AC System Design PDF

Level 4 Air Conditioning System Design RACM-405 The term *air conditioning* refers to a refrigeration process that is applied to comfort cooling. Air conditioning and comfort cooling processes are classified as **high-temperature** refrigeration systems. They operate with a saturated evaporator...

Level 4 Air Conditioning System Design RACM-405 The term *air conditioning* refers to a refrigeration process that is applied to comfort cooling. Air conditioning and comfort cooling processes are classified as **high-temperature** refrigeration systems. They operate with a saturated evaporator temperature above freezing. They operate with the same major components. - Compressor - Condenser - Metering Device - Evaporator **Types of Air Conditioning Systems**: *Evaporative Cooling* **General Operation** - Uses **100% outdoor air**, requiring an exhaust outlet (e.g., an open window) to prevent pressurization. - The incoming hot, dry air **absorbs moisture**, cooling as it passes through wet media. - Ideal for **hot, dry climates** where added humidity is beneficial. **Types of Evaporative Coolers** 1. **Drip-Type Swamp Cooler** - A **pump** lifts water to the top of an evaporative pad. - Water **drips down** the pad, keeping it moist. - Hot air passes through the pad, evaporating moisture and **cooling the air**. 2. **Spray (Slinger-Type) Swamp Cooler** - A **pump sprays water** onto the pad. - Air passing through the pad evaporates moisture, **absorbing heat** and cooling down. 3. **Rotary-Type Swamp Cooler** - A **motor rotates** an evaporative pad through a water sump. - As the pad **becomes wet**, it functions like other swamp coolers, cooling the incoming air. **RTU Packaged Units:** ***Installation:*** - Selection of unit location. - Roof penetration. - Installation of the curb and roof sealing. - Rigging unit to the roof. - Installation of power and control circuits. - Installation of fuel lines for heating if needed. - Economizer installation. - Ductwork. Unit placement should allow for proper air flow and service clearance (24"-36").  ***Economizers:*** **Fixed Dry-Bulb** -- Uses an outdoor temperature sensor to read ambient temperature, the setpoint is fixed. Ex. SP = 68 F. 70 F outdoor temperature -- no economizing. 65 F outdoor temperature -- unit is economizing. **Differential Dry-Bulb** -- Uses return air temperature sensor along with the outdoor sensor. If the return air is lower than the outdoor air temperature, no economizing. If the return air is warmer than the outdoor air, unit is economizing. **Fixed Enthalpy** -- Uses solid state temperature and humidity sensors, the enthalpy control measures the total heat content of the air. 28 btu/lb. is a typical. If the heat content of the outside air is greater than the setpoint, no economizing. If less than, unit is economizing. **Differential Enthalpy** -- Measures heat content of return air and outside side. If the heat content in the outside air is greater than the return air, it will not economize. If the return air heat content is greater than the outside air, unit will economize. Minimum Position: **10 %.** ***Tools Need for Calibrating Economizers:*** - **Thermometer -- calibrating temperature sensors.** - **Psychrometer -- calibrating humidity sensors.** - **Multimeter -- checking signal to controller.** ***Ventilation and Why We Need It:*** Ventilation is the process of supplying air by natural or mechanical means to a space. Ventilation is needed to provide fresh breathing air for spaces that are occupied. If no fresh air was introduced into a room or area with occupants, the oxygen getting consumed and not replaced becomes a problem. CO₂ is produced in our cells when breathing. When CO₂ levels rise in a space, cognitive function declines, breathing rate increases, blood acid level increases and these are just to name a few. To combat this, we supply fresh air to buildings. However, ventilation does have a negative effect on heating and cooling systems performance. During the winter, we are bringing in cold, dry air. During the summertime, we are bringing in warm, humid air. We can help recover some of this energy by employing an HRV or EEV. These are mechanical systems that mix the outdoor air with the already conditioned indoor air, recovering some energy that would be lost. ***HRV vs. ERV:*** **Heat Recovery Ventilators** -- these have two blowers, an **intake** and **exhaus**t blower. They are balanced systems, meaning the amount of air exhausted from the space should match the amount of air being drawn in. The air streams are pulled over a heat exchange core where the fresh air is tempered either warmer in the winter, or colder in the summer. HRVs only recovery **sensible heat**, now if you remember basic heat, you know that there are much more available BTUs in latent form. **Energy Recovery Ventilators** -- ERVs work in a similar principle, however they employ a **desiccant heat wheel**. The wheel actually rotates between both the exhaust and supply air streams increasing the heat exchange, In the winter months, the **latent heat** is recovered from the indoor air due to the desiccant heat wheel. In the summer months, the wheel helps dehumidify the fresh air prior to entering the structure. ***\ *** ***ASHRAE Standards 62 -- Minimum Ventilation Rates*** ***\ *** ***\ ***  ***Constant Air Volume (CAV):*** Older air-conditioning systems operated on a constant volume basis, circulating a fixed amount of air whenever the blower was on. These systems lacked zoning, meaning all areas received the same airflow regardless of their specific heating or cooling needs. ***Demand Control Ventilation (DCV)***: Increases or decreases ventilation brought into the space from a programmed schedule, or can be controlled automatically with CO₂ sensors. The more occupants, the higher the Co₂ content, the more ventilation needed. ***Variable Air Volume Systems (VAV)***: Varies the amount of air volume for each location in a conditioned space. **The volume of air depends on a number of factors, cooling/heating requirements of the space, ventilation requirements, occupancy levels in the space.