above restriction, frequent cycling of the fans should beavoided. Depending on the size of the fan motor, the number of starts mayhave to be restricted to between 6 and 12 starts per hour.

As stated earlier, the usual coil configuration is bare tube but it is possible toorder these types of condenser with finned coils. This is for those applicationswhich have a predictable heat of rejection in winter which could be satisfied by

the condenser running with air cooling only and which would allow the watercircuit and sump to be drained off completely thereby avoiding freezingproblems.

Figure 2 - Cooling Tower

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Cooling Towers

Systems using water cooled condensers are invariably connected to coolingtowers. Water is pumped from the sump of the tower through the condenserthen to the top of the tower where it is sprayed over the tower fill material andthen drains to the sump. Heat is removed from the water in the tower by heat

conduction from water to air and by evaporation of the water.

Since the water temperature leaving the tower is directly related to the ambientconditions, it is necessary to adjust the water temperature before it enters thecondenser. The first step in controlling water temperature is at the coolingtower and this is achieved by controlling the volume of air circulated by thetower fans, either by dampers or by fan speed control, as described inevaporative condensers above.

The second step is to force the water to recirculate by fitting a by-pass valvebetween the condenser outlet pipe and the pump inlet connection (See Figure1 Schematic (C)). The three way by-pass valve modulates the return water by

sensing the temperature of water entering the condenser or by sensing thecondensing pressure. As the temperature of the water entering the condenserfalls below a predetermined level, the valve seat gradually closes the outlet tothe tower and opens the outlet to the pump, thereby by-passing the tower andits water cooling capability.

With this arrangement the water flow rate through the water pump andcondenser are constant, a condition that satisfies the pump and condensermanufacturers, however it does result in considerable fluctuation of the waterflow through the tower. This condition is unsatisfactory to the cooling towermanufacturer as it is possible for the tower fill material and sparge pipes to dryout which results in a build up of deposits. (If the tower has not beenmaintained correctly, there would also be an increased risk of bacterialcontamination of the water remaining in the sump). When this condition islikely to occur, an auxiliary pump assembly should be included with the coolingtower. This pump would be controlled by a flow switch or an auxiliary contacton the three way by-pass valve controller. This method would ensure that thetower fill material was always kept wet.

NEXT MONTH: Part 29 - Control & Protection Devices - continued

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