Double Duct Mixing Box

PRODUCT INFORMATION

MODEL:

DDMB

APPLICATION:

Variflow double duct mixing boxes have been designed to be as trouble free as possible and to requre as little maintenance as possible over long periods.

All parts that could conceivably require maintenance namely the controls, are outside of the box. The only moving parts within the box are valves which seat just inside the high pressure air entries and the valve rods which actuate the valves. The valves rods have a straight push – pull operation from the motor and operate within tubes packed with Wynns Variplex Moly grease and are centered in self lubricating brushes. It is not anticipated that the valves would require attention within the life of the box and some have been operating for ever twenty years without attention.

The hot and cold air entries are staggered to ensure mixing prior to the air passing through the attenuating baffles.

The overall action of the controls are:-

The thermostat acting on the cold air valve provides the space with sufficient cold air to keep the occupied space cool.

The hot and cold air duct temperatures are of course controlled throughout relation to the outside temperatures so that in winter the hot air is warmer than in summer.

Should it be desired to operate the system as a variable volume system to conserve enerhy or reduce running costs it is only necessary to lower the setting of the air regulator down to the minimum air quantity that is required. The cold air valve will still provide enough air to keep the space cool within the capacity of the box but the amount of warm air used to reheat or makeup is reduced to the lower amount.

After the initial setting to work it is not usual for the box to require attention but at start up sometimes at the first change of season, faults in the control system may show up.

The faults most common are:-

Air leaks in the control piping. The particularly sensitive area is the branch line between the restrictor on the regulator line and the hot motor. The restrictor before the regulator only passes a very small amount of air and an extremely small leak can render this portion of the system inoperative. The restrictors are also liable to blockate by water etc, if the control is not clean. Sometimes the unions provided with the motors from overseas are not suited to the tubing used by the control piping contractor, and small leaks can occur at the connection to the motors.

The indication of a fault of this nature is usually excessive air and noise from the outlets connected to the offending box. Lack of sufficient air pressure in the ducts leading to the box. This shows up as a shortage of colling in the summer or shortage of heating in the winter or sometimes as erratic behavior of the controls.

It is usual to check the available static at the box on the end of the critical run at start up, but a check of the cold duct at the hottest part of the year and the hot duct on the coldest day is worth while.

It is of course essential to have the minimum static necessary for the box operation at this point at the time of maximum demand, but excess at this point only adds to the running cost, noise and load throughout the whole of the system for the whole of the year. Sometimes lack of air available to the box is due to sharp turns or more particularly offsets in the flexible duct connecting to the box and construction crews should be alerted for these at the time of installations as they are seldom accessible after the ceilings go up.

Access to the ceiling is most useful on the side where the regulator is fixed. This is about the centre of the box on the side of cold duct entry which is also the side mounting the hot duct motor.

The air quantity can be varied from this location and response of the hot motor and the static available to the cold duct can be checked from here.

The pressure feeler tubes influencing the regulator are also on this side of the box, and these are sometimes used to check the regulator operation without disturbing the set point of the regulator. The thermostat controlling the cold duct is usually checked and calibrated in the conditioning space and provided the branch line pressure is right the cold duct motor usually give no trouble.

The factory tests on the boxes are usually carried out a system equipped with orifice plates. The box is tested at the design air quantity. The design air quantity of the box and its related pressure drop across the tubes if then marked on the box.

The regulator is then set to this reading. Should gauges be installed in parallel with the regulator on the tapping tees, it is important to allow time for the system to settle down after each movement of the regulator set point. For accuracy at least five minutes should elapse between small movements of the regulator set point.

A variation of the air quantity can be done expeditiously by taking discharge velocity reading from a typical outlet or outlets on a box system, then changing the regulator set point until the required percentage increase or decrease is obtained at the typical outlets, or referring to the readings taken over the range of each box size and forecasting new settings with the use of a parallel rule used on the graph of these plotted readings.

Examine all connection for leaks.
Examine all pipe up for wrongs connection
Remove tapping plugs in feeler to static pressure regulator lines.
Turn adjusting knob on regulator slightly and slowly observce reaction of actuator - alreading on box lable.
This reading shoud produce the volume of air required at time of design.
Note and record the new reading on the instrument and file for future reference.
should the actuator fail to respond to S.P.R adjustment, check for a blcoked restrictor.
Replace restrictor and repeat procedure
Should the actuator still fail to respond, connect a pressure gauge to the line from regulator to motor and observe if pneumatic pressure is present.
If the gauge reads a maximum of mians pneumatic pressure when the S.P.R is adjusted to produce is maximum pressure, and the acutator still does not respond when reconnected, check for a perished bellows or a jammed valve rod. Rectify and repeat procedure until he desired setting is obtained.
Disconnect the instrument and re plug tees
Re adjust themostat to correct temperature setting

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Double Duct Mixing Box