# Questions tagged [hvac]

Questions about air heating, ventilation, and conditioning, including indoor air quality (IAQ).

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20 views

### Static pressure calculations for laser fume extration

I'm trying to estimate the static pressure (SP) of a exhaust system for a laser cutter in order to select an appropriate exhaust fan. From what I've read so far (https://www.centralblower.com/Pdf/...
10 views

### Help Needed: Looking for Sample Phase Change Material (PCM) Calculations

I am working on implementing a Phase Change Material (PCM) system for a cold storage facility. While I'm familiar with traditional refrigeration systems, I'm relatively new to PCM technology and would ...
40 views

### Modeling an HVAC system's energy

I'm trying to model the energy consumption on an HVAC system with intermittent stops. As my model main parameters I use the following : A temperature and RH sensor of ambient air The HVAC working ...
27 views

### Using the Psychrometric Chart to calculate volumetric flow rate

Blockquote The inside design conditions for a conditioned space are 20.1°C dry-bulb and 15.2°C wet-bulb. The air tem- perature increases 10.6°C as it passes through the conditioned space. The ...
94 views

### What's the lowest ambient temperature a refrigerator will work in?

Assuming a standard domestic refrigerator that uses R134a, I can see only two reasons a vapor-compression cycle wouldn't work. Either the ambient temperature is so low that the R134a freezes in the ...
• 103
1 vote
100 views

### Why don't AC units run condensate over the outdoor condenser coils?

As an AC cools the air it pulls condensate from inside. Ive noticed that this condensate is often discarded via drain a line away from the unit, why is this the case? Wouldn't it be more efficient to ...
1 vote
52 views

### How do I find the volume flow rate for a circular air duct?

My assumption to solve this problem is to find the average velocity on each circle then get the average velocity between the 1st and 2nd circle and then find the volume flow rate between circle 1 and ...
1 vote
96 views

### Refrigerant flow

I am an HVAC engineer. I know that in the refrigeration cycle the expansion valve controls the refrigerant flow rate but if flow through a positive displacement compressor is fixed, how does it do ...
57 views

### Can cascaded heat exchange system transfer more heat?

Suppose there is hot water supply at fixed flow rate and inlet temperature around 40~60C, also there is cold air to be heated by the water at fixed flow rate and inlet temperature around -10~20C. Heat ...
• 163
1 vote
80 views

### Running heat pump extra mid-day worth it?

I have a home with a heat pump on the south-facing side of my home, so it gets sun all day. I let my home cool off to 62℉ at night, and at 6 am, when it is usually the coldest outside, I warm my home ...
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29 views

### Temporary moving of Cooling Towers from roof

I am working on the replacement of 2 air cooled cooling towers at a hospital. the cooling towers are located on the roof, whilst the water-cooled chillers are located in a plantroom in the basement. ...
107 views

### What does a dynamic-pressure loss coefficient of more than 1 mean (for eg air going through bend)?

Air flow (eg in a duct) has a dynamic pressure, and when the air flow goes through e.g. a 90°-bend the bend has a dynamic-loss-coefficient. The dynamic-loss-coefficient is used to calculate the total-...
91 views

### Nusselt number equations for external flow over a single finned tube heat exchanger

I am designing a heat exchanger that will utilize a singular finned tube of around 2" diameter with 4 fins per inch that are 3/4" tall and .03" thick. I am looking at finding the ...
1 vote
418 views

### Single hose portable air conditioner with door open?

I have a strange question/theory that I'd like some advice on. I currently have a garden shed that is being used as a home office. On sunny days, the inside of the shed gets very hot - too hot for me ...
19 views

### Method to estimate indoor temperature from heat inputs, rather than A/C power from target indoor temperature

I'm looking to estimate the temperature drop I could expect in my house by installing cool roof tiles. The house does not has a regular air conditioning system. I have delved into HVAC references but ...
23 views

### What are the Sensors/Flow Meters Required for Automated Fault Detection & Diagnosis in a HVAC system?

Automated fault detection and diagnosis is a very promising area of implementation to reduce energy consumption by buildings. A lot of research has been done in this area, but most of it is about the ...
11 views