** The difference between a regular packaged unit is the blower. The blower is a variable speed motor, allowing a range of air movement. They are also equipped with a terminal units located in each zone. The terminal units are typically equipped a **damper, air pressure sensor, reheat coil, and devices needed to control damper position.** The reheat coil can be a hot water type, or electric heaters. The damper typically utilizes a 0-10VDC signal, 0 being closed to the minimum position and 10 being open to the maximum. Caution is needed when the reheat coil is hot water, or the system uses chilled water. To prevent these lines from freezing, a freeze stat can be installed in the outside ventilation, this can be used to enable heaters to preheat the outside air prior to it passing over the reheat/chilled water coils. If ambient reaches 32 F, the switch makes and powers the preheat contactor coil. ***Air Pressure Sensor VAV Box:*** The air pressure sensor in a VAV box is located just downstream of the damper. When the call for cooling at that zone is reduced, the damper starts to close, increased the pressure in the box. The sensor signal back to the main controller will tell the blower to reduce its speed. The key to this is it being located **downstream of the damper.** ***Static Pressure Sensor:*** The static pressure sensor is used to ensure adequate air pressure for the duct system. The signal will vary depending on the pressure it senses. Higher pressure will tell the blower to ramp down, the opposite with low pressure. The location of this is key, the closer to the blower the higher the reading. If we want to prove pressure for the duct system it should be as far away from the blower as possible. Typically, near the end of the longest duct run. If it is a return static pressure sensor, it would be located as far away from the start of the return duct. Ex. Duct run of 100 ft. Where would you locate the sensor? A. 25 ft. B. 30 ft. C. 50 ft. D. ***75 ft.*** ***Zoning a Central Split System:*** Zoning should be applied if there will be uncomfortable temperature swings in different areas of the home. Some two-story homes will have upwards of a 10 F temperature difference from the 2^nd^ to 1^st^ level. The thermostat is usually located on the main floor in a central area. In AC mode, the lower level of the structure will be much colder. If the thermostat was located on the 1^st^ floor or lower level, the upper levels will never get cool enough. To zone the system, you must ensure proper duct sizing and configuration or that the existing duct work can handle the different volumes of air. A zone damper will be installed on the branch line, a thermostat in the zone. These will be wired to a zone controller that will control the operation of the damper on a basis of the conditions in that actual zone. ***Terminal Reheat Zoning:*** A **terminal reheat system** in HVAC is a type of **zoned air distribution system** that delivers conditioned air (typically cool) to different zones and then uses **reheat coils** at the terminal units to fine-tune the temperature as needed. **How It Works:** 1. A central air handler supplies **cool, dehumidified air** to all zones. 2. Each zone has a **terminal unit** with a **heating coil** (electric or hot water-based). 3. If a zone requires heating, the terminal reheat coil warms the air before it enters the space. 4. The system allows for **precise temperature control** in different zones while still benefiting from central cooling. ***Induction Reheat System:*** An **induction reheat system** in HVAC is a type of **air distribution system** that combines primary air from a central air handling unit (AHU) with room air using an **induction unit**. It then reheats the mixed air as needed before supplying it to the space. **How It Works:** 1. The **central AHU** supplies **high-velocity primary air** (typically cooled and dehumidified) through ductwork. 2. This high-velocity air enters **induction units** located in each zone or room. 3. The induction unit **draws in room air** through a coil using the Venturi effect, mixing it with the primary air. 4. A **reheat coil** (hot water or electric) warms the mixed air before it is discharged into the space. 