# MER160 General Comments on Project 1 Some observations common to many design reports PGK, 1/22/05.

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MER160 General Comments on Project 1 Some observations common to many design reports PGK, 1/22/05

Correction: Project 2 4.... once you have decided on a system you must specify an actual pump and use its cost and efficiency. Please make sure it can record... 5.Please make sure flow meter can record the total flow of the wine between tanks to satisfy the state liquor authority. 6.Etc.

Choosing the thermodynamic limits Crucial first step. –A mnemonic diagram could help discuss relationship among variables –In particular, can help in specifying crucial parameters P, T at state points 2 & 3.

Top of the cycle Crucial picks T3 and T2. If P3 is high, so is T3. –This makes for a large compressor (capital + running costs) but a small HX (capital only). –But it’s obvious from thermo (entropy generation!) that a high T3 is wasteful if all you need is 40C. –But, running at 40C fluid temperature, requires an infinite HX – which is a mite too expensive.

Bottom of the cycle T2 > 0  C is engineering expertise. Needs justification! –How close to 0  C do you dare go? Run cases with T room < 20  C and see how much it depresses T2! –In general, you need to add variability to all major parameters, to see how stable your design really is. Real life not invariant. –So what ifs are helpful. For example: suppose the compressor sags (low voltage); suppose the water supply has a leak the town has to fix and the flow stops. Will the water freeze in your design?

Choosing the refrigerant Most important variable –Only R22 and R134a make cut physically. –Not much that different by economics (R-22 better than R134a) –Most of you figured this from the data. Good. But I would have always given my customer the choice of either by presenting the two cases (some of you did, some not). –Present both with caveats so customer can understand the trade-offs.

LMDT (ugh!!!) It does not matter which way the fluids are flowing, just that the heat is flowing from hot to cold and both fluids change temperatures linearly as a result (check the derivation in your heat transfer text).

LMDT (ugh!!!) You all missed part of the point. Less the formula (  T1-  T2) /ln(  T1/  T2) as much as when you can apply it. –Linear thermal duties on both sides

LMDT – the real issue  T’s are not the issue. Their linearity is! –What happens in the condenser? –So is heat exchange linear between points 2 and 3?

LMDT – the real issue Is heat transfer in condenser uniform?

LMDT – the real issue In the condenser we will have a sensible heat region where vapor cools and an isothermal 2-  region –Need two LMDT calculations!

Economics - some facts. Interest rates are deus ex machina (also by the world’s and the US’s overall economy) Annual operating costs cannot be further annuitized. Don’t use term “annuity” when applied to a particular annual charge (such as the annual cost of utilities)

Economics In project 1, PW is useful to your immediate customer –It’s the sum of the upfront capital costs + the PW of the annual operating costs for the utilities Your customer’s customer wants his annual costs –Add the annual costs of the upfront capital costs to the annual operating costs. This is the sum of the mortgage payments he will need + the utilities.

Do they make they make sense? COP –Q L /W elec for a refrigerator but it is Q H /W elec for a heat pump => you want heat after all and you buy electricity. Hours per year –You all figured correctly ~1500 hours per year. No one commented that this is almost two years of continuous duty over 10 year life. What are the implications for maintenance? It was worth telling your customer that reliability could be a concern.

Innovations You are selling technology – wouldn’t a follow up contract keep the wolves from the door? For example: a new consideration for your customer. –Can’t afford to leak refrigerant (+ any oil contained therein) into the water supply! –Suggest a possible solution to help your customer (as well as get yourself a follow-on contract!)

Innovations Here’s a device your customer might be able to develop –It will sell municipalities if they believe their supplies are protected. –At this stage plausibility is enough to get your foot in the door –Make it work later (follow-on contract).