Where city water is used for condensing purposes, an automatic water-regulator valve is supplied (usually by the manufacturer). Figure 13.42 shows a cross section of such a valve. The power element is attached to the hot-gas discharge of the compressor. As the head pressure builds up, the valve is opened more, allowing FIGURE 13.42 Water-regulating valve for condensing in an air-conditioning system. a greater flow of water to the condenser, thus condensing the refrigerant at a greater rate. When the compressor is shut off, the head pressure drops, the flow of water being cut off by the action of a power spring working in opposition of the power element. As the water temperature varies, the valve responds to the resulting head pressure and adjusts the flow automatically to maintain design head pressure. When a water tower is used, the automatic water-regulating valve should be removed from the circuit, because it offers too much resistance to the flow of water. It is not usually necessary to regulate the flow of water in a coolingtower system. For when the outside wet-bulb temperature is so low that the tower yields too low a water temperature, then air conditioning generally is not needed. Figure 13.43 shows a simplified wiring diagram for a tower system. Because of the interconnecting wiring between the magnetic starters, the tower fan cannot run unless the air-conditioning fan is in operation. Also, the circulating pump cannot run unless the tower fan is in operation, and the compressor cannot run unless the circulating pump is in operation. When the system is started, the air-conditioning fan should be operated first, then the tower via switch No. 1, the pump via switch No. 2, and the compressor via switch No. 3. When shutting down, switch No. 3 should be snapped off, then switch No. 2, then switch No. 1, and then the air-conditioning fan. FIGURE 13.43 Control circuit for a cooling-tower-type HVAC system. In buildings where one tower and pump are used to provide water for many units, pressure switches may be used in series with the holding coil of each compressor motor starter. No interconnecting wiring is necessary, for the compressor will not be able to run unless the pump is in operation and provides the necessary pressure to close the contacts on the pressure switch. Some city ordinances require that indirect-expansion, or chilled-water, systems be installed in public buildings, such as theaters, night clubs, hotels, and depots. This type of system is illustrated in Fig. 13.30. The refrigerant is used to cool water and the water is circulated through the cooling coil to cool the air. The water temperature should be between 40 and 45
F, depending on whether the occupancy is a high latent load or a low latent load. Because the cost of equipment is increased and the capacity decreased as water temperature is lowered, most designers use 45
F water as a basis for design and use a much deeper cooling coil for high-latentload

A good building design will be arranged so that the overall dimensions work to full brick sizes. But this is not always possible and, if varying the joint sizes doesnt make things fit, the bricklayer will have to use other methods. One method is to reverse the bond. This means that rather than the wall having matching headers (or stretchers) at either end, the bonding arrangement is reversed so that there will be a header at one end and a stretcher at the other end. Different Different Reverse bond If this doesnt produce a wall with uniform joints within the overall measurements, then the only choice is to cut the bricks to suitable dimensions. Placing cut bricks within a course of brickwork is called broken bond. The smallest cut allowed is known as a half-bat and measures 102.5mm, the same as the header face of a full brick. Half bat bats Broken bond Cut bricks in broken bond should be placed as near to the centre of the wall as possible. If there are doors or windows in the wall, the broken bond can be placed underneath one of them. When a half-bat is built into a course of brickwork, the course above and below it will contain two three-quarter cuts (or bats) to maintain the bond.

Special-Purpose Admixtures. The list of materials used from earliest times as admixtures for various purposes includes almost everything from human blood to synthetic coloring agents. Admixtures for coloring concrete are available in all colors. The oldest and cheapest is perhaps carbon black. Admixtures causing expansion for use in sealing cracks or under machine bases, etc., include powdered aluminum and finely ground iron. Special admixtures are available for use where the natural aggregate is alkali reactive, to neutralize this reaction. Proprietary admixtures are available that increase the tensile strength or bond strength of concrete. They are useful for making repairs to concrete surfaces. For special problems requiring concrete with unusual properties, detailed recommendations of Chemical Admixtures for Concrete, ACI 212.3R, and references it contains, may be helpful. For all these special purposes, a thorough investigation of admixtures proposed is recommended. Tests should be made on samples containing various proportions for colored concrete. Strength and durability tests should be made on concrete to be exposed to sunlight, freezing, salt, or any other job condition expected, and special tests should be made for any special properties required, as a minimum