Moisture In Hydrated Lime, Ca(OH)2

Moisture measurement is made to ensure that moisture levels do not exceed the 3% maximum.? If this level is exceeded the product will clump, be difficult to load and transport and cause blockages within the slaker.

Production of hydrated lime

Hydrated lime is obtained from hydrating calcium oxide or quick lime.? The hydration process is referred to as “slanking,” the water and calcium oxide undergo a controlled exothermic reaction resulting in “dry” free-flowing calcium hydroxide (hydrated lime powder).? ? Both temperature measurement within the reactor and moisture content are used to control the water addition to ensure moisture content is optimized but doesn’t exceed the 3% limit

Gauge location and Installation

The gauge is positioned at the exit of the slaker, 6-15″ above the hydrated lime.? Product flow should be continuous and of sufficient depth that the transporting medium isn’t seen by the gauge.? If flow is discontinuous a gating option can be used to enable measurement only when product is viewed.

Measurement Performance

Measurement? ? ? ? ? ? ? ? ? ? ? ? ? ? Location? ? ? ? ? ? ? ? ? ? ? ? ? ? ? Target Moisture ? ? Typical Accuracy

Hydrated Lime? ? ? ? ? ? Exit of the reactor? ? ? ? ? ? ? ? ? ? ? 0-3%? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? 0.2%

Asphalt Paving Dryers and Mixers

Asphalt dryer with IR thermometer mounted on exit chute

The Application

Asphalt used for road surfacing, parking lots etc., is manufactured in a two-stage process where crushed stone and sometimes crushed, recycled glass is mixed with tar and other binders.

First, the crushed material is dried and preheated in a continuous, rotary, horizontal furnace. It is then conveyed to a mixer where the binders are added and the mix is further heated. The source of heat in both cases is a single, gas or oil fired burner. When the temperature is correct, the mix is
dumped into a truck and taken to the laying sight.

Thermocouples are used to measure and control the temperature in the dryer and mixer, but are relative insensitive to rapid temperature changes and require frequent replacement. IR Thermometers are more responsive to process temperature changes, and do not need to be in contact with the abrasive material.

Asphalt Recycling Plants

Asphalt recycling plants are increasingly being used. Such plants operate on the highway, scrape up the existing road surface, crush it and pass it through a rotary heater where new binders are mixed in. The reconstituted material is then re-laid, all in one continuous operation.

The Importance of Temperature Control

The temperature at which the asphalt is laid down is critical to the life of the road surface, too low a temperature results in premature disintegration. If the temperature is too high, the material will sag and spread. This is particularly critical with high-speed, automatic laying machines.
Large highway construction and resurfacing contracts are carried out under the supervision of a representative from the highway authority, who monitors the laying temperature and the load in the trucks when they arrive, using a handheld IR thermometer.? If the temperature of the load is too
low, it is rejected and turned back to the supplier.

Application Tips

Plant operating conditions are extremely dusty. A good, well-maintained, filtered air purge is therefore essential, and periodic, manual lens cleaning may also be necessary. If compressed air cannot be made available, combustion air is an alternative, though less desirable.

In addition to connecting the purge air to the IR thermometer air purge unit, it should also be connected at points down the sight tube as shown in the picture below.

The length of the sight tube and angle should be such that ricocheting, crushed material cannot reach the IR thermometer lens. The thermometer should be shielded from direct sunlight, snow or rain. In some cases the
pyrometer is removed and stored during the off season.


IR Thermometer with air purge on exit chute of dryer

The Market

There are many, large asphalt paving contractors in the USA, and perhaps thousands of smaller ones with just one or two machines. Many of the smaller plants operate only spasmodically, as business dictates.

There are also a limited number of OEM?s worldwide.

The most efficient way to penetrate the market therefore is to focus on the corporate or regional engineering staffs of the larger contractors, and on the OEM?s. They have the authority to specify and mandate equipment purchase.? This is not to say that individual plants should be ignored, but
multiple sales are less likely from such sources.

Instrument Selection

Process Sensors Model PSC-SS-Laser with a 75:1 FOV a spot size of 0.63? at a 48? distance is used for continuous measurement at the mixer and dryer areas. Outputs: 4 -20mA, T/C and Ethernet

Process Sensors portable Models PSC-MS and PSC-PSLT can be used for spot measurements.