Refrigeration

| What is Refrigeration' Refrigeration is the removal of heat from a material or a space so that its temperature is lower than its surroundings. There are five (5) basic components that makes up the refrigeration system, in order for the system to work successfully, they are: 1. Evaporator 2. Compressor 3. Condenser 4. Expansion Valve 5. Refrigerant Below is an illustration displaying the Basic Refrigeration System How the Refrigeration System Works As specified above, each component above contributes in assisting the refrigeration system to operate efficiently. The refrigeration cycle begins with the refrigerant in the evaporator, the refrigerant is in a liquid format and used to absorb heat from the product, however when the refrigerant is leaving the evaporator the absorbed heat from the product becomes low pressure and low temperature vapor. The low pressure, low temperature vapor is drawn from the evaporator by the compressor, after being compressed the temperature rises. The compressor transforms the vapor from a low temperature vapor to a high temperature vapor which increases the pressure, the high temperature; high pressure vapor is then pumped from the compressor to the condenser; the surrounding air then cools it or in some cases a fan assists. The vapor in the condenser is cooled and becomes a liquid format again, the heat which was absorbed is conducted to the outside air and the liquid refrigerant passes through the expansion valve, the expansion valve reduces the pressure of the liquid refrigerant and reduces the temperature. The cycle is complete when the refrigerant flows into the evaporator from the expansion valve as a low pressure low temperature liquid. Cooling Tower Operations & Maintenance Issues There are two types of cooling towers, open and closed or they can be referred to as direct or indirect. The key components of the cooling tower are: 1. Water Distribution 2. Heat Transfer Medium (Fill) 3. Air Flow 4. Drift Eliminator Cooling Tower Operations For best operation of the cooling tower the manufacturers manual should be kept at arm’s reach, other methods used for efficient operation should be: 1. Use a preventive maintenance method which supports regular water treatment and maintenance of mechanical and electrical systems. 2. Ensure that the water leaving the cooling tower is as cold as the chiller manufacturer would allow for entering condenser water. 3. Cooling towers should be operated simultaneously because it would use less energy thus saving money.  4. The temperature set point of the water leaving the cooling tower should be at least 5 degrees F higher than the ambient wet-bulb temperature. If the Direct Digital Control (DDC) system has a wet-bulb temperature sensor, this can be done automatically. 5. The bypass fan should be closed before the cooling tower is started; if the bypass valve is not fully closed, the hot water leaving the chiller will short the circuits into the water returning to the chiller which would add unwanted load to the compressor. 6. A log should be created in order to keep track of the temperature of the water exiting the tower. Diagrams of a Cooling Tower Maintenance Issues Associated with the Cooling Tower 1. Bacteria, bio-film, scale build-up, solid and erosion can occur if the water is not properly treated. A regular chemical treatment should be implemented to solve this issue. 2. Clogged spray nozzles also effects the cooling tower from running smoothly, algae and sediments clogs the nozzles which causes the water to be distributed unevenly as well as inadequate airflow which reduces evaporation from occurring. 3. Poor Pump Performance; any loose connections, failing bearing, clogged strainers, excessive vibrations can cause reduced water flow and premature equipment failure. Having a proper water flow is important because it can achieve optimum heat transfer. Principles of Evaporators An evaporator is a heat transfer system, and is a part of the refrigerant cycle in which liquid refrigerant is evaporated for removing heat from a space. Evaporators come in many different types, shapes, sizes and designs and can be place in certain categories for their use, example: * type of construction * method of refrigerant feed * operating conditions * method of air (or liquid) circulation * type of controls * application There are two main types of evaporators: 1. The Shell Type 2. The Finned Type EVAPORATORS USED FOR CHILLING LIQUID | top | Evaporators used to cool liquids can be divided into five (5) catergories:- * double pipe coolers * shell and coil coolers * shell and tube coolers * baudelot coolers * tank type coolers The refrigeration evaporators are designed for about 3500 cfm/ton of air flow. Air entering the evaporator is roughly 10 degrees warmer and the evaporator metal fins and tubing are about 10 degrees warmer that the refrigerant. The 20 degree difference in temperature must occur when the system is running. When the off cycle is in process the evaporator temperature rapidly disappears and becomes the same temperature as the air entering the evaporator. The most common problem with the evaporators occurs when the thermostat is set at its coldest setting of 55 degrees, this prevents the system from ever shutting off, when the evaporator reaches 32 degrees F, (45 psig) the moisture on the evaporator freezes, reduces air flow, and causes a major freeze up within the system. Air Cooled Condenser An air condenser works by removing heat from the surrounding air roughly at about 45 to 50 degrees. The heat removal starts with refrigerant flowing into the compressor as a cooled gas, the compressor then in turns pressurizes the refrigerant and what was previously gas now becomes a hot liquid. This hot liquid refrigerant flows