### Different pump flow behaviour in common circuit?

I want to evaluate a system of closed circuit. Two hot water chillers are serving two different areas and have different capacities but their hot water source is common due to which we have combined ...
1 vote
46 views

### How will the motion of air be just before the isolation damper?

Let's say I have duct work shown as below. I have air moving from left to right with high velocity.There are two elbows and there is a damper of the same kind just before each of them and there is ...
50 views

### How much additional heat does sunlight add to a building? (estimate)

I'm adding air conditioning units the the second floor (UK) of a manor house, and estimating the output needed for AC units. I know the heat gain from walls, window, and roof, as I know the U-values ...
100 views

### Use of nozzle as a refrigeration device

When compressed air passes through a nozzle, its velocity increases while its temperature decreases due to the pressure drop. Why is this process not utilized as a refrigeration system similar to air ...
• 88
1 vote
49 views

### What is the impact of energy control in an AC in terms of cooling and power consumption?

Nowadays many inverter ACs come with energy saver modes claiming to run at 80% 60% etc… In this case wouldnt the cooling capacity also decrease? For eg if the AC is running at 60%, the AC compressor ...
• 111
8k views

### How do heat pumps output more heat kW than the electrical kW used to run the heat pump?

I understand that a 2 kW electric heater will use 2 kW of electricity and output 2 kW of heat (apart from small losses of energy as infra red, vibration etc.. but let's exclude those for this example ...
• 241
72 views

### heat accumulation tank: hot water IN from top or bottom?

should heated water from a wood boiler enter a vertical cylindrical heat accumulation tank from the top or the bottom? mine goes in from the top and can barely achieve more than 60C, whilst the boiler ...
• 213
35 views

### What factors limit the effiency of heat pumps to well below the Carnot efficiency?

Aircon SEER values top out around 23. SEER is an anachronistic unit, but it can be converted into a coefficient of performance (COP) by multiplying by 0.293. This gives a value of around 6.7, which ...
4k views

### Why isn't heatpump technology used for solar collector panels and boiler tanks?

All solar collector and boiler tank systems that I can find on the market use a liquid to circulate the heat between solar collector panels (or PVT panels) and boiler tanks. Is there a reason why they ...
• 265
37 views

### Air conditioner efficiency?

So I've been learning a lot about latent heat, and how the refrigerant is actually based on this concept. Just like dissolving water into air converts sensible heat into latent heat, spitting out ...
• 111
1 vote
29 views

### How does compressor ensure that rfgnt exiting the compressor exits at sufficient pressure such as its condensation tmp is always > otsd air temp?

I have very basic knowledge of the refrigerant cycle. I am talking about a Split system Home AC with an air cooled condenser. Assume a basic AC reciprocating or scroll compressor (i.e NOT a variable ...
112 views

### Why is there not a large power or performance difference between these two air conditioners?

My parents have these two window air conditioners that are the same brand and look the exact same on the outside. The control interface is the exact same as well. I'm not sure how old they are but I'm ...
39 views

### How would a gas flow switch react to falling pressure in the gas supply?

There's currently an ongoing discussion (not decision!) in Germany if households could be cut off from gas supply come winter, to make more natural gas available for industry. Of course media is full ...
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1 vote
90 views

### Why would you put covers in front of the output of the condenser fan on these mini-split units?

Seems like this would be bad for airflow and hurt efficiency? I'd think you would want to blow that hot exhaust air as far away from the intake as possible, and with as little static pressure as ...
• 111
1 vote
58 views

### portable humidifier (balancing a bucket of water on a bicycle)

I built a (relatively) portable air-blowing device that purifies room-temperature ambient air (HEPA, carbon filter), and then heats and humidifies it to 37 °C and RH85%. I use the machine to do ...
1 vote
80 views

### Can airflow through an HVAC duct be considered inviscid? Does viscid flow ever occur for HVAC?

Can airflow through an HVAC duct be considered inviscid? Does viscid flow ever occur for HVAC? If the answer to (1) is yes, how do we know airflow in an HVAC pipe is inviscid? In general, what's a ...
• 169
34 views

