Chapter 7 EOCQ
How are air handling units different from packaged air conditioning units?
• Air handling units (AHUs): consist of coils (through which steam/hot water or chilled water is circulated from central boilers and chillers), filters, fresh air intakes, exhaust air discharges, and sometimes humidification equipment. Generally located in building mechanical spaces, often remote from the areas actually being served. Connected to these areas via duct systems. • Packaged air condition units: operates separately and is essentially self-contained. A fuel provides heat and a refrigeration system produces cooling units because the evaporator is located directly in the conditioned airstream. Ductwork is less extensive tan that for AHUs and usually mounted on the roof.
Why is chemical treatment of cooling towers needed? What kinds of treatment are needed?
• Cooling tower: cascades water over various forms of tower fill and pulls/pushes air through the tower. Some of the water evaporates thereby cooling the remaining water. This cooled water is circulated back to the cooling system to remove more heat from the condenser. • Tower uses outside air, dirt, and other debris can get caught in the tower water. This debris needs to be removed from the tower basin and creates a breeding ground for algae and bacteria. • Proper chemical treatment controls the buildup of these contaminants. • Reduce the buildup of scale (dissolved salts) due to the evaporation of water • Involves using scale and corrosion inhibitors, dispersants to disperse sediments, biocides to kill or prevent multiplication of bacteria, disinfectants for a variety of types. • Failure to provide treatment: decline in tower performance, increased operating problems and energy usage, and premature tower failure; potential for Legionnaires' Disease • Metal should be inspected and treated for corrosion (periodic cleaning and painting with corrosion inhibitors)
What types of managerial decisions might be affected by growing concerns with ozone depletion in the earth's atmosphere?
• Discharge of chlorofluorocarbons (CFCs) and to a much lesser extent hydrochlorofluorocarbons (HCFCs) are a major contributor to ozone depletion • Alternatives: • Retrofit less harmful refrigerants to existing equipment • New equipment should be purchased using HCFCs with lower ozone depletion or alternative refrigerants such as hydrofluorocarbons (HFCs) having near zero ozone depletion ratings • Use an absorption (heat-driven) chiller • Decisions about refrigerant changes require careful evaluation and consultation with equipment suppliers. • New refrigerants may introduce safety concerns. There may be a need for refrigerant monitoring devices because of potential toxic effects of exposure to refrigerants. May have to purchase breathing apparatus to protect staff in the instance of leakages. • Montreal Protocol: consider the implications of HFC and HCFC regulations when performing maintenance and purchasing new equipment. Side benefit: using refrigerants with lowered or zero ozone depletion are more efficient. • Increased costs with CFC/HCFC: cost of CFCs has risen substantially due to new taxes and restricted availability. Production of CFCs had essentially been eliminated by the year of 2000. Production of HCFCs will probably be eliminated by 2020, and only recycled forms will be available. Scarcity and cost of recycling will be reflected in a higher price. • A large amount of the refrigerant discharged was due to either poor equipment maintenance or poor maintenance practices. Refrigerant leaks must be eliminated. • Proper equipment must be used to remove refrigerants from equipment and either recycle it or store it.
How is the efficiency of cooling equipment measured? Is using integrated part load values a good way to define efficiency of large cooling equipment? Why or why not?
• Efficiency of cooling equipment involves measuring the ratio of cooling achieved (the output) to the amount of energy used to operate the system (the input) • For smaller equipment, the ratio is expressed as the energy efficiency ratio (EER), which is calculated by dividing the rated cooling output of the unit in Btu/hr by the watts drawn by the unit. The higher the EER, the more efficient the unit. • Larger cooling equipment (e.g. building chiller) may have its efficiency expressed as an EER or more commonly, as a value of kilowatts per ton of cooling. Kw per ton is an inverse efficiency, as efficiency increase as kw per ton decreases. • Integrated part load values (IPLV) is significant because equipment operates for much of the time at partial loads. Selecting equipment with these part load efficiencies in mind can result in lower operating costs. • Efficiency of cooling equipment varies depending on the conditions of the evaporator and the condenser. The lower the evaporator temperature or the higher the condenser temperature, the less efficiently the equipment operates and the less cooling it delivers. If the evaporator is starved for air as a result of a clogged filter, its temperature will drop, heat transfer is reduced, and efficiency will suffer.