into the condenser, which allows heat to escape from the refrigerant through a fin-like vent in the condenser’s casing. When the refrigerant reaches the end of the coils it now becomes cooler and in liquid form, however it has pressure, the pressure is pushed into a small valve and leaves the condenser as a mist. Air cooled condensers require minimum maintenance, the unit coil requires periodic cleaning, clean the unit using a brush, vacuum cleaner, pressurized air stream. Graphical Components of an Air Cooled Condenser Various Metering Devices The metering device is the section of the refrigerator that separates the high and low pressures. Below is a picture illustrating this: There are two main types of Metering Device used: 1. Thermostatic Expansion Valve & 2. Capillary Tubes Thermostatic Expansion Valve (TXV) stands for Thermostatic Expansion Valve, which is a metering device that is used to control the flow of liquid refrigerant entering the evaporator. This valve is also used for both commercial and industrial facilities because it carries a high efficiency and can adapt to almost any type of refrigeration, it is the only valve that ensures that liquid cannot return to the compressor. The TXV valve is located at the entrance of the evaporator and it has a bulb sensor situated in the suction line at the exit of the evaporator. The bulb is connected to the TXV using a capillary tube, here is an illustration of the capillary tube; this is great for manually adjusting the flow of the refrigerant that enters the evaporator. | Capillary Tubes The Capillary Tube is a fixed metering device. It restricts the device and cannot be altered to affect the performance from the evaporator. This metering device is used in both residential and commercial systems. The thin copper tubing causes the liquid to back up creating a high pressure side and then drops to a low pressure side at its outlet. Various Compressors They are several types of compressors that I have learnt about during this course which I will further expand on, they are: 1. Reciprocating 2. Rotary 3. Scroll 4. Screw 5. Centrifugal The Reciprocating Compressor Reciprocating Compressors are positive displacement machines, which mean that they can increase air pressure by reducing its volume. It is compose of a cylinder and a piston, as the piston moves upwards and downward a change in the volume, the air is now compressed and converted into energy. Advantages of Reciprocating Disadvantage of Reciprocating 1. Easy to Install 1. Potential for vibration problems 2. Lower Initial Cost 2. Higher maintenance cost 3. Simple Design 3. Many are not design to run at full capacity 100% of the time. 4. Large range of horsepower 4. Many moving parts Various Compressors The Rotary Compressor Rotary Sliding Vanes Compressors are positive displaying machines as well; the compressor has a pump which consists of a rotor, stator, and 8 blades. The slotted rotor is arranged in the stator and creates a crescent shaped swept area between the intake and exhaust port. When the rotor turns the volumes goes from a maximum at the intake port to a minimum at the exhaust port. The vanes are forced outward from the rotor slots and place against the stator wall by rotational acceleration. Oil is then injected into the air intake and the stator walls to cool the air lubricate the bearings and vanes and seal between the vanes and stator walls. Advantages of Rotary Disadvantages of Rotary 1. Simple Design 1. Single stage designs 2. Low Maintenance cost 2. Difficulty with high Pressures 3. Low rotational speed 3. Single stage designs have lower efficiency 4. Easy to install 4. Oil free designs are Unavailable Various Compressors The Scroll Compressor The scroll compressor is also a positive displacement machine, it has an orbiting motion which creates a many gas pockets that travels between the two scrolls. The outer part of the scrolls, the pockets draw in gas and then moves to the center of the scroll, where the gas is then released. Advantages of Scroll Disadvantage of Scroll 1. Reliable 1. Cannot dissemble for maintenance 2. Very quiet operation 2. Incremental capacity control on system with multiple compressors The Screw Compressor Screw compressor work on air filling the void between the helical mated screws and their housing. The helical screws are turned, and the volume is reduced and increases the air pressure. These compressors inject oil into the bearing and compression area because of cooling, lubrication and forming a seal between the screws and housing to decrease leakage inside. Various Compressors Centrifugal Compressor The final compressor I will be describing is the Centrifugal Compressor are however not positive displacement machines. They use very high-speed spinning impellers to start the air then the diffuser to slow down or stop the air, this is known as dynamic compression, it uses velocity to increase the pressure. These compressors have intercoolers between each stage to cool the air and minimize impeller damage due to erosion. Advantages of Centrifugal Disadvantage of Centrifugal 1. Can reach pressures up to 1200 psi 1. High initial cost 2. Costs improves as size increases 2. Complicated monitoring & control systems 3. Designed to give lubricant free air 3. Limited capacity control modulation, requires unloading for reduced capacities Bibliography http://www.fridgesolutions.com/basics.shtml http://www.inspectapedia.com/aircond/aircond15.htm http://xpedio.carrier.com/idc/groups/public/documents/marketing/811-20065.pdf http://www.engineeringtoolbox.com/air-compressor-types-d_441.html http://www.refrigers.com http://www.refrigerationbasics.com http://www.acel.biz/doc/download/AcqueIndustriali/CTI_ASHRAE.pdf http://www.actechelp.com/Mech_Refrig/metering_device6.htm