### Compressor ratio and addition of battery cooling loop (chiller) in the existing HVAC circuit

Imagine if i want to change my IC car to an electric vehcile, with the obvious addition of battery, how can i use existing HVAC loop to cool the battery using a chiller. HVAC was obviouly desinged ...
1 vote
27 views

### Intra-facility Heat Pump? (HVAC)

Does anyone know if there exists a type of heat pump for moving heat from one indoor area to another? In this scenario there is a room where excess heat is being generated and could be used in heating ...
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300 views

### Heating pump/refrigerator - calculate efficiency

I'm wondering how I can calculate the overall thermal efficiency of my heating/refrigerator pump. Actually I have this AC/heating pump unit from Samsung (P/N: AR09TXFYBWKNEE). I've placed a power ...
• 101
697 views

### Can an AC compressor used as a Hydraulic pump?

Can a car ac compressor be used as a Hydraulic pump ? Does it require some modifications to do so or is it impossible to begin with and the compressor can't handle the oil?
• 103
184 views

### Cooling system to maintain 0.5 °C at 95% humidity

I am looking for some guidance on a redneck cooling system. I need to store seeding trees at a temperature just above freezing, and at a humidity of close to saturation. I only need the system for ...
62 views

Pic for reference - any HVAC pros around? https://i.sstatic.net/vMIGR.jpg The return duct matches the supply duct width, but the intake drop looks so small in comparison. Do I need to get this fixed ...
71 views

### Identify this textbook

I would really like to buy this textbook, but only see a scan of one chapter online. Could anybody help me identify the book? Thanks!
• 3
539 views

### Is it feasible to build a pedal-powered air conditioning booth?

Imagine it's hot and humid outside. You see a pedal-powered air conditioning booth. You get in the booth, close the door, and start pedaling. The pedals are connected to a shaft-driven (automotive-...
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1 vote
70 views

### What is the efficiency of direct electric heating versus water heated via electricity?

I am attempting to keep plants warm in a greenhouse. Therefore, I assume that heating the soil should be most efficient, as that's what plants seem to care about the most (i.e. the temperature of the ...
• 111
94 views

### What would happen if the air conditioning failed in a skyscraper for an extended period?

I'm writing some fiction which involves the AC in a modern glass curtain skyscraper (50-80 floors?) in an American city failing for a period of weeks, maybe even for up to two months. Could anybody ...
1k views

### Can Peltier Modules be used as an Air Conditioner? [duplicate]

Air Conditioners are very expensive so I'm planning to build a split type air conditioner using Peltier modules for my sealed room that is 6.5 length, 7 feet height, and 5.6 wide. I plan to use nine ...
627 views

### Is the use of direct-expansion split systems to heat or cool exit enclosures permitted under the US model building Codes?

Most multistory buildings have enclosed exit stairways somewhere within them to provide safe egress from their non-discharge floors. In a building that relies on hydronic heating/cooling, steam ...
1 vote
47 views

### HVAC Condensor Liquid Level/Ratio Estimate

Let's say I have a simple condenser inside of a HVAC system with 1 point of entry(Super-heated Vapour) and 1 point of exit(Saturated Liquid) for my refrigerant r134a. A pipe passes through it with ...
39 views

### Pressure loss of a shape change of an ar duct, keeping the area the same

I have an air duct 400x400 mm that will attach to an engine with a ventilation opening 981 x 167 mm. I assume a smooth funnel of about 500m length between the two shapes (that's about as much space as ...
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120 views

### Why is there limited use of Y (Wye) fittings in a refrigeration system?

I'm a Mechanical Engineering intern currently working in HVAC and Refrigeration for grocery stores. I'm looking into ways, with lower energy losses, to encourage equal flow distribution at liquid pipe ...
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1 vote
111 views

### What is the max speed and duty cycle of an air conditioner's magnetic clutch?

I am considering using an automobile ac compressor to control the resistance of an exercise machine by engaging and disengaging the magnetic clutch to let it slip. The max torque should be about 500 ...
1 vote
44 views

### Why does my heat pump condenser outlet make a single pass through the evaporator before the expansion valve?

I'm hoping to modify a hot water heat pump to capture both the hot and cold side. I'd work with an HVAC shop to make the modification but want to understand the circuit. It's not the simplest of units ...
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