What factors affect guest and employee comfort? What is the comfort zone and how is it useful?
• Factors that affect building comfort 1. Room air temperature 2. Room air movement 3. Relative humidity of room air or wet bulb temperature 4. Activity level in the room 5. Clothing worn by room occupants 6. Temperature of the room surfaces • Comfort zone: a region of temperature and relative humidity that provides a simplified way to view thermal comfort. A range of conditions in which 80% of the population tested was "comfortable." As air temperature rises, range of comfort shifts somewhat to lower levels of humidity. 2. What are the two elements of furnace and boiler efficiency? How can you determine whether your furnace or boiler is operating efficiently? • Two elements of furnace and boiler efficiency: 1. Efficient combustion of fuel: requires the correct mix of combustion air and fuel. Too little combustion air causes the fuel to burn incompletely, which leads to waste. Too much combustion air will reduce the temperature of the combustion gases and the amount of heat that can be removed from these gases. • Maintenance staff/outside contractors should check as part of semi-annual or annual maintenance of the furnace/boiler. More frequent checks when on-site personnel are used and large furnaces or boilers are installed. 2. Efficient transfer of that combustion heat to the air or water being heated • Combustion efficiency is checked by measuring the oxygen or carbon dioxide content and temperature of the flue gas. • Measuring content will show whether the unit is receiving the correct amount of oxygen. Larger commercial units now use a controlled combustion process in which air (oxygen) use is controlled and adjustable. • Flue gas temperature may indicate efficiency problems (low temperature: too much air is being supplied; high temperature: combustion heat is not being transferred efficiently)
What is a building automation system (BAS)? What benefits does it provide the hotelier?
• Large modern hotels have complex HVAC systems with system components distributed throughout the building. Requires a large amount of information and the coordination of a number of system components. • Building automation systems: collect this information and effectively coordinate the operation of the components; uses info to make decisions as to system operation. • Use a digital signal that provides info about the operating status and conditions of a system • Operator can provide control of systems from off-site • Maintenance needs: cleaning of sensors and proper calibration of controllers, transmitters, and gauges.
Why is makeup air required? What problems may result if the ventilation system provides insufficient makeup air to a guestroom?
• Most guestrooms operate at a slight negative pressure due to the bathroom ventilation fan. The air removed by this fan must be replaced from somewhere. • May rely on airflow under the guestroom/window leakage to provide makeup air • Decentralized HVAC units adjust the amount of outside air they admit • If outside air is not conditioned and controlled operational problems can result • During summer, hot and humid outside air can result in moisture condensation in, and deterioration of, walls • In winter, warm, moist interior air deposit moisture in the walls as it migrates through the walls • Other related problems include mold growth. • Solution: sealing and caulking (temporary), redesign of building systems and often a resizing and reselection of the guestroom HVAC units.
What sorts of controls are needed on HVAC systems? What is the difference between mechanical and digital controls?
• Thermostat: Senses the temperature, converts the temperature into a signal (usually on/off) that is sent to the HVAC unit conditioning the space. • Commonly encountered. • Signal can be on/off type or proportional to the measured temperature. "On" tells HVAC to do something (heat or cooling). • Some thermostats provide more control, such as time-of-day control, startup and shutdown, and staged heating. • Thermostats may have adjustable "dead bands" that establish a range over which neither heating nor cooling will be required. • May also have differential control that establishes range of likely operation around a setpoint. Appropriate differential avoids excessive rapid cycling of the unit, while maintaining temperature within acceptable limits. • Enthalpy or economizer: Senses temperature and relative humidity. Determines when the temperature and humidity of outside air make it a potential source of cool air for the building. Mechanical cooling system can be turned off and cooling system can be turned off and cooling accomplished using outside air • Older control sensors rely on mechanical sensing and response to parameters being measured • More modern systems use sensors that measure changes in electrical properties, relying on an electrical rather than a mechanical connection between the sensor and the controller. • Digital systems: use a control algorithm to analyze the signals and control the operation of equipment and systems. Allow the building operator to change the operation of the building from the central control computer. • Can monitor system operation from the computer, determining the operating status and conditions of equipment/systems • Makes variable output operation of equipment more feasible • Variable air volume (VAV) systems vary the amount of air flowing into a zone based on sensor input of the load in the zone.