5. The system maintains **temperature and humidity control** while providing adequate ventilation. ***Zoning a Hydronic System:*** Zone control valves, or zone valves, regulate water flow in heating systems based on thermostat control. When powered, heat is generated, causing a bimetal strip to warp and open the valve. Many zone valves include a manual override for troubleshooting and temporary heating. In operation, when a zone requests heat, its valve opens, triggering an end switch that activates the circulator pump. Heated water is then distributed until the thermostat is satisfied, at which point the valve closes and the circulator stops. ***Zone valves come in two types:*** - ***Two-port valves:*** Control individual heating zones but may increase supply header pressure when multiple valves are closed. A pressure differential bypass valve can help mitigate this issue. - ***Three-port valves:*** Maintain a constant water flow, redirecting it back to the boiler when certain zones do not require heat. ***Sizing Humidity Control Equipment:*** 1\. Determine the Required Humidity Load **The first step is calculating the moisture load, which depends on:**\ ✔ Building size & volume (sq. ft. and ceiling height)\ ✔ Climate conditions (indoor vs. outdoor humidity levels)\ ✔ Number of occupants (people generate moisture through breathing and activities)\ ✔ Ventilation rates (fresh air introduction affects humidity levels)\ ✔ Appliances & activities (cooking, showers, laundry, etc.) Dehumidifier Sizing (For High Humidity Control) Dehumidifier capacity is measured in pints per day (PPD) of moisture removal. - **Manual J or ASHRAE guidelines** help determine the moisture load. - A rough estimate can be made using space size: - 500-1,500 sq. ft. → 20-30 PPD - 1,500-3,000 sq. ft. → 30-50 PPD - Large spaces/commercial → 50+ PPD or whole-house dehumidifier (\~90-150 PPD) ✔ Basements & crawl spaces often require higher capacity due to poor ventilation.\ ✔ High-efficiency models use less energy per pint of moisture removed. Humidifier Sizing (For Low Humidity Control) **Humidifiers are rated in gallons per day (GPD) of water output.** - General guidelines based on home size: - Up to 1,000 sq. ft. → 3-5 GPD - 1,000-2,000 sq. ft. → 6-8 GPD - 2,000-3,000 sq. ft. → 9-12 GPD - Larger homes → Whole-house humidifier (12-18+ GPD) ✔ Steam humidifiers offer precise control but require more energy.\ ✔ Bypass & fan-powered humidifiers integrate with HVAC for even distribution. 2\. **Select the Right Equipment Type** Dehumidifiers - Portable dehumidifiers → For small areas, easy to move. - Whole-house dehumidifiers → Integrated into HVAC, ideal for large homes. - Commercial dehumidifiers → High-capacity models for industrial spaces. Humidifiers - Portable humidifiers → Room-by-room control. - Whole-house humidifiers → Works with HVAC system for consistent humidity. 3\. **Monitor & Control Humidity Levels** - Ideal indoor humidity: 30-50% - Use hygrometers and thermostats with humidity control to maintain proper levels. - Adjust settings seasonally based on outdoor conditions. ***Split System Piping:*** Split systems can come pre charged for a certain length of piping, or can come dry and need to be field charge. ***Always follow manufacturers specifications for piping and refrigerant charge***. For example, a 410a system pre-charged for 25 ft has a piping run of 49 ft. The suction line is 1 1/8" with a 1/2" liquid line. We find the chart and it states for 1 1/8" suction line the additional charge is 0.202 oz/ft. The 1/2" liquid line column states 0.94 oz./ft. 0.94 + 0.202 x 24 ft = **27.4 oz.** 27.4 oz/16 oz. = **1.7 lbs**. or **1 lb. 11.2 oz**. We need to add the additional charge required to fill the liquid line after the evacuation. ***Equipment Start Up:*** **Verify incoming voltage is within range +/- 10 %.** **Verify correct phase balance, all three legs need to be within 2%.** **Condensing Temperature Over Ambient (CTOA):** This refers to how much higher the temperature of the refrigerant in the condenser is above the surrounding air. - 6-9 SEER = 30 F CTOA. - 10-12 SEER = 25 F CTOA. - 13-14 SEER = 20 F CTOA. - 15-18 SEER = 15 F CTOA. - 18+ SEER = 10 F CTOA. **Evaporator Design Temperature Difference (DTD)**: difference between the saturated refrigerant temperature in the evaporator and the return air temperature. - @ 400 CFM/Ton = - 35F - Oversized evaporators/increased airflow above 400 cfm/ton results in a LOWER DTD. - Airflow less than 400 CFM/Ton = HIGHER DTD. **Temperature Difference (TD)** -- The difference in temperature from the return air to the supply air. 20 F rule of thumb only if operating @ 75 F, 50 % RH, and 400 CFM/Ton. **Target Superheat**: Can be found in manufacturers literature, however 5 -- 15 F super at the evaporator outlet is standard. **Target Subcooling:** Can be found in the manufacturer's literature as well. TXV systems will usually have 8-14 F subcooling, where piston or fixed orifice will have around 5-20 F of subcooling. **Discharge Superheat (DSH) -- Discharge Line Temperature - Saturated Condensing Temperature.** This is can be used to verify charge in most systems. Readings should be between **27 F and 72 F** although the number can vary slightly per manufacture. Overcharge/flood back causes a **low DSH**, where as an undercharge/starved evaporator causes a **high DSH.** Ex. **14 SEER** AC system with a **95 F** condensing temperature, and a discharge line temperature of **210 F.** DSH = 115 F. This would lead to look for an **undercharge or starved situation**, or **lack of oil return** if there is a separate oil return circuit. Ex. **14 Seer**, **5-ton** split system, **400 CFM/Ton**, **90 F** outdoor ambient temperature, subcooling of **12 F**, superheat of **10 F**, return air temperature is **75 F.** If the system is operating as designed, coils clean, airflow correct, etc. What would you expect the pressures to be? What would the expected liquid line temperature be? What would the expected suction line temperature be? The temperature difference between the supply and return is 23 F, and we have a change in enthalpy of 5.5 btu/lb. what is the system capacity? What is the compression ratio? What can cause a low evaporator DTD? What can alter the temperature difference from return to supply if the airflow rate stays the same? The system is rated for 208/230V, what is the voltage tolerance?