Thermostat condition

The water sample is thermostatically conditioned until equilibrium is... 

To obtain the analog of Clausius s formula from the Boltzmann equation we will impose an additional condition on the interactions of the gas molecules with the walls. We will assume that there exists a local temperature of the wall t wCp ) at every point p such that if the distribution of molecules incident on the surface at p, is a Maxwell-Boltzmann distribution with temperature T (ps), then the distribution of the reflected molecules is also a Maxwell-Boltzmann distribution with the same temperature. Using Eq. (33) we may express this thermostat condition as a condition on K w, v ) as... 

Boltzmann s form of the //-theorem, dH/dt 0, is recovered whenever the boundary conditions are such that dOfdt = 0. If the molecules are specularly reflected at the walls, then this condition is certainly satisfied, as it is if the distribution function at the walls is a Maxwell-Boltzmann distribution with the same temperature for both incident and reflected particles. In addition, we can also verify that if the walls are at a uniform temperature Ty, and if the thermostat condition is satisfied, then the equilibrium distribution function, Eq. (38), is a solution of the Boltzmann equation, Eq. (36), and for this distribution df/dt = 0 and dH/dt = 0. Therefore, according to the Boltzmann equation, if in the course of time the system reaches an equilibrium state with/ given by Eq. (38), it will remain in equilibrium for all later times. 

Returning to our original problem, we would like to prove that for any initial distribution /(r, v, t) and for boundary interactions that satisfy the thermostat condition, Eq. (40a), with a constant wall temperature at all points, the gas will eventually reach a state of total equilibrium in which the distribution function has the Maxwell-Boltzmann form given by Eq. (38) with the temperature equal to the wall temperature. 

Thus we have shown by means of the H-theorem that if the walls are maintained at a constant wall temperature 7, and the molecule-wall interaction satisfies the thermostat condition, or if there is no energy flow into or out of... 

When the motor overheats, the thermostat opens, interrupting motorline current. Pilot thermostats mounted on the windings of larger motors trip the motor starter rather than interrupt line current. This method gives good protection for sustained overloads, but because of the thermal time lag between the copper winding and the thermostat it may not provide adequate protec tion for stalled conditions or severe overloads. 

To control the room temperature llow of chilled w atei is controlled in AHU, Figure I 3.49(e). A thermostat senses the room condition and iictivates a motorised solenoid valve in AFIU coil which in turn adjusts the flow of... 

An air conditioning system uses an assembly of equipment to treat air. Normally the assembly includes a heating system for modifying winter indoor temperature and humidity a refrigeration system for modifying summer temperature and humidity, a means to maintain indoor air quality (i.e., air filters and fresh air intake) a method of distribution of conditioned air and a control system, such as a thermostat, to maintain desired comfort conditions. 

Many air conditioning systems use simple thermostats to cycle equipment however, more sophisticated control systems employing electronics and microprocessors can reduce energy consumption. 

During prolonged heat waves which can cause blackouts and brownouts, electric utilities always request that customers limit energy consumption by raising thermostats and turning off the air conditioning when not home. It is uncertain if these requests are heeded, and if they arc, it is uncertain whether conformity is out of altruism (help the utilities) or self-interest (save money). 

The combination of lower thermostat settings, reduced ventilation, and lower lighting levels created environmental quality problems in many buildings. Even today, some building owners associate energy consei-vation with poor indoor environmental conditions. 

Energy factor is a measure of average service efficiency at a specified condition and hot water draw pattern. It includes the effects of both standby losses and rccnvei y efficiency of the water heater. Currently, water heaters are shipped from the manufacturers with thermostats set at 120°F (48.9°C) to reduce the risk of scalding. This is a drop from values of 140°F (60°C) that were reported from the early 1970s. 

The definition of an automatic burner is A burner where, when starting from the completely shut-down condition, the start-gas flame is established and the main gas safety shut-off valves are activated without manual intervention. This means that a burner is only automatic if it is ignited by means of a remote interlock (e.g. thermostat, timeswitch, etc.) closing. A burner is not automatic if it has a pilot burner that remains ignited in the off condition. Nor is a burner strictly automatic if a start button needs to be pushed, even though the controls may comply with all requirements for automatic burners. 