What are the two elements of furnace and boiler efficiency? How can you determine whether your furnace or boiler is operating efficiently?
• Two elements of furnace and boiler efficiency: 1. Efficient combustion of fuel: requires the correct mix of combustion air and fuel. Too little combustion air causes the fuel to burn incompletely, which leads to waste. Too much combustion air will reduce the temperature of the combustion gases and the amount of heat that can be removed from these gases. • Maintenance staff/outside contractors should check as part of semi-annual or annual maintenance of the furnace/boiler. More frequent checks when on-site personnel are used and large furnaces or boilers are installed. 2. Efficient transfer of that combustion heat to the air or water being heated • Combustion efficiency is checked by measuring the oxygen or carbon dioxide content and temperature of the flue gas. • Measuring content will show whether the unit is receiving the correct amount of oxygen. Larger commercial units now use a controlled combustion process in which air (oxygen) use is controlled and adjustable. • Flue gas temperature may indicate efficiency problems (low temperature: too much air is being supplied; high temperature: combustion heat is not being transferred efficiently)
What are the operating characteristics, advantages, and disadvantages, of two-pipe, three-pipe, and four-pipe centralized HVAC systems? Are guests likely to know the differences between these systems? Why or why not?
• Two-pipe system: allows both heating and cooling, but only one at a time. • Disadvantage: Must decide whether to provide heating or cooling on a given day. Converting from one option to the other usually takes several hours or a day or two. Not capable of dealing with both heating and cooling over course of a day. • Advantages: Operating costs are low, allow shutdown of the boiler or chiller during seasons of the year, and provide fuel economies due to the operation of fossil fuel heating and an efficient central chiller. • Guests/management may be frustrated by lack of flexibility in meeting varying conditions (e.g. cool outside air temperature but abundant sunshine or widely varying outside temperatures from night to day, it will be very difficult to maintain comfortable conditions in all guestrooms). May install small electric resistance heaters if there are short heating seasons or using a boiler. • Steam may be used in older designs, but this means it is not possible to connect the fan coil to a chiller. • Three-pipe system: provides both hot and cold water to the fan coil units at all times and mixes the return water from the fan coils; relatively uncommon. • Advantages: Provide good guestroom comfort, since heating and cooling can be provided as needed • Disadvantage: Since both use lukewarm return water, efficiencies are reduced, causing operating costs to rise. Must use energy to bring lukewarm return water back up and down. • Four-pipe system: provides same level of comfort as three-pipe system, but keeps cold and hot water returns separated • Advantages: greater boiler and chiller efficiency, since less energy is needed to reheat and recool the return water • Disadvantages: most expensive central system option to install, since it requires more extensive piping and two coils in the fan coil unit. • Guests cannot tell the different in room comfort between three-pipe system and four-pipe system. 6. How can heat pumps be used to provide both heating and cooling in guestrooms? What is the difference between an energy efficiency ratio and a coefficient of performance? • Reduce cost of space heating may be to use a heat pump unit instead of electric resistance heating. Heat pump uses the refrigeration cycle to be more efficient by two or more times greater than electric resistance heating • Heat pump unit uses the refrigeration cycle for space cooling and space heating • Reversing the direction of refrigerant flow causes the evaporator and the condenser to switch functions • Heat can be removed from outside air and added to inside (and vice versa) • Coefficient of performance (COP): heat delivered in Btu/hr (output) divided by the heat equivalent of the electric energy input (with each watt of input the equivalent of 3.413 Btu/hr) • The COP is less of a constant than the EER because the temperature conditions of operation vary considerably over the heating season.