A starter or contactor with manual push-button or thermostatic operation to start and stop the fan normally controls simple systems. More complex systems that incorporate components that need control or monitoring are normally operated from purpose-built central control panels. The most common functions provided are fan motor stop, start and speed control, damper control, filter-condition indication and heater battery control. For optimum control, the system should be automatically controlled from thermostats or other sensors and a timeswitch. 

Make no excuse for checking whether the sun shines on the thermostats or someone has washed the floor. Another common occurrence is that while the room is not being controlled as specified, the plant appears to be operating correctly. Here a series of simultaneous readings of air flows and wet and dry bulb temperatures should be taken around the whole of the air circuit - on and off coil, condition after fan, condition at inlet grilles, etc. Referring these to a psychrometric chart will usually indicate where the fault or leak lies. 

The test load must be calculated and is applied in two parts. The lights should be as in normal use and all plant controls normal and automatic. The background heaters (if installed) can be called as load but it is essential that thermostats and overloads do not reduce the applied load unobserved. Electrical measurements should be taken, particularly at the full-load condition, and for this purpose, trust not put in rating plates. 

The thermostatic expansion valve can also be used to maintain a liquid level. The phial and a heater element are both clamped to a bulb at the required liquid level. Illiquid is not present, the heater warms the phial to a superheat condition and the valve opens to admit more liquid. 

Since the purpose of a refrigeration or air-conditioning system will be to reduce or maintain temperature, a thermostat will usually be fitted to stop the equipment or reduce its capacity when the required condition is reached. The following types are in use ... 

Systems are normally designed to work satisfactorily during maximum amhient conditions, and the condenser will he sized for this. In colder weather, the condensing temperature and pressure will fall and the resulting lower pressure difference across a thermostatic expansion valve may lead to malfunction. A drop of pressure difference to half the normal figure may reduce mass flow helow that required, and it will he necessary to prevent the condenser pressure from falling too low. 

However, since the suction gas to the compressor would then be hotter than its normal slightly superheated condition, the compressor may overheat and the discharge gas become too hot for correct and safe working. This form of capacity reduction is usually combined with a liquid injection valve, thermostatically operated, which introduces liquid also into the suction to keep it cool. The fitting of dual interdependent controls of this sort, both of which have inherent fail-unsafe possibilities, should be approached with caution. 

Unless a thermostatic expansion valve is very tightly rated, the system will operate satisfactorily at a lower condensing condition in cool weather, with a gain in compressor duty and lower power input. A growing awareness of energy economy is leading to more careful application of this component. Suppliers are ready to help with advice and optimum selections. 

A greater difficulty arises where the compressor may go down to 33% or 25% capacity and the thermostatic expansion valve is called upon to control a much reduced flow Under such conditions, the thermostatic expansion valve maybe unstable and hunt , with slight loss of evaporator efficiency Since the required duty is less, this is of no great importance. It is possible to fit two expansion valves in parallel, one for the minimum load and both for the full load, but this arrangement is not usually necessary... 

In the first of these methods, the reduction in air mass flow is limited by considerations of distribution velocities within the rooms, so at light load more air may need to be used, together with more re-heat, to keep air speeds up. Within this constraint, any proportion of sensible and latent heat can be satisfied, to attain correct room conditions. However, full humidity control would be very wasteful in energy and a simple thermostatic control is preferred. 

Control of the indoor cooled condition will be by thermostat in the return airstream, and thus based on room dry bulb temperature. The resulting humidity level will depend on coil characteristics and air flow. Packaged air-conditioners for tropical applications commonly have a design coil sensible/total ratio in the order of 0.7 with entering air at 50% saturation, and will give indoor conditions nearer 45% saturation if used in temperature climates with less latent load (see Chapter 35). 

---